Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Hydrolysis and Group Transfer Reactions 99
OH
O
R*OH
OH
+ R*OAc
O
O
O
+
OH
O
HO
HO
porcine pancreatic
lipase
vinyl acetate
THF, Et 3 N
AcO
HO
98% enantiomeric excess
52% yield
Figure 5.21 Enantioselective acylation reaction using a serine esterase. Et 3 N, triethylamine; THF,
tetrahydrofuran.
transferred to an alcohol acceptor. High-yielding regio- and enantio-speciWc
acylation reactions have been developed using vinyl acetate as a solvent. Formation
of the acetyl–enzyme intermediate is accompanied by formation of the
enol form of acetaldehyde, which rapidly tautomerises to give acetaldehyde,
making the reaction irreversible. Attack of the alcohol functional group then
gives the acetylated product. An example is illustrated in Figure 5.21 in which
an achiral meso-substrate is regioselectively acylated to give a single enantiomeric
product.
5.4 Acyl transfer reactions in biosynthesis: use of
coenzyme A (CoA)
How do living systems synthesise the amide bonds found in proteins, or the
ester functional groups found in lipids, oils and other natural products
The general strategy, shown in Figure 5.22, is to make an activated acyl
derivative containing a good leaving group, and then to carry out an
acyl transfer reaction.
good leaving group
R
OH
O
activation
R
X
O
acyl
transfer
acceptor Y
Figure 5.22 Use of an activated acyl group for acyl transfer.
R
Y
O