Post on 11-May-2015
Br OH
Cl
Cl
Br
OH
CF3 OH
SH
NH2
CO2HCHO
Assign a configuration, R or S to each of these compounds
N+
Et
HMe
Cl
N+
Me
HMe
ClN
H
Me
Br
Problem: Compound I has two stereoisomers, but compound II and III exist as single compounds. Explain?
I II III
Problem: The following compound has only one chirality center, why then dose it have four stereoisomers
OH
OH
Me
HBr
HMe
1.
(R)
KMnO4
A (chiral) + B (achiral)
Identify A & B
2.
(R)
H2/NiC (chiral) + D (achiral)
Both C & D are C8H18, identify them.
3. Two stereoisomers are obtained from the reaction of HBr with (3S, 4S)-4-bromo-3-methyl-1-pentene.one of the stereoisomer is optically active and the other is not. Explain?
+ HBr
CO2H
CO2H CO2HHO2C
CO2H
CO2H
CO2HHO2C CO2HHO2C
Problem: Feist's acid (C6H6O4) is a cyclopropane based natural product and having twostereocenter. The compound is chiral. Based on this information predict its correct structure?
Hint:
X X
right structure
NHCO2Me
Me
OH
O
Ph
SMe SMeO
NH
O
Discuss the stereochemistry of the following compounds and mention about their optical activity with good diagrams.
Me Me Me
Me
Me Me Me
Me
Me
MeCl
Me
Me
Cl
Cl
Me
Me
Cl
Cl
Me
Me Cl
Cl
Problem: Which of the following are chiral and why?
a(p) a(p) c a(p)
ca(p) a(c) a(p)
Me Me Me
Me
Me
Me
Problem: Draw all the isomers with mol. formula C6H-12 that
contain a cyclobutane ring, and comment on their chirality
Hint: the base structure is dimethyl cyclobutane [1,2 (cis & trans) and 1,3 (cis & trans) also 1,1]; there also exist 1ethyl cyclobutane.
CO2HHO2C
CO2H
MeMe
CO2H
CO2HHO2CCO2H CO2H
Problem: 2,5-dimethyl-1,1-cyclopentane dicarboxylic acid exist as two optically inactive isomers (A & B; differ in mp),Upon heating (mono decarboxylation A yields two 2,5-dimethylcyclopentanecarboxylic acid and B yields one.Assign structures of A & B.
Hint:
heat+
heator
A
Bsame
A & B are diastereomers
Me
CH2OHEt
H
CH2OH
EtH
Me Me
Et
ClH
HOH
Et
Me
ClH
HOH
Me
Me
ClH
HOH
Me
Me
HCl
OHH
Cl Cl H Cl
Cl
Cl Cl
H H
Cl H
ClH
H Cl
Cl H
Cl H
ClH
Et Me
CH2Cl
H
Me CH2Cl
Et
H Br
BrH
H
Br
BrH
H
H
Br H Br
Indicate wheather each of the following pairs of compounds are identical or enantiomers, diastereomers or constitutional isomers.
anda. b. and
c. and d. and
ande.
f. and
g. and h. and
i.
andj. and
OH
OH
OH
OH
OHOH
OH OH
OH
OH
OHOH
OH
OH
OH
OH
OH OH
OH
OH
Problem: Which of the following compounds are resolvable, and which are non resolvable?Which are truly meso?
a) cis-1,2-cyclohexane diol; b) trans-1,2-cyclohexane diol;c) cis-1,3-cyclohexane diol; d) trans-1,3-cyclohexane diol;e) cis-1,4-cyclohexane diol; f) trans-1,4-cyclohexane diol.
Non resolvable (easily interconvertible by flipping)
Hint:
trans (resolvable)
cis (meso) trans (resolvable)
achiral (absence of chirality center)
Ph
Ph
BrH
HBr
H Br
HBrPh
Ph
Br
Br
HPh
PhH
Br
H
HPh
BrPh
Br
Ph
PhPh
Ph
BrH
BrH
H Br
BrHPh
Ph
Br
Br
HPh
HPh
Ph
Ph
Problem: On treatment with the aromatic base pyridine, racemic 1,2-dibromo-1,2-diphenyl ethane loses HBr to yield trans-1-bromo-1,2-diphenyl ethane; In contrast the meso dibromide loses Br2 to yield trans-1,2-diphenyl ethene. Suggets a mechanism?
Hnit:
Racemic
Br2 loss is not favored
meso
-HBr
-Br2
O
OH
H
H
OH
B
H
Al
H
HH
H
LiAlH4Li-sBu3BH
Problem: Reduction of 4-t-butyl cyclohexanone with LiAlH4 gives exclusively trans alcohol; whereas with lithiumtri-sec butylborohydride yields cis`alcohol exclusively? explain.
Hnit:
O OHOH
O
O
O OHOH
O
O
R
RO
R
ROH+
OHOH
R
ROH
O
OH
R
RO
+
OH
R
R
O
O+
H
O O
R R
OH
O
OHOH
O
OH
H+
A
H+
A
Hint: H+
H+
-H2O
It is more difficult to form an acetal of compound A than B?
OH
HBr
OH
HBr
Me
H
OH
Br
Me
H
OHBr
Problem: Which of these two compounds would forman epoxide on treatment with base?
A B
Hint:
A B
OO
Br
OO
OO
Br
OO
OO
Br
OO
Br
Problem: Only one of the diastereomeric bromides shown hereeliminates to give alkene A. Why? Neither bromide gives alkene B. Why not?
