Organic Chemistry

Section 18.15 Enzyme-Catalyzed Additions to a,b-Unsaturated Carbonyl Compounds 773

PROBLEM 39

Give the major product of each of the following reactions:

18.15 Enzyme-Catalyzed Additions to

-Unsaturated Carbonyl Compounds

Several reactions in biological systems involve addition to carbonyl
compounds. The following are examples of conjugate addition reactions that occur in
biological systems. Notice that the carbonyl groups are either unreactive or
have low reactivity (the CoA ester) toward the nucleophile, so conjugate addition
occurs in each case. The last reaction is an important step in the biosynthesis of fatty
acids (Section 19.21).

-methylaspartase

OO

−OCCH CHCO−

+NH 3 CH 3

+ NH 3

crotonase

OH

O

CH 3 (CH 2 )nCHCH 2 CSCoA

O

CH 3 (CH 2 )nCH CHCSCoA + H 2 O

O

O

CC

H CO−

−OC CH

3

O

O

fumarase

CC

H CO− OO

−OCCHCH

2 CO

−

OH

OH OPO 32 −

+ H 2 O

enolase

O

+ H 2 O CH 2 CHCO−

−OC H

O

CC

H CO−

H OPO 32 −

(COO - )

a,b-unsaturated

A,B

O

C
CH 2 CH OCH 2 CH 3

O

C

CH 3 CH 2 CH 2 NHCH 2 CH 2 OCH 2 CH 3

+ CH 3 CH 2 CH 2 NH 2

O

C

CH OCH 3

1. LiAlH 4


2. H 3 O+

a.

O

C

Cl

1. LiAlH 4


2. H 3 O+

b.

O

C

CH OCH 3

c. NH^3

O

C

Cl

excess

d. NH^3

CH 3 CH

CH 3 CH

CH 3 CH

CH CH 3 CH CH