Strategies for the Synthesis of Taxol
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Transcript of Strategies for the Synthesis of Taxol
Mohima Begum Roomi Chowdhury
CHEM3004
1
Strategies for the Synthesis of Taxol
by
Mohima Begum Roomi Chowdhury
Abstract
Paclitaxel, known as Taxol (commercially known as Taxol®)
[1] is one of the most revered anti-cancer drugs to be
developed. After its discovery in the 1960’s, it has occupied the minds, laboratories and countless journals of
scientists all over the world. The story of Taxol dominated many different forums; from the science community
and politics to environmental and wildlife media. With the problem of cancer being ever-evident, and the
extraction of naturally occurring Taxol proving inefficient, insufficient and unsustainable, the issue of developing
the compound came in to the limelight. There was no doubt that the obtention of the then potential
chemotherapeutic drug, Taxol, ought to be pursued – prompting the necessity for chemical synthesis.
It took around three decades from the discovery of Taxol and the elucidation of its structure to the completion of
the total synthesis – with many semi-syntheses and syntheses of various analogues along the way. This long
process of the drug’s development highlights the difficulties faced by the many scientists who have attempted to
find a viable synthetic route – whether the partial or total chemical synthesis, the biosynthesis, synthesis from
cell tissue culture, or any of the other attempted methods.
When developing a viable strategy to fully synthesise Taxol, how and when to implement each part of the
structure – the rings and the regio- and stereospecific functional groups – was carefully planned, and many routes
were possible. Discussion and analysis of the various strategies for the total synthesis of Taxol will be carried
out. Each of the methods has advantages and disadvantages – which will be discussed and analysed.
Mohima Begum Roomi Chowdhury
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Table of Contents
1. Introduction
2. Total synthesis of Taxol
a. Introduction
b. Holton group
i. Introduction
ii. Strategy
iii. Results and discussion
c. Nicolau group
i. Introduction
ii. Strategy
iii. Results and discussion
d. Danishefsky group
i. Introduction
ii. Strategy
iii. Results and discussion
e. Wender group
i. Introduction
ii. Strategy
iii. Results and discussion
f. Kuwajima group
i. Introduction
ii. Strategy
iii. Results and discussion
g. Mukaiyama group
i. Introduction
ii. Strategy
iii. Results and discussion
3. Discussion and conclusion
a. Comparison of strategies
Mohima Begum Roomi Chowdhury
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*This is an estimate and is rounded to the closest 10.
Data collated from: Cancer Facts & Figures 2005, 2006,
2007, 2008, 2009 and 2010, American Cancer Society.
1. Introduction
The problem of cancer and its incurability has encouraged
much research in various different fields for many years. With a
general increase in the number of people being newly diagnosed
with cancer from year to year [4]
(see table), research will need to
continue and one avenue of interest will be nature and the products
it has to offer. The propensity of nature to supply useful medicines
has been known since ancient times. Nowadays with funding from
public and private sectors in countries such as the USA, nature’s true
potential has been and can be further exploited.
Many natural products have been discovered with potential
for pharmacological and therapeutic use, for instance as anti-cancer agents [10]
.
Amongst them are Epothilone B, which was found in ‘Sorangium cellulose’, a
myxobacterium (soil-dwelling bacterium); and Podophyllotoxin, which was extracted
from the roots of the ‘American Mayapple’ (Podophyllum peltatum) and various other
species of the Podophyllum genus. Camptothecin is another natural anti-cancer agent,
isolated by Wall and Wani from the bark and stem of the ‘Happy Tree’ (Camptotheca
acuminata). Other naturally occurring compounds which possess cytotoxic properties (ability to kill cancer cells)
include Vinblastine, Roscovitine, Silvestrol [5]
and many others.
Some of these compounds are already used clinically; others are at the stage of
clinical and preclinical development and of course many others are yet to be discovered.
One particular compound which was found to have cytotoxic properties in the 1960’s
gained vast interest worldwide when extraction from natural sources proved inefficient,
insufficient and unsustainable.
Paclitaxel (known as Taxol) is a complex functionalised diterpene. Diterpenes are organic compounds
comprising of four isoprene units and are produced by many plants. In the 17 years that Taxol®
has been on the
market, it has been one of the best-selling anti-cancer drugs worldwide – some have dubbed it “the best-selling
anti-cancer drug ever” [3]
. Since its introduction to the pharmaceutical market, it has been used by millions of
cancer sufferers all over the world.
Year Number of new cases of
cancer in the US*
2005 1,372,910
2006 1,399,720
2007 1,444,920
2008 1,437,180
2009 1,479,350
2010 1,529,560
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The structure of Paclitaxel (Taxol) elucidated by M. E. Wall and
M. C. Wani in 1971.
Taxol was the first phytochemical (a biologically active compound found in plants) drug product to gain
approval from the US Food and Drug Administration (FDA) in more than 25 years [3]
. In search of natural
compounds with potential medicinal uses, samples of tree bark, leaves, seeds, shrubbery and the like were
collected from many trees and plants, including the Pacific Yew tree (Taxus brevifolia); a slow-growing
evergreen tree, native to the Pacific Northwest of America. When the collection was sent for routine tests for
biological activity, the Yew tree samples showed an ability to kill cancer cells; hence attempts to isolate the active
substance began [16]
.
The cell-killing essence was first isolated, and its
structure was elucidated by Wall and Wani in 1971 [1]
.
Wall named the compound ‘Taxol’ as it comes from the
genus ‘Taxus’ and contains the alcohol functional group.
The confirmed structure shows that Taxol comprises of
the taxane ring system (a 15-membered tricyclic ring
system), a rare four-membered oxetane ring and an ester
side chain [2]
.
Extraction and structure elucidation of the active
compound took several years as Wall and Wani were
faced with many problems – including running out of
material regularly, poor yields (of less than 1%) and dated structure-elucidating lab equipment. The technique
used to characterise Taxol’s structure was the ‘heavy atom method’ (also known as the Patterson technique) [1] [12]
which is one of two X-ray crystallographic methods. The theory behind the method is that the heavy atoms
impose their diffraction pattern on the rest of the molecule; this is then analysed by X-ray Diffraction
Crystallography [13]
.
As well as problems with characterising the structure of Taxol, many problems were encountered with
developing a viable and sufficient route to obtaining the active cytotoxic product. As mentioned, the extraction
from the Yew tree bark, for instance, provided abysmal yields – sacrificing a 100 year old tree afforded 300kg of
bark which provided about one dose of Taxol (around 300mg) [2]
. As the necessity for a viable synthetic route
became apparent, many scientists showed interest in Taxol and attempts were made to synthesise the cytotoxic
compound by means of total and semi-syntheses; as well as other methods, such as cell tissue culture studies.
Chemists at the forefront of research into the synthesis of Taxol included R. A. Holton, K. C. Nicolau, S J.
Danishefsky, P. A. Wender, P. Potier, F. Guéritte-Voegelein, D. Guérnard, Kuwajima, Mukaiyama, D. G. I.
Kingston and many others.
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The challenge to artificially synthesise the Taxol, dominated many scientific studies – from chemical
synthesis and biosynthesis to botanical and cell culture studies. Also, an avenue that required studying was the
mode of action by which Taxol killed cancer cells. It was first thought that Taxol killed cancer cells by inhibiting
microtubule assembly – hence interrupting the ability of cells to divide. However, in 1979, cell culture tests
carried out by Susan B. Horwitz, a pharmacologist based in New York, confirmed that in fact Taxol worked by a
completely unique mechanism that had never been seen before [2]
.
The mode of action by which Taxol works was found to be opposite to that predicted whereby Taxol
actually stimulates the growth of microtubules. These microtubules would usually form at the beginning of cell
division and go on to break down and form tubulin (building blocks of the cell structure) and hence cancer cells
would divide – but Taxol stops this process from occurring. When Taxol is present, formation of microtubules
goes in to overdrive and with the process being irreversible, the microtubules are unable to disassemble. This
leads to mitotic arrest; hence cancer cells collapse and die (by apoptosis) [2] [7]
. This newly-discovered means of
cytotoxicity has since been found in the modes of action of many other naturally occurring products, such as
Epothilones, Eleutherobin, Discodermolide and others.
