chemoselective for the cleavage of a phenylacetate group in its natural substrate,
penicillin G, which is industrially used for the production of 6-aminopenicillanic
acid (6-APA) (Scheme 2.37). This reaction has become a paradigm of how biocatalysis can contribute to make a chemical process more environmentally friendly.
Chemical hydrolysis of 1 ton of Pen G requires ~10 tons of organic solvent
(half of which is chlorinated) and problematic reagents, such as PCl 5 (600 kg)
and amines (1 ton). The enzymatic hydrolysis using immobilized (reusable)
Pen G acylase only requires a base to neutralize the acid formed [279]. Due
to its specificity for a phenylacetate moiety, this enzyme can be employed in
enzymatic protecting group chemistry [280, 281]. For instance, phenylacetyl
groups can be removed in a highly chemoselective fashion in the presence of
acetate esters (Scheme 2.37) [282, 283]. Furthermore, it can be used for the
resolution of esters of primary [284] and secondary alcohols [285] as long as
the acid moiety consists of a phenylacetyl group or a structurally closely
related (heterocyclic) analog [286–288]. Some structural similarity of the alcohol
moiety with that of the natural substrate penicillin G has been stated as being an
advantage.
Along the same lines, a protease derived from Aspergillus oryzae, which
has hitherto mainly been used for cheese processing, has proven useful for
the resolution of sterically hindered substrates such as α,α,α-trisubstituted carboxylates [289] (Scheme 2.38). While ‘traditional’ proteases such as subtilisin were
plagued by slow reaction rates and low selectivities, the α-trifluoromethyl mandelic
ester (which constitutes a precursor of the chiral derivatization agent ‘Mosher’s
acid’ [290]) was successfully resolved by Aspergillus oryzae protease [291].
O
O
R
Ph
O
O
OAc
OAc
O
AcO
AcO
O
OAc
OAc
HO
AcO
AcO
O
Ph
O
COOH
O
H
N
S
N
OH
R
O
O
R
O
OH
O
O
HO
O
O
R
O
rac
4
5
buffer
penicillin
acylase
82
80
e.e. [%]
penicillin
acylase
buffer
buffer
penicillin
acylase
+ R-ester
+ 4S,5R-ester
R = H
R = Et
penicillin G
+
rac
e.e. = 90%
4R,5S
e.e. = 90%
penicillin
acylase
buffer
S
R
CH 3
CO 2 Me
O
Ph
O
O
Ph
Ph
O
Scheme 2.37 Chemo- and enantioselective ester hydrolyses catalyzed by penicillin acylase
2.1 Hydrolytic Reactions
71
penicillin G, which is industrially used for the production of 6-aminopenicillanic
acid (6-APA) (Scheme 2.37). This reaction has become a paradigm of how biocatalysis can contribute to make a chemical process more environmentally friendly.
Chemical hydrolysis of 1 ton of Pen G requires ~10 tons of organic solvent
(half of which is chlorinated) and problematic reagents, such as PCl 5 (600 kg)
and amines (1 ton). The enzymatic hydrolysis using immobilized (reusable)
Pen G acylase only requires a base to neutralize the acid formed [279]. Due
to its specificity for a phenylacetate moiety, this enzyme can be employed in
enzymatic protecting group chemistry [280, 281]. For instance, phenylacetyl
groups can be removed in a highly chemoselective fashion in the presence of
acetate esters (Scheme 2.37) [282, 283]. Furthermore, it can be used for the
resolution of esters of primary [284] and secondary alcohols [285] as long as
the acid moiety consists of a phenylacetyl group or a structurally closely
related (heterocyclic) analog [286–288]. Some structural similarity of the alcohol
moiety with that of the natural substrate penicillin G has been stated as being an
advantage.
Along the same lines, a protease derived from Aspergillus oryzae, which
has hitherto mainly been used for cheese processing, has proven useful for
the resolution of sterically hindered substrates such as α,α,α-trisubstituted carboxylates [289] (Scheme 2.38). While ‘traditional’ proteases such as subtilisin were
plagued by slow reaction rates and low selectivities, the α-trifluoromethyl mandelic
ester (which constitutes a precursor of the chiral derivatization agent ‘Mosher’s
acid’ [290]) was successfully resolved by Aspergillus oryzae protease [291].
O
O
R
Ph
O
O
OAc
OAc
O
AcO
AcO
O
OAc
OAc
HO
AcO
AcO
O
Ph
O
COOH
O
H
N
S
N
OH
R
O
O
R
O
OH
O
O
HO
O
O
R
O
rac
4
5
buffer
penicillin
acylase
82
80
e.e. [%]
penicillin
acylase
buffer
buffer
penicillin
acylase
+ R-ester
+ 4S,5R-ester
R = H
R = Et
penicillin G
+
rac
e.e. = 90%
4R,5S
e.e. = 90%
penicillin
acylase
buffer
S
R
CH 3
CO 2 Me
O
Ph
O
O
Ph
Ph
O
Scheme 2.37 Chemo- and enantioselective ester hydrolyses catalyzed by penicillin acylase
2.1 Hydrolytic Reactions
71