Utilizing steroid substrates, MEH was able to hydrolyze not only epoxides, but
also the corresponding heteroatom derivatives such as aziridines to form trans-1,2aminoalcohols albeit at slower rates (Scheme 2.87) [647]. The thiirane, however,
was inert towards enzymatic hydrolysis. The enzyme responsible for this activity
was assumed to be the same microsomal epoxide hydrolase.
Although many studies have been undertaken with hepatic epoxide hydrolases
due to their importance in detoxification mechanisms [648], enzymes from these
sources are unsuitable for preparative-scale transformations, since they cannot be
obtained in reasonable amounts. In contrast, epoxide hydrolases from microbial
sources are easy to produce by overexpression.
Microbial Epoxide Hydrolases
Although it was known for several years that microorganisms possess epoxide
hydrolases, they were only scarcely applied to preparative organic transformations
[649–652]. Thus, the hydrolysis of epoxides, which was occasionally observed
during the microbial epoxidation of alkenes as an undesired side reaction causing
product degradation, was usually neglected, and systematic studies were undertaken
later on. It should be emphasized, that for practical reasons, many preparative-scale
reactions were performed by using whole-cell preparations or crude cell-free extracts
containing an unknown number of epoxide hydrolases. Some microbial epoxide
hydrolases have been purified and characterized [653–657].
As a result, an impressive amount of knowledge on microbial epoxide hydrolases from various sources – such as bacteria, filamentous fungi, and yeasts – has
n (CH 2 )
O
OH
n (CH 2 )
OH
epoxide hydrolase
buffer
cis-meso
R,R
[OH - ]
OH
n (CH 2 )
OH
S,S
or
[OH - ]
n
Enzyme
Diol
Config.
E.e. [%]
1
microsomal EH
R,R
90
1
cytosolic EH
R,R
60
2
microsomal EH
R,R
94
2
cytosolic EH
R,R
22
1
Rhodotorula glutinis CIMW147
R,R
>98
2
Rhodococcus erythropolis DCL14 (mutant)
S,S
97
2
Sphingomonas sp. HXN-200
R,R
99
3
Rhodococcus erythropolis DCL14 (mutant)
S,S
98
Scheme 2.86 Desymmetrization of cyclic cis-meso-epoxides by hepatic epoxide hydrolases
HO
HX
X
R
R =
buffer
microsomal
epoxide
hydrolase
NH, N-CH 3 : slow reaction
S: no reaction
X = O: fast reaction
Scheme 2.87 Enzymatic hydrolysis of steroid epoxides and aziridines by microsomal epoxide
hydrolase
2.1 Hydrolytic Reactions
119
also the corresponding heteroatom derivatives such as aziridines to form trans-1,2aminoalcohols albeit at slower rates (Scheme 2.87) [647]. The thiirane, however,
was inert towards enzymatic hydrolysis. The enzyme responsible for this activity
was assumed to be the same microsomal epoxide hydrolase.
Although many studies have been undertaken with hepatic epoxide hydrolases
due to their importance in detoxification mechanisms [648], enzymes from these
sources are unsuitable for preparative-scale transformations, since they cannot be
obtained in reasonable amounts. In contrast, epoxide hydrolases from microbial
sources are easy to produce by overexpression.
Microbial Epoxide Hydrolases
Although it was known for several years that microorganisms possess epoxide
hydrolases, they were only scarcely applied to preparative organic transformations
[649–652]. Thus, the hydrolysis of epoxides, which was occasionally observed
during the microbial epoxidation of alkenes as an undesired side reaction causing
product degradation, was usually neglected, and systematic studies were undertaken
later on. It should be emphasized, that for practical reasons, many preparative-scale
reactions were performed by using whole-cell preparations or crude cell-free extracts
containing an unknown number of epoxide hydrolases. Some microbial epoxide
hydrolases have been purified and characterized [653–657].
As a result, an impressive amount of knowledge on microbial epoxide hydrolases from various sources – such as bacteria, filamentous fungi, and yeasts – has
n (CH 2 )
O
OH
n (CH 2 )
OH
epoxide hydrolase
buffer
cis-meso
R,R
[OH - ]
OH
n (CH 2 )
OH
S,S
or
[OH - ]
n
Enzyme
Diol
Config.
E.e. [%]
1
microsomal EH
R,R
90
1
cytosolic EH
R,R
60
2
microsomal EH
R,R
94
2
cytosolic EH
R,R
22
1
Rhodotorula glutinis CIMW147
R,R
>98
2
Rhodococcus erythropolis DCL14 (mutant)
S,S
97
2
Sphingomonas sp. HXN-200
R,R
99
3
Rhodococcus erythropolis DCL14 (mutant)
S,S
98
Scheme 2.86 Desymmetrization of cyclic cis-meso-epoxides by hepatic epoxide hydrolases
HO
HX
X
R
R =
buffer
microsomal
epoxide
hydrolase
NH, N-CH 3 : slow reaction
S: no reaction
X = O: fast reaction
Scheme 2.87 Enzymatic hydrolysis of steroid epoxides and aziridines by microsomal epoxide
hydrolase
2.1 Hydrolytic Reactions
119