With such an effective novel mode of action, it was undeniable that an alternative route to produce Taxol
was necessary and many scientists around the world engaged in the challenge. This resulted in many semi-
syntheses from Taxol precursors, studies on cell cultures (investigations of bacteria present in various Taxus
species), and perhaps most interestingly, routes of total synthesis – first achieved simultaneously by Holton and
Nicolau groups in 1994. The challenge of synthesising Taxol from scratch was fraught with drawbacks; from the
difficult tricyclic carbon framework to the complex stereochemistry of the molecule (Taxol has 11 chiral centers
and hence 2048 diastereomeric isomers) [8]
. Alterations in the configurations were found to lead to important
changes in the biological activity; hence it was essential to ensure the correct configurations were achieved.
Various synthetic strategies for the total synthesis of Taxol will be investigated. The route of total
synthesis carried out by Holton, Nicolau, Danishefsky, Wender, Kuwajima and Mukaiyama and their groups will
each be discussed and compared.
Mohima Begum Roomi Chowdhury
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2. Total Synthesis of Taxol
From its discovery in the 1960’s, Taxol was recognised as a vitally important compound which
unfortunately could not be sustained naturally. This led to the need for a viable synthetic route to make Taxol
artificially. By the 1980’s, more than 30 research groups were working towards becoming the first ones to
achieve the total synthesis of Taxol. In 1994, Holton and Nicolau groups simultaneously published different
routes for the total synthesis. Other total syntheses have since been achieved by the groups of Danishefsky,
Wender, Kuwajima and Mukaiyama.
The taxane ring system posed many problems when synthetic strategies were designed, for instance, the
rigid arched structure of the whole molecule made functionalisation very difficult. Also, the specific regio- and
stereochemistry of the molecule required careful planning of which reagents and conditions to use for each step,
as well as what order to carry out the functional group additions; using conformational control elements to direct
functionality stereoselectively. Each of the groups adopted unique strategies where the construction of rings and
functionalisation around the periphery were carried out in a different sequence. A common theme in all the
strategies was that the addition of the side chain at C-13 was carried out at the end; all but one group used
Ojima’s β-lactam as the side chain precursor.
Ojima developed the synthesis of many β-lactams, including (±)-cis-1-benzoyl-3-
triethylsilyloxy-4-azetidin-2-one, which was used as the side chain for Taxol. These can be
synthesised using the chiral lithium ester enolate – imine cyclocondensation strategy [ref 14
in Ojima paper]; which gives a high yield in just three steps with an enantiomeric excess of
almost 100%
Each strategy for the total synthesis of Taxol, from commercially available and/or naturally abundant
starting materials, has limitations and many different problems were faced by each of the chemists aiming to
achieve an efficient and practical route to artificial Taxol. Many drawbacks were experienced due to the complex
and intricate configuration of the Taxol molecule – it was found that even slight modifications of just one
functional group could change the effectiveness of Taxol as a cytotoxic compound.
Mohima Begum Roomi Chowdhury
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Holton’s total synthesis of Taxol
Holton and his group followed a linear synthesis strategy whereby the functionalised AB-ring system was
first formed, followed by cyclisation to obtain the C-ring and then instalment of the oxetane ring. The synthesis
of Taxol was completed by the addition of the side chain at C-13 using Ojima’s β-lactam. The starting material
used was (-)-camphor (to obtain (-)-Taxol), which is readily available and abundant from a number of sources,
such as, the distillation of turpentine oil. To synthesise (+)-Taxol, (-)-patchino was used as the precursor.
The naturally abundant (-)-camphor was converted to diol (1) which is
the starting point of Holton’s strategy. This diol can also be obtained from β-
patchoulene oxide (patchino), which is a commercially available natural
product and can also be obtained from patchouli alcohol.
A major advantage of using this compound (1) as the precursor is that it
already contains 15 of the 20 carbon atoms which make up the taxane ring framework.
Holton achieved the total synthesis of Taxol in 1994; however along the way, major
milestones were achieved. For example, in 1984 Holton and his group completed the
synthesis of the taxane ring system, using patchoulene oxide as the starting material and then went on to
successfully synthesise the compound Taxusin in 1988.
The synthesis of Taxusin, along with various other molecules with
the bicyclo[5.3.1] skeleton, was achieved using this “epoxy alcohol
fragmentation” strategy (fig ?). This fragmentation is one of the first steps in
Holton’s total synthesis of Taxol.
Synthetic route
The functionalised AB-ring system (16) was formed in 12
steps from diol (1), which is an intermediate in the preparation of
taxusin and can be synthesised from (-)-camphor or β-patchoulene
oxide. As shown in figure ?, (1) can be obtained from β-patchoulene
oxide: by firstly refluxing β-patchoulene oxide (A) with tert-
butyllithium, enol (B) is obtained. Following epoxidation using t-
BuOOH and Ti(iPrO)4, the diol (1) is obtained by epoxy alcohol rearrangement using boron trifluoride and
trifluoro-methanesulfonic acid. In order to obtain the correct enantiomeric series, Holton used (-)-camphor as the
CH3CH3
CH3CH3
OHOH
(1)
Mohima Begum Roomi Chowdhury
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starting material for the synthesis of (-)-Taxol. The reaction
scheme below outlines the transformations carried out to
obtain the functionalised AB-ring system of Taxol.
Once (-)-camphor was converted to diol (1),
silylation using triethylsilylchloride (TESCl) gave (2)
which then underwent epoxy alcohol fragmentation via the
epoxy alcohol intermediate (3) to give the bicyclo[5.3.1]
skeleton of the AB-ring system (4). Treatment with
TBSOTf to protect the C-13 hydroxyl group was followed
by the diastereoselective aldol condensation of the
magnesium enolate of (5) to give ethyl carbonate (6), after
direct protection using phosgene and ethanol.
Hydroxylation at C-2 was then achieved using LDA
and (R)- or (S)-camphorsulfonyl oxaziridine to convert (-)-
or (+)-(6), respectively, to hydroxy carbonate (7).
Reduction of (7) using Red-Al gave the non-isolated 2, 3,
7-triol intermediate, which was treated with phosgene
and pyridine to afford carbonate (8), which underwent
Swern oxidation using dimethylsulfoxide, oxalyl
chloride and triethylamine to give ketone (9). Then
using lithium tetramethylpiperidide (LTMP), a Chan
rearrangement was carried out to give hydroxy lactone
(10) with a C-3 α-OH group. The conformational
alignment of this hydroxyl group allowed for its
reductive removal using samarium diiodide (SmI2).
Enol (11) was treated with silica gel and trans-
and cis-lactones, (12) and (13) respectively, were
obtained in a 1:6 mixture. A process of recycling was
used where trans-lactone (12) was treated with tBuOK in
THF and quenched with acetic acid to give back enol
(11), which in turn was treated with silica gel again –
this achieved 91% yield of the cis-lactone (13). The cis-
Mohima Begum Roomi Chowdhury
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lactone (13) was then treated with LTMP to deprotonate
at C-1, rather than C-3 surprisingly.
Then addition of (±)-camphorsulfonyl oxaziridine
gave (14) and (15) both containing the C-1 β-hydroxyl
group. Reduction of (15) using Red-Al in THF resulted
in the formation of trans-C1/C2 diol and some cis-C1/C2
diol; which was worked up under basic conditions to
give the desired trans-diol. The trans-diol containing the
C-2 α-hydroxyl group was then treated with phosgene to
obtain carbonate (16).
Ozonolysis of the terminal olefin of lactone (16),
followed by oxidation using potassium permanganate
and esterification using diazomethane afforded methyl
ester (17) and Dieckmann cyclisation resulted in the
formation of enol ester (18). Before
decarbomethoxylation was carried out, the hydroxyl
gro
up
at C-7 was protected using p-toluenesulfonic acid and 2-
methoxypropene. The MOP-protected enol ester (19) was
treated with PhSK and dimethylformamide to give ketone
(20). A more robust protecting group at C-7 was employed
by treating (20) with benzyloxymethyl (BOM) chloride and
the next step was used to introduce a TMS-ether group at
C-5, via an enol ether intermediate, using LDA and
trimethylsilyl chloride. Oxidation using mCPBA then gave
ketone (22).
