Amino Acids Peptides And Proteins Page 14
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Amino Acids, Peptides, and Proteins
CHAPTER 24
SOLVED PROBLEM 24-1
Show how you would use a Strecker synthesis to make isoleucine.
SOLUTION
Isoleucine has a sec-butyl group for its side chain. Remember that
CH
¬ CHO
undergoes Strecker synthesis to give alanine, with
3
CH
as the side chain. Therefore,
sec-butyl ¬ CHO
should give isoleucine.
3
CH
O
CH
NH
CH
NH
3
3
2
3
3
+
NH
, HCN
H
O
3
3
CH 9 C 9 H
CH 9 C 9 H
CH 9 C 9 H
CH
CH
CH
CH
CH
CH
3
2
3
2
3
2
H
O
sec-butyl 9 CHO
2
C # N
COOH
(2-methylbutanal)
(
,
)-isoleucine
D
L
Hint
problem-solving
PROBLEM 24-13
In the malonic ester synthesis,
(a) Show how you would use a Strecker synthesis to make phenylalanine.
use the side chain of the desired
(b) Propose a mechanism for each step in the synthesis in part (a).
amino acid (must be a good
S
2
N
substrate) to alkylate the ester.
In the Strecker synthesis, the
PROBLEM 24-14
aldehyde carbon becomes the
a
Show how you would use a Strecker synthesis to make
carbon of the amino acid: Begin
(a) leucine
(b) valine
(c) aspartic acid
with [side chain] ¬ CHO.
S U M M A R Y
Syntheses of Amino Acids
1. Reductive amination (Section 24-5A)
N 9 H
O
NH
2
excess NH
H
R 9 C 9 COOH
3
R 9 C 9 COO
2
R 9 CH 9 COO
NH
4
Pd
a-ketoacid
a-amino acid
imine
2. Amination of an a-haloacid (Section 24-5B)
O
Br
O
NH
O
2
NH
(1) Br
/PBr
3
R 9 CH
9 C 9 OH
R 9 CH 9 C 9 OH
R 9 CH 9 C 9 O
2
3
NH
2
4
(large excess)
(2) H
O
2
a-bromo acid
carboxylic acid
(
,
)-a-amino salt
D
L
(ammonium salt)
3. The Gabriel – malonic ester synthesis (Section 24-5C)
temporary ester group
CO
2
O
O
COOEt
COOEt
COOH
H
+
H
O
(1) base
heat
3
N
CH
N
C
R
H
N
C
R
H
N
C
R
(2) R 9 X
3
3
heat
COOEt
COOEt
COOH
COOH
O
O
a-amino acid
N-phthalimidomalonic ester
alkylated
hydrolyzed
4. The Strecker synthesis (Section 24-5D)
O
NH
NH
2
3
+
H
O
H
O
3
R 9 C 9 H
2
R 9 C 9 H
R 9 C 9 H
NH
HCN
3
C # N
COOH
a-amino nitrile
a-amino acid
aldehyde
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