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## Microsoft word - pps 25 - 30 _final_11.doc

Problem 25: Cyclohexanes

B forms in the reaction of A with a strong, non nucleophilic base. B reacts with bromine to
form racemic C. The final products D (major) and E (minor) form by the reaction of C with a
strong, non nucleophilic base.

25.1 Draw a 3-D structure of A in its most stable conformation. Circle the atoms that
are possibly involved in the reaction to B.
25.2 Draw the structure of B.
25.3 Draw a 3-D structure of C (only one enantiomer needs to be drawn) in its most
stable conformation .Circle the atoms that are possibly involved in the reaction
of C to D and E.

25.4 Draw the structures of D and E.

Problem 26: Chiral Compounds
There are a number of compounds with the molecular formula C4H8O but only a few of them
are chiral.
26.1 What are the compounds having at least one chiral centre (asymmetric carbon
atom)? Draw their structural formulae (line-bond structures) with all chiral
centres marked by an asterisk.

26.2 Some of them show (S)-configuration at all their chiral centres. Draw their
configurational formulae.

26.3 If there is a
meso-compound with this molecular formula draw its configurational
formula.

Problem 27: Monosaccharides
A monosaccharide A has a molecular weight of 150 Da. The two stereoisomers B and C,
that are both optically inactive, form when A reacts with NaBH4.
27.1 Draw the structures of A, B and C according to the Fischer projection.

27.2 Determine at all stereocenters in A up to C the absolute configuration according

to the CIP (R/S) nomenclature.

27.3 Show all different stereoisomers of B and indicate their stereochemical
relationship.
Problem 28: Epibatidine

Epibatidine, isolated from tropical frogs, is about 200 times more effective as a pain reliefer
than morphine and is not addictive. In the synthesis towards epibatidine, A is converted to B
by an intramolecular SN2 reaction.

28.1 Mark all asymmetric stereocenters in A by an *.
28.2 Determine the absolute configuration of A according to the CIP (R/S)
nomenclature at all stereocenters.

28.3 Draw a 3-D structure of A indicating from where the reaction to B takes place.

Indicate the course of the reaction by an arrow between the reaction centres.
28.4 Draw a 3-D structure of B.

Problem 29: Crixivan®
Amino alcohol B is an important intermediate in the synthesis of Crixivan® that is a potent
HIV protease inhibitor. Chemists from Merck wanted to use the epoxide A as a starting
material of the synthesis process.

29.1 Upon treatment of A with benzyl amine in the presence of a weak acidic catalyst,
they obtained mainly the undesired amino alcohol C along with some of the
desired product D that could serve as a precursor of B. Draw the structure of C
and a mechanism leading to this compound. Take into account stereochemical
and regiochemical issues.

29.2 After the treatment of A with concentrated H2SO4 and acetonitrile under
thermodynamic conditions, only E formed that was subsequently hydrolyzed to
B. Draw the structure of E and a mechanism leading to this compound. Take into
account stereochemical and regiochemical issues.

Problem 30: Stereoselective Reduction
In 2001, Prof. R. Noyori received the Nobel prize for his development of stereoselective
reductions of C=C and C=O double bonds.
Let us consider a simplified model reaction to understand some of the basic control elements
necessary to achieve a stereoselective hydrogenation reaction.
E. g., racemic β-ketoester A can be reduced by hydrogen to racemic B with a high
diastereoselectivity in the presence of a metal catalyst that will chemoselectively
hydrogenate C=C double bonds. Enantiomerically pure β-ketoester C is diastereoselectively
reduced to racemic B. An additive (1 equivalent) such as lithium chloride proved to be highly
important for the high diastereocontrol in the reaction.

30.1 Draw the structures of the enantiomers forming racemic B.

30.2 There are two diastereomers to the compounds of 30.1 that form a racemic B*.

Draw their structures.

30.3 Develop a model showing that in the reaction described above only B (but no B*)

forms .