An α-methyl group was added at C-4 using a methyl
Grignard reagent in methylene chloride and tertiary alcohol
(23) was then treated with Burgess’ reagent, followed by an
acidic workup to achieve elimination to give allylic alcohol
(24). Oxetanol (29) was obtained via two different routes.
Mohima Begum Roomi Chowdhury
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Labelled AB-ring of taxol, with B-ring in the chair-chair conformation
Osmylation of (24), followed by treatment with tosyl chloride gave diol tosylate (27); subsequent cyclisation
using base DBU provided oxetanol (29). The second route involved initial protection of the α-hydroxy group at
C-5 using mesyl chloride, followed by osmylation to give the diol mesylate (28); subsequent cyclisation using
diisopropylethylamine gave oxetanol (29).
Acetylation at C-4 using DMAP, acetic anhydride and pyridine, followed by removal of the triethylsilyl
protecting group at C-10 using HF-pyridine and MeCN
gave C-10 hydroxy oxetane (30). Treatment with
phenyllithium gave the C-2 benzoate; then oxidation at C-
10 was achieved using N-methylmorpholine-N-oxide
(NMO) and TPAP. Oxidation at C-9 using tBuOK and
benzeneseleninic anhydride was followed by direct
acetylation using tBuOK again, then acetic anhydride,
pyridine and DMAP to achieve direct acetylation at C-10,
affording 7-BOM-13-TBS baccatin III (32).
In preparation for the addition of the side chain, the
C-13 hydroxy group was deprotected using TASF. 7-BOM
baccatin III (33) was then treated with lithium
bis(trimethylsilyl)amide followed by (±)-cis-1-benzoyl-3-
triethylsilyloxy-4-azetidin-2-one (β-lactam) and acetic
acid. Taxol was then obtained after deprotection of the
silyl and BOM groups using HF-pyridine and
hydrogenolysis (H2, Pd/C and ethanol), respectively.
Results and Discussion
By controlling the conformation of the
bicyclo[5.3.1] ring system, functionality was
achieved at C-1, C-2, C-3, C-7 and C-8 in 12
steps from diol (1) to obtain carbonate (16) in
40% overall yield. Having successfully
carried out the formation of the AB-ring
system, completion of the C ring was
Mohima Begum Roomi Chowdhury
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achieved by a Dieckmann cyclisation (via a protocol pioneered by Holton himself). Instalment of the oxetane
ring, final functionalisations and addition of the side chain at C-13 afforded synthetic Taxol; with an overall yield
from diol (1) of ca. 4-5%.
Problems and limitations were encountered during this challenging synthesis; for instance, conventional
chemical transformations were at times unsuccessful and required the use of protecting groups to avoid unwanted
reactions – making the synthesis longer with protection and deprotection steps. Deprotonation at C-8α, for
instance, was unsuccessful at first and required C-10α protection with a silyloxy group. Furthermore, a large
epimerisable C-3α substituent had to be introduced to favour the boat-chair conformation to carry out the
formation of the C-2α alcohol, so subsequent epimerisation could return the molecule to the chair-chair
conformation for further functionalization.
The C-7 stereocenter was introduced early on, in the absence of the C-9 carbonyl group, to avoid
epimerisation. Conformational control of the 8-membered ring to shift groups around the periphery to equatorial
positions, hence minimising non-bonding interactions, was used throughout to direct functional group additions
selectively. Due to the C-4 carbonyl group being very hindered, formation of the acetoxy oxetane (30) was
difficult as addition of C-20 failed with most nucleophilic reagents with the carbonate being present. This was
resolved by introducing a robust protecting group at C-7, addition of C-20 from the α-face and indirect
reconfiguration to achieve the C-20β component via formation of an allylic alcohol intermediate (26).
Holton achieved this unique synthetic route by utilising some reactions that were still in development.
The Chan rearrangement, for example, was used for the first time in a cyclic system and it was found to be a very
stereoselective method. The Dieckmann cyclisation carried out to convert methyl ester (17) to enol ester (18)
followed a protocol which was developed by Holton.
Addition of the ester side chain at C-13 was carried out using the (+)-lactam with (-)-(33) and (+)-lactam
was used with (+)-(33) to achieve the desired stereochemistry of the final product. This step proceeded with no
problems. The removal of the TBS group, prior to addition of the β-lactam, almost posed a problem as the α-face
of the molecule was very crowded; the deprotecting agents, HF-pyridine proved ineffective and Holton then
found that TASF worked effectively to cleave the TBS group. Holton’s synthesis required 41 steps and an overall
yield of ~4-5% was achieved.
Mohima Begum Roomi Chowdhury
CHEM3004
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Nicolau’s total synthesis of Taxol
Nicolau and his group followed a convergent strategy whereby rings A and C were first prepared, then
merged to form the B ring. Once the ABC-tricyclic system was obtained, the oxetane ring was installed and the
various substituents were added around the peripheries. The side chain was then added at C-13 by esterification,
after oxygenation, and the total synthesis of Taxol was completed.
The two fragments, rings A and C – which were prepared by Diels-Alder reactions using simple
precursors – were coupled by a Shapiro reaction and a McMurry pinacol coupling.
Before Nicolau and his group published their route for the total synthesis of Taxol in 1994, they made a
number of breakthroughs along the way. The total synthesis was gradually achieved by successfully synthesising
the A- and CD-ring systems in 1992, and going on to form the ABC-taxoid ring system in 1993. Complete
functionalisation of the tetracyclic framework and the addition of the side chain at C-13, hence total synthesis of
Taxol, were then achieved in 1994.
Synthetic route
The fully functionalised A-ring system was
achieved by Nicolau in 1992. The synthesis was started by
heating diene (1) at 135°C with 2-chloroacrylonitrile (2)
for 96 hours in a sealed tube. This Diels-Alder reaction
resulted in the formation of adduct (3), which was
converted to ketoacetate (4) using potassium hydroxide
and t-butanol, followed by acetic anhydride and DMAP. In
order to couple to the CD-synthon, hydrazone functionality was introduced at C-1 using 2,4,6-
triisopropylbenzenesulfonylhydrazide in methanol, affording (6).
Preparation of the CD ring system involved an initial Diels-Alder reaction between dienophile (7) and 3-
hydroxy-2-pyrone (8). Dienophile (7) was prepared in four steps from
allyl alcohol: (i) silylation using tBuPh2SiCl-imidazole, (ii) ozonolysis,
(iii) condensation using Ph3P=CH(Me)-CO2Et and finally (iv)
desilylation using n
Bu4NF. The Diels-Alder reaction was made
intramolecular using phenylboronic acid which carried the reaction
through an intermediate shown in fig. ?. This cycloaddition reaction
Mohima Begum Roomi Chowdhury
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achieved the correct regiochemistry to give compound (9), which
was transformed to the requisite C-ring aldehyde (14) through a
number of steps.
Firstly, the hydroxyl groups were protected using TBSOTf
and the ester group was selectively reduced with LiAlH4 to give
alcohol (10). The next step was used to add the benzyl group at
the C-7 and to protect the primary alcohol at C-9. This was
carried out by deprotecting the hydroxyl group at C-7 using an
acid catalyst, followed by selective TPS-protection at C-9 and
then addition of the benzyl group at the secondary alcohol.
Compound (11) was obtained and treated with LiAlH4, which
reductively opened the lactone and concomitantly deprotected the
hydroxyl group at C-4 to give triol (12). Treatment with 2,2-
dimethoxypropane afforded acetonide (13) which was then
oxidised using TPAP in the presence of N-methylmorpholine-N-
oxide (NMO) to give the desired aldehyde (14).
