Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 99

transesterification of 6-alkyl substituted CLs (Fig. 10 c ) [ 27 , 84 ] . A decrease in the
enantioselectivity was observed in the transesterification of 6-substituted CLs upon
increasing the substituent size, but

-selectivity was present in all lactones. van
Buitenen et al. evaluated the transesterification of lactones differing in ring size
with the methyl substituent at the

( S )

-position (Fig. 10 d ) [ 85 ]. Remarkably, small
lactones (up to seven-membered ring) showed reactivity for the

ω

( S ) -enantiomer or

for both enantiomers, whereas in the case of larger rings the ( R ) -enantiomer was

the faster reacting (Fig. 11 ) . In fact, the ( R ) -selectivity for the transesterification

of ω -methyl-substituted lactones is what one would expect on the basis of the

( R ) -selectivity found in the esterification of secondary alcohols. The observed high

( -enantioselectivity in the hydrolysis or transesterification of these large ring

lactones can be explained by the fact that the enantioselective step in the reaction
sequence is merely the reverse of the acylation of a chiral secondary alcohol because
the enantioselective step in the reaction sequence goes through the same transition
state [ 87 ] .
The

R )

-selectivity for 6-MeCL in Novozym-435-catalyzed hydrolysis and trans-
esterification reactions was unexpected. Recent molecular modeling studies by Veld
et al. suggest that the cisoid ester conformation in lactones of ring size below eight
could be responsible for the switch in enantioselectivity [ 88 ] . In contrast, the ester

( S )

200

S -enantiomer

150

R -enantiomer

100

50

0

O

O

O

O O O O

O

O

O O

O

Fig. 11 Enantioselectivity in the CALB-catalyzed transesterification of ω -methyl lactones [ 75 ]