If, in place of the D-lactate monomer, the OH group of the D,D-dimer attacks
EM, a D,D,D-trimer will be formed, and the repetition of this type of reaction
results in the formation of higher D-oligomers. Since the L-lactate was not
consumed, the reaction of EM with the OH group of L-lactate does not occur
and the reaction shown in step (c) does not take place. On the other hand,
hydrolysis of D-lactate also needs activation to form EM. Then, EM reacts with
water to give D-lactic acid, as shown in step (d).
Concerning the reactions of L-lactate monomers, alkyl L-lactates were not
consumed at all in the oligomerization. In the hydrolysis, alkyl L-lactates were
hydrolyzed to give L-lactic acid [step (h) in Fig. 3]. This is a clear indication that
step (e) actually took place to produce (S)-acyl–enzyme intermediate
EM. However, neither the OH group of D-lactate nor the OH group of L-lactate
was allowed to attack EM to give L,D-dimer via step (f) or L,L-dimer via step (g).
Although hydrolysis steps (d) and (h) in Fig. 3 (both deacylations) are not
selective due to no chirality in the water molecule, esterification steps (b), (c), (f),
a
(R)
HO
H CH 3
O
OR
(R)
HO
H CH 3
O
OR
OH
O
(R)
HO
H CH 3
O
HO
H 3 C
O
OR
(R)
HO
H CH 3
O
OH
H
(S)
H 2 O
b
HO
H 3 C
O
OR
H
(S)
HO
H 3 C
O
H
(S)
O
OH
(S)
HO
H 3 C
H
O
OH
H 2 O
(R)
HO
H CH 3
O
OR
HO
H 3 C
O
OR
H
(S)
(R)
(R)
(S)
HO
H CH 3
O
O
(S)
HO
H CH 3
O
O
HO
H 3 C
H
O
O
HO
H 3 C
H
O
O
H
CH 3
H
CH 3
(R)
(S)
(R)
(S)
OR
O
OR
O
H
OR
O
OR
O
H
CH 3
CH 3
Lip
Lip
D-lactate
(R)-acyl-enzyme intermediate
(a)
- ROH
(b)
(c)
D,D-dimer
D,L-dimer
(d)
Lip
Lip
(e)
- ROH
(S)-acyl-enzyme intermediate
(h)
(f)
(g)
L,D-dimer
L,L-dimer
L-lactate
(enzyme-activated monomer: EM)
(enzyme-activated monomer: EM)
Fig. 3 Lipase-catalyzed reaction pathways of D-lactates (a) and L-lactates (b): acyl–enzyme
intermediate formation steps a and e, subsequent dimer formation steps b, c, f, and g, and
hydrolysis steps d and h. ○ denotes that the step takes place, whereas  denotes that the step
does not take place. In steps b, c, d, f, g, and h, the lipase leaving group is omitted
Green Polymer Chemistry: Recent Developments
157
EM, a D,D,D-trimer will be formed, and the repetition of this type of reaction
results in the formation of higher D-oligomers. Since the L-lactate was not
consumed, the reaction of EM with the OH group of L-lactate does not occur
and the reaction shown in step (c) does not take place. On the other hand,
hydrolysis of D-lactate also needs activation to form EM. Then, EM reacts with
water to give D-lactic acid, as shown in step (d).
Concerning the reactions of L-lactate monomers, alkyl L-lactates were not
consumed at all in the oligomerization. In the hydrolysis, alkyl L-lactates were
hydrolyzed to give L-lactic acid [step (h) in Fig. 3]. This is a clear indication that
step (e) actually took place to produce (S)-acyl–enzyme intermediate
EM. However, neither the OH group of D-lactate nor the OH group of L-lactate
was allowed to attack EM to give L,D-dimer via step (f) or L,L-dimer via step (g).
Although hydrolysis steps (d) and (h) in Fig. 3 (both deacylations) are not
selective due to no chirality in the water molecule, esterification steps (b), (c), (f),
a
(R)
HO
H CH 3
O
OR
(R)
HO
H CH 3
O
OR
OH
O
(R)
HO
H CH 3
O
HO
H 3 C
O
OR
(R)
HO
H CH 3
O
OH
H
(S)
H 2 O
b
HO
H 3 C
O
OR
H
(S)
HO
H 3 C
O
H
(S)
O
OH
(S)
HO
H 3 C
H
O
OH
H 2 O
(R)
HO
H CH 3
O
OR
HO
H 3 C
O
OR
H
(S)
(R)
(R)
(S)
HO
H CH 3
O
O
(S)
HO
H CH 3
O
O
HO
H 3 C
H
O
O
HO
H 3 C
H
O
O
H
CH 3
H
CH 3
(R)
(S)
(R)
(S)
OR
O
OR
O
H
OR
O
OR
O
H
CH 3
CH 3
Lip
Lip
D-lactate
(R)-acyl-enzyme intermediate
(a)
- ROH
(b)
(c)
D,D-dimer
D,L-dimer
(d)
Lip
Lip
(e)
- ROH
(S)-acyl-enzyme intermediate
(h)
(f)
(g)
L,D-dimer
L,L-dimer
L-lactate
(enzyme-activated monomer: EM)
(enzyme-activated monomer: EM)
Fig. 3 Lipase-catalyzed reaction pathways of D-lactates (a) and L-lactates (b): acyl–enzyme
intermediate formation steps a and e, subsequent dimer formation steps b, c, f, and g, and
hydrolysis steps d and h. ○ denotes that the step takes place, whereas  denotes that the step
does not take place. In steps b, c, d, f, g, and h, the lipase leaving group is omitted
Green Polymer Chemistry: Recent Developments
157