Coupling of the two fragments (6) and (14) to form
the complete ABC framework of Taxol (19). Fragments (6)
and (14) were coupled using n-butyllithium via a Shapiro
reaction whereby the hydrazone precursor (6) was treated
with nBuLi to form a vinyllithium intermediate (fig ?). This
carbanionic species then attacked the carbonyl of the
aldehyde (14) to give a single disastereoisomer of
compound (15). Epoxidation was carried out at the C1,C14
double bond using tert-butyl hydroperoxide and VO(acac)2
to give epoxide (16). Regioselective opening of the
epoxide was achieved using LiAlH4 to obtain tran-1,2-diol
(17). This was then converted to a cyclic carbonate in
preparation for later transformations. Phosgene and KH
were used to achieve this cyclisation and dialdehyde (18)
was obtained by desilylation followed by oxidation using
TBAF and TPAP (respectively).
Mohima Begum Roomi Chowdhury
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Dialdehyde (18) was the appropriate structure to undergo
the McMurry pinacol coupling using titanium trichloride and a
reducing agent (zinc-copper alloy on this case). This achieved
cyclisation to form the ABC-taxoid ring system (19) which then
underwent further functionalisations. Acetylation of the C-10
hydroxyl group using acetic anhydride and oxidation of the C-9
hydroxyl group using TPAP afforded (20), which then underwent
hydroboration using BH3-THF. Treatment with basic hydrogen
peroxide was then carried out and a mixture of regioisomers of
the hydroxyl group at C-5 were obtained. The acetonide group
was removed using acid and the subsequent triol (21) was
acetylated at the C-20 hydroxyl group to afford (22).
The benzyl protecting group at C-7 was replaced with a
triethylsilyl group and selective monodeacetylation was carried
out using potassium carbonate in methanol to afford triol (23).
The next step achieved construction of the oxetane ring through silylation of the primary alcohol and triflation of
th secondary alcohol, followed by treatment with mild acid. This gave oxetanol (24) which was subsequently
acetylated at the tertiary C-4 hydroxyl group to give oxetane (25), ready for the final transformations.
In order to obtain Taxol, enantiomerically pure compound (25) was used. Treatment with excess
phenyllithium regioselectively opened the cyclic carbonate and hydroxy benzoate functionality was achieved at
C-2. This was followed by the introduction of a carbonyl group at C-13 using PCC-Na(OAc) and benzene. The
C-13 carbonyl group was then stereospecifically reduced to an alcohol using excess NaBH4. Esterification and
hence addition of the side chain at C-13 was carried out using NaN(SiMe3)2 and Ojima’s β-lactam. Taxol was
then obtained by deprotecting the C-7 and C-2’ hydroxyl groups, using HF-pyridine.
Mohima Begum Roomi Chowdhury
CHEM3004
15
Results and Discussion
In Nicolau’s paper published in 1992, the A-ring synthesis was continued from the ketoacetate (4) to
achieve functionality at the Taxol C-13 position. However in Nicolau’s first total synthesis, published in 1994,
the A-ring synthon used was hydrazone (6) where the hydroxyl group at C-13 is introduced much later. Also, in
Nicolau’s paper published in 1993, describing the synthesis of the ABC-taxoid ring system, the protecting group
employed at the Taxol C-10 position was methoxyethoxy methyl (MEM) ether, whereas in the total synthesis,
Nicolau used the TBS group. These slight modifications made throughout the path towards total synthesis of
Taxol and the fact that many different fragments which could be used highlight the complexity of the molecule.
Similar alterations were made en route to the C- and CD-ring synthons. Nicolau presented a completed
oxetane system in the precursor synthesised in his 1992 paper, whereas an acetonide was actually used as the C-
ring fragment in the total synthesis. This could be due to the sensitivity of the oxetane ring to the transformations
carried out when coupling the two fragments.
Throughout the synthesis, mixtures of stereoisomers were achieved and chromatographic separation was
used to obtain the desired isomeric forms. For instance, when forming the triol prior to oxetane formation, a
mixture of regioisomeric alcohols was afforded, which were then separated using a silica column to obtain the
wanted regioisomer.
Danishefsky’s total synthesis of Taxol
Danishefsky and his group focused on the synthesis of baccatin III, and
between 1992 and 1996 gradually developed the various fragments to build
together the taxane tetracyclic core. A convergent strategy was used where the A-
and C-ring fragments were first prepared and then coupled, forming the B-ring
by a Heck reaction cyclisation. The oxetane ring was introduced at an early
stage, before the coupling of the A- and C-rings was carried out.
The Wieland-Miescher ketone was utilised as the predominant starting
material and as fig ? shows, it provides the structure of the C-ring with easy
functionalisation at C-7 and the angular methyl group (C-19) at C-8. It was a
suitable starting compound as it was easy to prepare by L-Proline-induced aldolisation of a trione which is
prochiral; hence a chiral compound is formed in one step from an achiral compound. (Tetrahedron Letters 41
(2000) 6951-6954)
Mohima Begum Roomi Chowdhury
CHEM3004
16
The A-ring fragment was formed from trimethylcyclohexane-1,2-diol; which can be prepared from
readily the available compounds 2-methyl-3-pentanone and acryloyl chloride. (Hargreaves et al J. Chem. SOC.
(C), 1968, 2599-2603, also quote J. Org. Chem 1992 57 4043-4047, use scheme 1?). It was found that
lithiation could be carried out on the A-ring fragment (at the Taxol C-1 position), which allowed for coupling to
the aldehyde group introduced at the CD-fragment (at the Taxol C-2 position). The ACD-structure then required
cyclisation to form the B-ring, which was carried out using a Heck reaction. This was followed by appropriate
functionalisations of the tetracyclic framework, including the hydroxyl group at C-13 to obtain baccatin III;
which was then converted to Taxol using Ojima’s β-lactam.
Synthetic route
The starting point for the A-ring fragment, 2,2,4-trimethylcyclohexane-1,3-dione, was prepared from 2-
methyl-3-pentanone and acryloyl chloride (Hargreaves). Monohydrazone (2A) was prepared from (1A) by
reaction with hydrazine. Treatment with iodine converted (2A) to iododienone (4A) in a
Barton reaction [ref] via the monoene
iodide intermediate (3A). This was then
treated with TMSCN and catalytic
potassium cyanide to obtain racemic
cyanohydrin (5A). The hypothetical target molecule (7)
required a metal at the Taxol C-1 position and this was
achieved by lithiation of (5A) to give the A ring fragment (6).
To obtain the CD synthon, the Wieland-Miescher ketone was
treated with sodium borohydride which reduced the carbonyl
group at C-7 to a hydroxyl group. The deconjugated ketal (2C)
was then obtained by treatment with acetic anhydride and
DMAP, followed by ethane-1,2-diol and naphthalenesulfonic
acid, then sodium methoxide. The C-7 hydroxyl group was
then protected using TBSOTf and functionality at C-3 was
attempted. Using diborane followed by hydrogen peroxide,
oxidation to an alcohol was carried out. Then oxidation with
potassium dichromate converted the alcohol to ketone (3C)
after treatment with sodium methoxide.
CH3 O
CH3
CH3
OH
M(7)
Mohima Begum Roomi Chowdhury
CHEM3004
17
The next target was to install the oxetane ring via a triol intermediate (5C). A methylene group was first
introduced at the C-4 position and two different routes were investigated from (3C) to (4C). One route was
proven inefficient as many intermediates were formed. This route involved triflation at C-4 followed by
conversion from the vinyl triflate to an α,β-unsaturated ester. The second route, however, proved more efficient
(Scheme ?). Treatment of (3C) with sulfonium ylide Me3S+I- and KHMDS resulted in a spiroepoxide
intermediate which was then converted to allylic alcohol (4C) by Lewis-acid-induced epoxide ring opening using
aluminium isopropoxide and toluene. Osmylation of the allylic alcohol (4C) using osmium tetroxide and N-
methylmorpholine-N-oxide (NMO) gave triol (5C).
Due to the hindered β-face, osmylation was
expected to be stereospecific, however around 15%
of the unexpected triol was also formed; the
mixture of triols was easily separated and the target
triol (5C) was obtained.
The next step involved the actual formation
of the oxetane ring. This was carried out by: (i)
silylation of the primary alcohol using
trimethylsilyl chloride, (ii) activation of the
secondary alcohol by triflation and finally (iii)
refluxing with ethylene glycol to give oxetane (6C).
