and reliable procedures [615, 1289]. Although high selectivities have been achieved
for the epoxidation of cis-alkenes, the selectivities achieved with trans- and terminal olefins were less satisfactory using the latter methods.
In contrast, the strength of enzymatic epoxidation, catalyzed by
monooxygenases, is in the preparation of small and nonfunctionalized epoxides,
where traditional methods are limited [617, 1290]. Despite the wide distribution of
monooxygenases within all types of organisms, their capability to epoxidize
alkenes seems to be associated mainly with alkane- and alkene-utilizing bacteria,
whereas fungi are applicable to a lesser extent [1175, 1291–1295].
Like C-H hydroxylation, epoxidation of alkenes catalyzed by monooxygenases
is preferably performed on a preparative scale with whole microbial cells. Toxic
effects of the epoxide formed, which accumulates in the cells, where it reacts with
cellular enzymes, and its further (undesired) metabolism catalyzed by epoxide
hydrolases in whole cells (Sect. 2.1.5) can be minimized by employing biphasic
media. Alternatively, the alkene itself can constitute the organic phase into which
the product is removed, away from the cells. However, the bulk apolar phase tends
to damage the cell membranes, which reduces and eventually abolishes all
enzyme activity [1296]. In any case, these methods require bioengineering skills
[1297].
Once the problems of product toxicity were surmounted by sophisticated
process engineering, microbial epoxidation of alkenes became also feasible on
an industrial scale [1298, 1299]. The latter was achieved by using organicaqueous two-phase systems or by evaporation of volatile epoxides. For instance,
the epoxy-phosphonic acid derivative ‘fosfomycin’ [1300], whose enantiospecific
synthesis by classical methods would have been extremely difficult, was obtained
by a microbial epoxidation of the corresponding olefinic substrate using Penicillium spinulosum.
The most intensively studied microbial epoxidizing agent is the ω-hydroxylase
system of Pseudomonas oleovorans [1301, 1302]. It consists of three protein
components: the actual nonheme iron ω-hydroxylase and the electron-transport
chain consisting of rubredoxin and NADH-dependent rubredoxin reductase. It
catalyzes not only the hydroxylation of aliphatic C–H bonds, but also the epoxidation of alkenes [1303, 1304]. The following rules can be formulated for epoxidations using Pseudomonas oleovorans (Scheme 2.155).
• Terminal, acyclic alkenes of moderate chain length (e.g. 1-octene) are converted
into (R)-1,2-epoxides of high enantiomeric excess along with varying amounts
of ω-en-1-ols or 1-als [1305], the ratio of which depends on the chain length of
the substrate [1306, 1307]. In contrast, alkane hydroxylation predominates over
epoxidation for short (propene, 1-butene) and long-chain olefins.
• α,ω-Dienes are transformed into the corresponding terminal (R,R)-bis-epoxides.
184
2 Biocatalytic Applications
for the epoxidation of cis-alkenes, the selectivities achieved with trans- and terminal olefins were less satisfactory using the latter methods.
In contrast, the strength of enzymatic epoxidation, catalyzed by
monooxygenases, is in the preparation of small and nonfunctionalized epoxides,
where traditional methods are limited [617, 1290]. Despite the wide distribution of
monooxygenases within all types of organisms, their capability to epoxidize
alkenes seems to be associated mainly with alkane- and alkene-utilizing bacteria,
whereas fungi are applicable to a lesser extent [1175, 1291–1295].
Like C-H hydroxylation, epoxidation of alkenes catalyzed by monooxygenases
is preferably performed on a preparative scale with whole microbial cells. Toxic
effects of the epoxide formed, which accumulates in the cells, where it reacts with
cellular enzymes, and its further (undesired) metabolism catalyzed by epoxide
hydrolases in whole cells (Sect. 2.1.5) can be minimized by employing biphasic
media. Alternatively, the alkene itself can constitute the organic phase into which
the product is removed, away from the cells. However, the bulk apolar phase tends
to damage the cell membranes, which reduces and eventually abolishes all
enzyme activity [1296]. In any case, these methods require bioengineering skills
[1297].
Once the problems of product toxicity were surmounted by sophisticated
process engineering, microbial epoxidation of alkenes became also feasible on
an industrial scale [1298, 1299]. The latter was achieved by using organicaqueous two-phase systems or by evaporation of volatile epoxides. For instance,
the epoxy-phosphonic acid derivative ‘fosfomycin’ [1300], whose enantiospecific
synthesis by classical methods would have been extremely difficult, was obtained
by a microbial epoxidation of the corresponding olefinic substrate using Penicillium spinulosum.
The most intensively studied microbial epoxidizing agent is the ω-hydroxylase
system of Pseudomonas oleovorans [1301, 1302]. It consists of three protein
components: the actual nonheme iron ω-hydroxylase and the electron-transport
chain consisting of rubredoxin and NADH-dependent rubredoxin reductase. It
catalyzes not only the hydroxylation of aliphatic C–H bonds, but also the epoxidation of alkenes [1303, 1304]. The following rules can be formulated for epoxidations using Pseudomonas oleovorans (Scheme 2.155).
• Terminal, acyclic alkenes of moderate chain length (e.g. 1-octene) are converted
into (R)-1,2-epoxides of high enantiomeric excess along with varying amounts
of ω-en-1-ols or 1-als [1305], the ratio of which depends on the chain length of
the substrate [1306, 1307]. In contrast, alkane hydroxylation predominates over
epoxidation for short (propene, 1-butene) and long-chain olefins.
• α,ω-Dienes are transformed into the corresponding terminal (R,R)-bis-epoxides.
184
2 Biocatalytic Applications