Base
Base
A
No alkene AAlkene B
Hint:
N CH3I N CH3+ + I
Explain the relative rate
NMeN
NMe MeN
Me
1.00 2.27 0.47 0.042
Ph
HO2C
H
HPh
HO2C
Ph
HO2C
heatheat
Explain the reaction sequences with proper explanation.
O
H
H
MgBr
O
O
a)
b) KH, 60oC
?175oC
?
Identify the missing products in the following reaction sequence.
O
O
H
H
H
H
Each of the following reactions involves one or more concerted steps that take place in accordance with theWoodward-Hoffmann rules. In each case, prdeict exactly what is happening, with stereochemistry?
1. 25oCHint: (ring junction cis-beta)
2.100oC light 25oC
cis-beta trans
3. 140oC 220oC
Me
Me
Me
Me
H
Me
Me
HMe
Me
H
H
Me
Me
Me
Me
MeO2C CO2Me
Problem: Account for the difference in conditions required to bring about the following transformations
176oC
400oC
Problem: Give stereochemical structures of A and B and tell exactlywhat process is taking place in each reaction
cis, cis, cis-cycloocta-1,3,5-triene100oC
A (C8H10)
A + B (C14H16O4)
Bheat
Cyclobutene + dimethylphthalate
OHO
OHPhMeCOCl OH
Ph O
O
O
CO2Me
O
Me
HO
CO2Me
Predict the products in the following reactions;
1.
KH
THF, heat?
H+
?
2.
Et3N?
LDA?
heat
H+?
Ireland-Claisen rearrangement
3. Suggest a mechanism for this reaction?
+
H
R
H
R
H
HR
R
H
HEt
MeH
H
EtMe
Each of the following transformations is believed to proceed by the indicatedsequence of concerted reactions. Show just what each step involves, and give structures of each intermediates
1. heat
Aheat
Both electrocyclic closure
2. 200oCB
260oCC
1,5-H shift, electrocyclic opening.
3. 170oC
D170oC
electrocyclic opening, electrocyclic closure
D D
D
heat
The deuterium scrambling has been accounted for on the basis ofintramolecular Diels-Alder and retro DA reactions. Show how might this occur?
Hint: look for an intermediate that is symmetrical except for the presence of deuterium.
CO2-
N2
O
?
+
?
??
Br I
Cl O Cl
Br Br
Br Br
Br Br
BrBr
Br
Br
Which alkyl halide would you expect to be more reactive in an SN2/SN1 reaction with a given nucleophile?
a.and
b. and
c. and
d. and
e. and
f. and
g. and
Me
Br
Me
Br
Me OH
Me
Br
Me Me OHMe
OH
But Br But Br
Problem: When cis-1-bromo-4-methyl cyclohexane undergoes an SN2 reaction, only trans-4-methylcyclohexanolis obtained, where as under SN1 reaction conditionboth cis and trans product is obtained. Explain
cis
OH-
SN2 OH-
cis
H2O
SN1
+
trans cis
trans
Hint:
Problem: Which of the following will react faster in an SN1 reaction?
Me Br
Me
Me
Me
Br
Br
Br
OTsBut OTs
Problem: Which of the following compounds would you expect to bemore reactive in an SN2 reaction?
A B
Hint:
A B
Ans: A
Problem: Which of the following compounds would you expect to bemore reactive in an SN2 reaction?
A B
Ans: A
NH2
OH
CHO
O
O
Br
OH CHO
O
OSO2Ar
O
OSO2Ar
Problem1 NaNO2, H2O
H2SO4
NaNO2, H2O
H2SO4
Problem 2
AgNO3
Problem 3: Kinetic measurements reveal that solvolytic displacement of sulfonateis about 5 X 105 faster for 3B than for 3A
3A3B
OO
O
I II III
Transannular participation of ether oxygen
. Unfavorable polar effect of the C-O bond
.III is more favorable than either I or II
XO
XO
X
O
X
Relative rate 1.0 0.014 0.14 4.85 x 10 4
Q.
Explain
OH
NMe2
OPh
NMe2 OH
NMe2
DEAD, TPP
PhOH
DEAD, TPP
PhOH
A B
Both A and B gave the same priduct when subjected to Mitsunobu conditions with phenol as nucleophile
OH
NO2
NO2
H
H
H
DEAD, TPP
Explain the reaction
N+M e
M e
OPh
NMe2
OH
NO2
OP+Ph3
NO2
OP+Ph3
C NO2
H
NO2
common intermediate
PhOH
TPP
DEAD
O
BrOH CHOH2O
O
BrC
+
O O
OH
OH CHOO
+
hemiacetal
Q. Describe the stereochemistry of the products of these reactions.
Cl
S
O LiAlH4
HS
O
S
S
OH_( + )
(a)
(b)
enantiomerically pure
base
Both are intramolecular SN2
RS- forms, inversion takes place (SN2) at C-atom containing Cl, forms cis-fused product
O
NH
OTs OTs
Product
KOH/H2S
Product
O
Ph
+NH
N
OH
Ph
(a)
(b)
OTs OTsKOH
H2SS
SH OTs
HS SN2
S OTsS
Q. Suggest a mechanism for the following reaction
PhN
Ph
O
O
Cl H2OPh N
H
O
O
O
Ph
CH3CN, heat
N
O
Ph
O
Ph
Cl
O
NPh
O
Ph
O
NPh
O
Ph
OH
Ph NH
O
O
O
Ph
CH2OHMeO
NH
N
I2, Ph3P
CH2OHMeO
NBS, TPP
Predict the final product
For each of the compounds A through H indicate the number of gauche butane interactions present in the most stable chair conformation.
Me
OH
Me
OH
OH
H
OO
OH Br
OHBut But
OH
H
OH OO
OHBr