Once the oxetane ring was obtained, the tertiary
hydroxyl group at C-4 needed to be protected by a
group which would be removable later and also
distinguishable from the other hydroxyl groups. A
benzyl protecting group was installed using benzyl
bromide, sodium hydride and tetrabutylammonium
iodide (TBAI). This benzyl ether was then treated
with p-toluenesulfonic acid to cleave the ketal and
obtain ketone (7C). Many different degradation options were looked at in an attempt to form a suitable CD
fragment which would be coupled with the lithiated cyanohydrin ring A synthon (6). Scheme ? outlines the
transformations chosen to convert (7C) to the CD fragment (15); this route was used due to its scaleability.
Mohima Begum Roomi Chowdhury
CHEM3004
18
Ketone (7C) was first treated with TMSOTf
to give silyl enol ether (8C), which was then treated
with 3,3-dimethyl dioxirane followed by
camphorsulfonic acid to achieve hydroxylation at C-
10 to give (9C). Then, lead tetraacetate was used to
fragment the ring which resulted in formation of
(10C) with a methyl ester at C-2 and an aldehyde at
C-10. The aldehyde was converted to the dimethyl
acetal using methanol and 2,4,6-collidine p-
toluenesulfonate (CPTS) and (11C) was obtained.
The methyl ester at c-2 was then reduced to a
carbinol (12C) using lithium aluminium hydride. Then, to prepare the C-2 position for oxidation to an alkene,
carbinol (12C) was treated with o-nitophenylselenium cyanide and tributylphosphine to give o-nitrophenyl
selenide (13C). This then underwent oxidation using hydrogen peroxide to obtain alkene (14C). Finally,
ozonolysis of the alkene afforded aldehyde (15) which was used as the CD synthon.
The ACD ring structure was then formed
by coupling the two prepared fragments (6) and
(15). This was started by treating the lithiated
cyanohydrin (6) with THF then adding the
aldehyde (15), which formed the new C1-C2
bond. Treatment with TBAF deprotected the
ketone at C-11 to give the first ACD compound
(16), which was further functionalised and
prepared for ring closure to form the B ring.
Epoxidation of (16) with mCBPA gave epoxide
(17) which underwent hydrogenation and a
hydroxyl group was introduced at C-1, hence diol
(18) was formed. The C-1 and C-2 hydroxyl
groups were then protected as a cyclic carbonate
(19) using carbonyl diimidazole and sodium
hydride.
Mohima Begum Roomi Chowdhury
CHEM3004
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Conjugated reduction of the enone (19) using
organoborane L-selectride was then carried out to
obtain ketone (20). In preparation for the cyclisation
by the Heck reaction, triflation at C-11 gave vinyl
triflate (21) which was hydrolysed using pyridinium
p-toluenesulfonate (PPTS) to cleave the dimethyl
acetal, giving aldehyde (22). Then, following a Wittig
olefination, alkene (23) was formed and the
palladium-catalysed Heck reaction was carried out to
form the C10-C11 bond, and hence the tetracyclic
(24) was obtained.
The next focus became functionality at various
positions and it was found, by model probes, that the
TBS group at C-7 would be too difficult to remove after
further functionalisation around the tetracycle.
Therefore using TBAF and then TESOTf, the C-7
hydroxyl group was deprotected and reprotected
(respectively). The triethylsilyl ether was then treated
with mCPBA to give epoxide (25). Deprotection and
acetylation at C-4 gave compound (26) which was then
treated with phenyllithium to cleave the cyclic
carbonate and hence benzoate functionality was
achieved. Osmylation of (27), followed by treatment
with lead tetraacetate gave ketone (29) via osmate ester
(28). The epoxide oxygen was then removed using
samarium diiodide and acetic anhydride to give (30)
with the desired alkene functionality at C11-C12.
Ketone (30) was then treated with potassium tert-
butoxide and phenylseleninic anhydride followed by
Mohima Begum Roomi Chowdhury
CHEM3004
20
acetic anhydride to achieve the desired functionalities at C-9 and C-10 to give acetoxy ketone (31).
The final task was to functionalise C-13 and add the side chain. This was carried out by introducing a
carbonyl group at C-13 using pyridinum
chlorochromate (PCC) via an allylic
oxidation to give (32). Sodium
borohydride was then used to reduce the
ketone to an enol (33). This could then be
deprotected using HF-pyridine to give
Baccatin III or reacted with Ojima’s β-
lactam to add the side chain at C-13. Once
the side chain was added, desilylation was
carried out using HF-pyridine and
synthetic Taxol was obtained.
Results and Discussion
The (+)-Wieland-Miescher ketone was used as the starting material for the C-ring as it supplies a simple
route to a single optically active Taxol enantiomer, because of its single chiral centre. It also contains a C-7
carbonyl group which made functionalisation easy; furthermore the C-19 angular methyl group was already
present, saving further instalment steps.
Addition of the sulphur ylide to ketone (3C) to form the spiroepoxide intermediate was effectively carried
out via the Johnson-Corey-Chaykovsky reaction; this allowed for addition of C-20 prior to formation of the
oxetane ring. Another prominent reaction carried out in Danishefsky’s sequence was the Heck reaction where a
palladium catalyst induced the cyclisation to form the B-ring of the taxane skeleton.
Careful selection of protecting groups was vital throughout the synthesis as it was important to choose
effective, but cleaveable groups. For instance, many protecting groups were investigated for the protection of the
C-4 α-hydroxy group of compound (6C) and it was found that the most effective, and removeable, was the
benzyl protecting group; added using benzyl bromide, sodium hydride and tetrabutylammonium iodide (TBAI).
Limitations to this strategy include the vast number of intermediates which were generated, making the
synthesis less than optimal. Furthermore, one of the major problems encountered was the failure to protect some
groups in the presence of others – hence steric crowding hindered effective protections to be carried out; which at
times left the development of the strategy at a standstill.
Mohima Begum Roomi Chowdhury
CHEM3004
21
Wender’s total synthesis of Taxol
Wender and his group developed their synthetic strategy from the naturally abundant and inexpensive
compound, pinene; which is one of the most widely available natural products, found from various sources, such
as pine trees and industrial solvents like turpentine. In 1992, Wender successfully synthesised the ABC tricyclic
taxane framework by convergence of A and C ring precursors. Pinene was used as the building block for the A
ring fragment and proved to be an effective starting material as it possesses 10 of the 20 carbon atoms of the
Taxol system and could supply the correct chirality for the Taxol core, hence a concise sequence could be
achieved. It was anticipated, at the time, that a total synthesis of Taxol could be achieved by the end of 1992, but
changes in strategy and various different routes had to be investigated before a successful total synthesis was
achieved in 1997.
A linear strategy was eventually adopted where the air-
oxidation product of pinene, verbenone, provided the A-ring
fragment and using readily available compounds, a 6-
membered seco-B ring was formed. Then fragmentation gave
the AB-bicyclic precursor which was elaborated into the ABC-
tricyclic core. Functionalisations were carried out along the way and instalment of the oxetane ring and the side
chain completed the synthesis of Taxol.
Synthetic route
Pinene (1) was oxidised in air using a
cobalt catalyst to give verbenone (2). Using
potassium tert-butoxide, the dienolate of
verbenone was formed and underwent alkylation
with 1-bromo-3-methyl-2-butene. Selective
ozonolysis at the more electron-rich alkene gave
aldehyde (3); which underwent photorearrangement to achieve the correct connectivity of the A ring. The lithium
salt of ethyl propiolate was then added to the C-9 carbonyl group of aldehyde (4) and following protection with
trimethylsilyl chloride, adduct (5) was formed. Then, the conjugate addition of Me2CuLi served two purposes:
addition of the C-8 methyl group and formation of a carbanion at C-3 which effected cyclisation to form
tricarbocycle (6).
Mohima Begum Roomi Chowdhury
CHEM3004
22
With (6) being conformationally rigid
and due to its stereochemistry, various
functionalisations were carried out before the
taxane B ring was completed. The C-9
hydroxyl group was first oxidised to ketone (7)
using N-methylmorpholine-N-oxide (NMO)
with catalytic RuCl2(PPh3)3; although
oxidation could also have been carried out
with Dess-Martin periodinane. The
stereocontrolled introduction of the C-10 hydroxyl group was then achieved by deprotonating (7) with KHMDS
and then adding Davis’ oxaziridine to give α-hydroxyketone (8); which then underwent reduction using LiAlH4
to obtain tetraol (9). Acetonide (10) was then
formed by treating (9) with PPTS and 2-
methoxypropene after TBSCl and imidazole.
The next step achieved the AB ring
system via epoxy alcohol fragmentation using
mCPBA to first form the epoxide and then
fragmentation was prompted using the catalyst
DABCO. The hydroxyl group introduced at C-13
was directly protected with the triisopropylsilyl
(TIPS) group and the AB ring system (11) was
ready for functionalisation. Triol (12) was
obtained by a multi-process step where ketone
(11) was treated with potassium tert-butoxide and
triethyl phosphite in an oxygen atmosphere;
ammonium chloride and methanol were used in
situ to remove the TBS group and sodium
borohydride was utilised to reduce the C-2
ketone. Then, using Crabtree’s catalyst, the C3-
C8 alkene was hydrogenated and a cyclic
carbonate was formed using triphosgene, after
the C-4 hydroxyl group was protected using
Mohima Begum Roomi Chowdhury
CHEM3004
23
TMSCl and pyridine; hence (13) was obtained. The C-4 center was then oxidised to an aldehyde using PCC, in
preparation for rings C and D.
Aldehyde (14), the fully functionalised taxane AB system, was then transformed into the tricyclic ABC
system starting with the addition of a CH2 unit (homologation) at C-4 using
methoxymethylenetriphenylphosphine (Ph3PC(H)OMe). Using hydrochloric acid and sodium iodide, the
acetonide and enol ether groups were hydrolysed, leading to aldehyde (15). The hydroxyl group at C-9 was
protected using triethylsilyl chloride and the C- 10 hydroxyl group was oxidised to a ketone. Using
dimethylmethylideneammonium iodide (Eschemoser’s salt) and excess triethylamine, C-20 was introduced to
give enal (16). Then, Grignard addition of allyl magnesium bromide, in the presence of zinc dichloride, added the
remaining carbon atoms needed to complete the taxane system. Protection of the C-5 hydroxy group using
benzyloxymethyl chloride (BOMCl) gave ether (17), with the cyclic carbonate unaffected by the Grignard
reagent, thanks to the presence of ZnCl2. The next step involved deprotection using ammonium fluoride and
acetylation at C-9, and also the cleavage of the cyclic carbonate and formation of the C-2 benzoate using
phenyllithium; to give acetate (18). In order to
functionalise C-9 and C-10 correctly, (18) was
treated with a guanidinium base to achieve
transposition. Acetoxyketone (19) then
underwent ozonolysis of the terminal alkene to
give aldehyde (20); which was then treated with
excess DMAP to effect aldol cyclisation.
Tricyclic (21) was obtained after protection of
the C-7 hydroxyl group using 2,2,2-
trichloroethyl chloroformate (TrocCl).
Instalment of the oxetane ring was then
started by removal of the BOM group using
sodium iodide and hydrochloric acid, and
leaving group (bromide) was to be added at C-5.
Alcohol (22) was mesylated and (23) was then
treated with lithium bromide to give bromide
(24). Osmylation then introduced oxygen at C-4
and C-20, and instead of direct oxetane
formation, the C-2 benzoate group migrated to
Mohima Begum Roomi Chowdhury
CHEM3004
24
the C-20 hydroxyl group. To continue towards cyclisation of the D ring, Wender used imidazole to complete the
migration of the benzoate group, triphosgene was then used to form a cyclic carbonate at C1 and C2 and
potassium cyanide was used to remove the benzoate group – giving diol bromide (25). N,N-
diisopropylethylamine (Hünig’s base) was used to form the oxetane ring and following acetylation using acetic
anhydride and DMAP, tetracycle (26) was obtained. TASF was then used to remove the C-13 protecting group
and using phenyllithium, the C-2 benzoate was re-established. Hence a mixture of baccatin III (27) and 10-
deacetylbaccatin III (28) was obtained. 10-DAB (28) could be acetylated using acetic anhydride and DMAP to
give baccatin III.
Finally, the addition of the side chain at C-13 was carried out according to Ojima’s method whereby
baccatin III (27) was converted to Taxol in three steps. Sodium hydride was used to form the sodium salt at C-13,
followed by addition of Ojima’s β-lactam to give Taxol, after deprotection at C-2’ using dilute hydrochloric acid.
Results and Discussion
Taxol was synthesised in the correct enantiomeric form in 37 steps from verbenone; making Wender’s
strategy of total synthesis of Taxol the shortest and most concise to be reported. One of the unique factors of
Wender’s strategy is the use of a photorearrangement [ref 11 from Wender 2755] to achieve the correct carbon
connectivity which then accommodated the subsequent fragmentation and formation of the AB-ring system. The
rigidity of the tricarbocycle was used advantageously to selectively introduce functionality.
As with the other strategies, results from previous studies allowed for methods and conditions to be
designed appropriately. For instance, Wender found that aldol cyclisation could not be carried out on
ketoaldehyde (20) with a cyclic carbonate present at C1-C2, hence cleavage of the cyclic carbonate and
concomitant formation of the hydroxybenzoate was carried out before aldolisation.
Other problems experienced during this strategy include the attempts made o form the oxetane ring. It
was found that by placing a leaving group at C-20 did not achieve cyclisation, so a different approach was tried,
where a leaving group was placed at C-5 and was attacked by the nucleophilic C-20 hydroxyl group. This
approach successfully formed the oxetane ring.
Mohima Begum Roomi Chowdhury
CHEM3004
25
Kuwajima’s total synthesis of Taxol
Kuwajima and his group achieved the enantioselective total synthesis of (-)-Taxol in 1998 via a
convergent strategy. The optically active A-ring fragment was prepared from linear starting materials and two
different attempts were made for the C-ring fragment. Either a cyclohexadiene derivative or an aromatic
fragment was used as the starting point for the C-ring synthon. The diene route gave unwanted polymerisation
by-products during a number of transformations, so the aromatic route proved more efficient and scaleable.
Rings A and C were coupled to achieve the tricarbocycle, functional group manipulations were carried out and
the oxetane ring was then formed. (-)-Taxol was obtained after addition of the C-13 side chain using Ojima’s β-
lactam.
Introduction of the C-19 methyl group proved interesting in Kuwajima’s synthesis as two routes were
attempted. In the first, Kuwajima tried to utilise a method used in his synthesis of (+)-taxusin [ref 8b 2000 paper]
involving cyclopropanation of the C3-C8 double bond. This route was inefficient as it required many protecting
group exchanges along the way. The second – more viable option – was the conjugate addition approach;
introducing the C-19 methyl group using a cyano group. Another interesting feature of Kuwajima’s synthesis was
the Birch reduction of the aryl C-ring to obtain the diene-type C-ring precursor used in the conjugate addition
approach.
Hence, Kuwajima’s synthesis involved preparation of the A-ring which was coupled to a readily available
aromatic C-ring and then cyclisation of the B-ring, induced by stannic chloride, formed the ABC-tricarbocycle.
Birch reduction of the C-ring was then carried out and the precursor for the conjugate addition of the C-19
methyl group was prepared. Following conjugate addition, functional group manipulations were carried out and
then the C-ring was dihydroxylated, ready for the formation of the oxetane ring. The addition of the side chain at
C-13 and final deprotections completed the synthesis of Taxol.
Mohima Begum Roomi Chowdhury
CHEM3004
26
Synthetic route
Lithiated propargyl ether was added to propanal,
followed by hydrogenation using Lindlar’s catalyst to give
alkene (3). Ketone (4) was obtained by Swern oxidation
followed by the conjugate addition of isobutyric ester enolate.
Keto ester (5) was then treated with potassium tert-butoxide
and a Claisen-like cyclisation occurred followed by the
formation of the pivaloate using pivaloyl chloride. Aldehyde
(7) was obtained by removing the THP group and carrying out
Swern oxidation using oxalyl chloride, dimethylsulfoxide and
triethylamine and then treatment with TIPSOTf, DBU and
DMAP gave the silyl enol ether (8). Then the chiral ligand
DHQ-PHN was employed in Sharpless’s asymmetric
dihydroxylation to form α-hydroxy aldehyde (9), but recovery
proved
difficult.
N,N’-diethylmethylenediamine was added to crude (9) in
hot benzene and purification by a silica gel column gave
pure monomeric (9). The pivaloyl protecting group was
replaced with a TIPS group and following a silica gel
column, enone (10) was obtained. Dienol silyl ether (11)
(the A-ring fragment) was then formed, as a mixture of the
geometrical isomers, via a Peterson olefination by treating
(10) with N,N’-diethylmethylenediamine, then
PhSCH(Li)TMS followed by a silica gel column.
The A-ring fragment (11) was then coupled to the lithiated
C-ring fragment, 2-bromobenzaldehyde dibenzylacetal
(12), in the presence of magnesium (II) ion, in a chelation-
controlled addition to give adduct (13) as a single isomer.
The C1,C2-diol was protected as a borate ester (14) and
the B-ring was formed by cyclisation induced by tin
Mohima Begum Roomi Chowdhury
CHEM3004
27
tetrachloride to give tricarbocycle (15) after deprotection
of the borate ester using pinacol and DMAP. Next, the aryl
ketone (18) was prepared to undergo Birch reduction of the
C-ring. Conversion of (15) to (18) involved reduction of
the C-13 carbonyl group using DIBAL, then TBS-
protection of C-2 and C-13, followed by cleavage of the
benzyl and phenylthio groups and finally Swern oxidation to attain a C-9 carbonyl group. Aryl ketone (18)
underwent Birch reduction using a potassium/ammonia electride
salt and a highly-substituted alcohol, 2,2,4-trimethyl-3-isopropyl-
3-pentanol, as the proton source to give the desired diene (19).
Unwanted C-9 reduction product (20) was recycled back to
(18) via Swern oxidation and further Birch reduction afforded
diene (19). The C1,C2-diol was then protected with a benzylidene
group, following deprotection using TBAF. The C-9 carbonyl was
then reduced to an alcohol using sodium borohydride and allylic
alcohol (23) was obtained after a Rose bengal-catalysed photo-
oxidation step.
Triol (23) was used as the precursor for the conjugate
addition of the C-19 methyl group using a cyano group. Before diethylaluminium cyanide was added, enone (24)
was formed by protecting the C7,C9-diol with a p-methoxybenzilydene group and oxidising the C-2 hydroxyl
group using Dess-Martin preiodinane. Hence, treatment
of enone (24) with Et2AlCN gave enol (25) which was
then protected at C-2 to give enol ether (26).
Transforming the cyano group to a methyl group went
via a C-19 alcohol following treatment with DIBAL and
LiAlH4. Attempts to introduce a leaving group failed and
cyclopropane derivative (28) was formed. On treating
(28) with acid, cyclopropyl ketone (29) was obtained and
then treatment with samarium diiodide and TBAF gave
stable enol (30) containing the C-19 β-methyl group.
Using sodium methoxide and methanol, enol (30) was
converted to ketone (31) with a C-3 α-proton.
Mohima Begum Roomi Chowdhury
CHEM3004
28
Next, functional group manipulations
were carried out, starting with protection of the
C13-OH with a TBS group. This step had to be
conducted via a boron ester of the C7,C9-diol
to ensure selective protection at C-13. The
boron ester was cleaved using hydrogen
peroxide to give (32) which was selectively
oxidised at C-9 using Dess-Martin periodinane.
Ketone (33) was obtained after the C-7
hydroxyl group was protected as the MOP-
ether. The next target was to add the final
taxane carbon atom (C-20) at C-4 via an enol
triflate (34). This was formed by treatment
with KHMDS to give the C-4 enolate which was
quenched using Tf2NPh. Using the Grignard reagent
TMSCH2MgCl, in the presence of catalyst Pd(PPh3)4,
enol triflate (34) was converted to (35). Treatment with
N-Chlorosuccinimide gave alkene (36) containing an α-
chloride group at C-5, ready to be displaced later on
formation of the oxetane ring.
Functionality at C-10 was then introduced to aid
dihydroxylation of the C4-C20 alkene. Lithium
diisopropylamine was used to form the C9-C10 enolate
which was then oxidised to an alcohol using
MoO5·pyr·HMPA (MoOPh), to give (37α), after
acetylation. The C-10 α-OAc group was then isomerised
using DBN to attain (37β) containing the C-10 β-
acetoxy group. Osmylation was then carried out and the
desired C4,C20-diol (38) was obtained and the oxetane
ring was formed by DBU-induced cyclisation.
Mohima Begum Roomi Chowdhury
CHEM3004
29
Acetylation of the C-4 α-hydroxyl group could not be achieved using acetic anhydride and DMAP, so protecting
group exchanges were made. The C1,C2-benzyl protecting group was changed to a cyclic carbonate to sterically
aid acetylation of C-4. A more robust TES-group was installed at C-7 and the C1,C2-benzylidene group was
removed using H2 and Pd(OH)2 and triphosgene was used to form the cyclic carbonate. Acetylation then gave
(41), which was treated with phenyllithium to add the α-benzoate functionality at C-2. Then (42) was obtained by
replacing the C7-TES group with a Troc-group to avoid deprotection in the next step, when TASF was used to
cleave the TBS group at C-13. To complete the enantioselective total synthesis of (-)-Taxol, the ester side chain
was added at C-13, using Ojima’s β-lactam, and deprotection at C-7 and C-2’ was carried out.
Results and Discussion
Mohima Begum Roomi Chowdhury
CHEM3004
30
Mukaiyama’s total synthesis of Taxol
Mukaiyama and his group published their ‘Asymmetric Total synthesis of Taxol®’ in 1999, where they
adopted a convergent strategy starting with the formation of the cyclooctane B-ring from optically active linear
precursors. Taxol synthesis was achieved via BC to ABC to ABCD ring construction. Where the first five total
syntheses utilised Ojima’s β-lactam to add the C-13 ester side chain, Mukaiyama prepared his own by an
enantioselective aldol reaction.
- Two different B-ring structures tried: one cyclised to BC, the other didn’t
- Two routes to pre-precursor
- Why Si bulky groups used? To sterically position the alkyl chain???
Mohima Begum Roomi Chowdhury
CHEM3004
31
Synthetic route
The synthesis was started
with the construction of the B-
ring; which required the
preparation of an optically pure
linear precursor from two
subunits: ketene silyl acetal (4)
and an optically active aldehyde
(9). The latter was prepared in two
ways: the first involved the
conversion of commercially
available methyl 3-hydroxy-2,2-
dimethylpropionate (1) to
aldehyde (9) via aldehyde (7) and
required seven steps. The second route involved the conversion of L-serine to the same aldehyde (9) via the
protected dihydroxyaldehyde (11) and comprised of five steps.
Propionate (1) was converted to a dimethyl acetal (2) via a Swern oxidation, followed by reduction of the
ester using LiAlH4 giving aldehyde (3) after a subsequent Swern oxidation. Then the asymmetric addition of
aldehyde (3) to ketene silyl acetal (4) using a chiral promoter consisting of tin(II) triflate and a chiral diamine
afforded the optically active ester adduct (5). This aldol-adduct was then protected at the secondary alcohol with
a PMB group and reduction with LiAlH4 gave alcohol (7) as the seprataed single stereisomer. On protecting the
primary alcohol with a TBS group and cleaving the diacetal group using acetic acid, chiral aldehyde (9) was
obtained.
Preparation of this aldehyde (9)
from L-serine occurred via dihydroxyester
(10) which was reduced to aldehyde (11)
by treatment with DIBAL, following
protection of the primary and secondary
alcohols with TBS and benzyl groups,
respectively. Then, the lithium enolate of
methyl isobutyrate was reacted with
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aldehyde (11) via a stereoselective
aldol reaction to give aldol product
(12). This was subsequently
converted to the desired aldehyde (9)
by protection of the hydroxyl group
with a PMB group, followed by
reduction with DIBAL and a Swern
oxidation.
An aldol reaction between chiral
aldehyde (9) and ketene silyl acetal (4)
in the presence of MgBr2·OEt2 afforded
adduct (14), which was treated with
TBSOTf to protect the hydroxyl group
to give (15). Reduction of the ester
function using DIBAL and successive
Swern oxidation provided aldehyde
(17), which was then treated with
methylmagnesium bromide to give the
B-ring precursor (18) after a Swern
oxidation.
The optically pure methyl ketone
(18) was transformed into the C8-
methyl 8-membered B-ring (23) in five steps. The
first two steps achieved the formation of the
brominated silyl ether (20) via TMS enol ether
(19), by treatment with LHMDS and TMSCl,
followed by N-bromosuccinimide to give (20).
The α-position (20) was then methylated using
LHMDS and methyl iodide, giving intermediate
(21) after deprotection of the TBS group using
acid, followed by Swern oxidation. Cyclisation to
form the B-ring was achieved by an
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intramolecular aldol reaction induced by excess
samarium diiodide. The B-ring fragment (23) was
obtained following acetylation and treatment with
DBU. The addition of the C-ring was achieved by
Michael addition of a cuprate reagent made in situ from
tBuLi, copper cyanide and TES-protected 2-bromo-5-
penten-1-ol. Adduct (24β) was then deprotected using
acid followed by TPAP and cyclisation of the
corresponding ketone (25β) to form the C-ring was
carried out via an intramolecular aldol reaction using
sodium methoxide to give (26α). Epimerisation of the
diastereomer containing a C-7 α-hydroxyl group was
achieved using sodium methoxide to give the target β-
alcohol (26β) which was then used as the ABC-
tricarbocycle precursor.
Construction of the A-ring was started by
protecting the C7,C9-diol with isopropylidene acetal
after reducing the ketone using AlH3 in tolune.
Deprotection of the PMB group followed by oxidation
using PDC gave the conformationally-rigid C-1 ketone
(28), which then underwent alkylation using
homoallyllithium reagent in benzene. Deprotection of
the C-11 hydroxyl group using TBAF then gave cis-diol
(29) which was then treated with three different
dialkylsilyl compounds, adding to the C1,C11-diol to
give (30a-c). Methyllithium was then used to cleave the
Si-O bond at C-11 (restoring the C-11 hydroxyl group)
to form the tetravalent trialkylsilyl protecting group at
C-1, sterically positioning the α-alkyl chain at C-1 for
cyclisation. Then using TPAP and N-
methylmorpholine-N-oxide, the C-11 alcohol group was oxidised to form ketones (32a-c). Wacker oxidation
using PdCl2 was then used to oxygenate the C-12 position forming diketones (33a-c), which then underwent
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34
titanium-mediated intramolecular pinacol
coupling using TiCl2 and LiAlH4, to form the
ABC-ring structure (34a-c). The target
pentaol (35), possessing the exact
stereochemistry of baccatin III (and hence
Taxol) was then formed by deprotection of the
benzyl and alkylsilyl groups using Na/NH3
and TBAF, respectively.
Pentaol (35) required final
functionalisations and the addition of the
oxetane ring, in order to become baccatin III,
the precursor of Taxol. First the C1,C2-diol
was protected using
bis(trichloromethyl)carbonate and acetylation
of the C-10 hydroxyl group using acetic
anhydride and DMAP, to give (36). The
acetonide group was then cleaved using 3M
hydrochloric acid, followed by TES-
protection at C-7 and oxidation using TPAP
and NMO to form the C-9 ketone (37). Alkene
functionality at C11-C12 was then achieved
by treatment with thiocarbonyl diimidazole
(TCDI) and DMAP, followed by
desulfurisation using trimethylphosphite, to
give compound (38). Functionality was then
introduced at C-13 by oxygenation using PCC
and NaOAc to give and enone which was
reduced using K-Selectride to give α-alcohol
(39) after protection with TESOTf. Then in
preparation for oxetane ring formation, a
bromine leaving group was introduced at C-5
via allylic bromination using CuBR and CH3CN and PhCO3tBu, followed by CuBr and Ch3CN, giving (41).
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35
Osmylation was then used to introduce the dihydroxy group at C-4 and C-20 from the α-face, to give diol
bromide (42). Oxetane ring closure was induced by DBU and oxetanol (43) was obtained after acetylation.
Baccatin III was then formed by treatment of (43) with phenyllithium to benzoylate the C-2 position and
cleavage of protecting groups using HF-pyridine.
Unlike the first five strategies
discussed, where Ojima’s β-lactam was
utilised as the C-13 side chain
precursor, Mukaiyama prepared his
own from benzaldehyde and TMS-
protected S-ethyl benzyloxy-
ethanethioate (44). The
enantioselective aldol reaction was
carried out using a chiral promoter
prepared from Sn(OTf)2, a chiral
diamine and nBu2Sn(OAc)2 to give
adduct (45). The next step replaced the
β-hydroxyl group with an amine group
via a Mitsunobu reaction using
hydrogen azide, triphenylphosphine
and diethyl azodicarboxylate (DEAD),
followed by reduction of the resulting
azide using triphenylphosphine to give amine (46). Benzoyl chloride and DMAP were then utilised to give amide
(47) which was treated with aqueous silver nitrate to hydrolyse the thiol ester, affording the desired side chain
(2R,3S)-3-Benzoylamino-2-benzyloxy-3-phenylpropionic acid (48). Other side chains were also synthesised by
modifying protecting groups and chiral centers to investigate the effect on reactivity when attaching the side
chain to 7-TES baccatin III to obtain Taxol; the attachment of side chain (48) to 7-TES baccatin III was the next
step.
Using a mixture of O,O’-di(2-pyridyl) thio carbonate (DPTC) and DMAP, dehydration condensation of
side chain (48) and 7-TES baccatin III was carried out to give ester (51). Then, to obtain a good yield, this
procedure was repeated three more times after 7-TES baccatin III was recovered from the resulting mixture using
a silica gel column. Ester (51) was then converted to Taxol by deprotecting the benzyl and silyl groups using
palladium hydroxide on carbon (in a hydrogen atmosphere) and HF-pyridine, respectively. When modified side
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36
chains (49) and (50) were used, the dehydration condensation step did not need to be repeated, however in the
case of (49), the isopropylidene protecting group could not be deprotected. Mukaiyama’s synthesis involved a
number of
enantioselective aldol
reactions using chiral
precursors and by
careful
implementation of
functionality as the
four rings were
gradually constructed
afforded the desired
end product, Taxol.
Results and Discussion
Discussion and Conclusion
Each group demonstrated a unique strategy for the synthesis of Taxol from commercially available and/or
naturally abundant starting materials. Holton’s linear synthesis strategy was based on controlling the
conformation of the B-ring which was formed after epoxy alcohol fragmentation of a bicyclo precursor,
developed from camphor or β-patchoulene oxide. By using control elements (substituents around the ring),
conformation of the B-ring – hence whether the chair-chair, chair-boat, boat-chair or boat-boat conformation was
used – functionalisations were carried out in a regio- and stereospecific manner. Such control elements were used
in the strategies developed by others as well.
Nicolau’s convergent strategy, where the A- and C-ring fragments were coupled by a Shapiro reaction and
then a McMurry pinacol coupling, utilised the Diels-Alder reaction to form the starting points for the two
fragments. One major drawback which resulted in low yields was the need for enantiomer separation a number of
times in Nicolau’s synthesis. However and advantage of using a convergent strategy as opposed to a linear
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strategy is that derivatisations could be carried out later on in the synthesis; hence minimising the effort required
to retain one functionality whilst installing another.
Another convergent strategy was carried out by Danishefsky, who used the Wieland-Miescher ketone as
the precursor for the C-ring and used a lithiated cyanohydrin as the A-ring synthon. Coupling of the A- and C-
rings were carried out