The following crude guidelines for the asymmetric bioreduction of activated
alkenes using ene-reductases can be delineated:
• Only C¼C-bonds which are ‘activated’ by electron-withdrawing substituents
(EWG) are reduced (Scheme 2.133) [1112], electronically ‘isolated’ double
bonds are not accepted [1113]. In a rough approximation, the activating capability of an EWG goes in line with its electron-withdrawing strength. With
activated, conjugated 1,3-dienes only the α,β-bond is selectively reduced, leaving the nonactivated γ,δ-bond behind (Scheme 2.133). In a similar manner,
cumulated 1,2-dienes (allenes) mainly give the corresponding 2-alkenes. A
rearrangement of the allene to give an acetylene may be observed
occasionally [1114].
• Acetylenic triple bonds yield the corresponding (E)-alkenes [1115]. The latter
may be subject to further (slow) reduction.
The following functional groups may serve as ‘activating’ groups:
• α,β-Unsaturated carboxaldehydes (enals) are quickly reduced in a clean fashion
yielding saturated aldehydes when pure ene-reductases are used. In contrast,
whole-cell reductions are heavily plagued by competing carbonyl reduction,
which often outcompetes the ene-reductase to furnish the corresponding allylic
alcohol (thereby depleting the substrate) and/or the saturated prim-alcohol (via
over-reduction of the desired product) [1116, 1117]. These undesired sidereactions sometimes allow to use an allylic alcohol as substrate, which is
transformed via the corresponding enal by whole cells [1118] (Scheme 2.133).
• α,β-Unsaturated ketones (enones) are good substrates for ene-reductases. With
whole cells, competing carbonyl-reduction is slower as compared to enals and
the product distribution depends on the relative rates of competing carbonyl- and
ene-reductases [1119, 1120] (Scheme 2.134).
• α,β-Unsaturated nitro compounds can be readily transformed into chiral nitroalkanes. Depending on the type of OYE, reductive biodegradation may occur via
the Nef-pathway [1121]. Due to the high acidity of nitroalkanes, any chiral
center at Cα is prone to racemization, whereas Cβ-analogs are perfectly stable
[1122] (Scheme 2.135).
• Cyclic imides, such as maleimide, are readily reduced without competing side
reactions.
• α,β-Unsaturated carboxylic acids or esters have to be regarded as
‘borderline’-substrates:
Simple α,β-unsaturated mono-carboxylic acids or mono-esters are not readily
reduced by OYEs (they are substrates for ‘enoate-reductases’). However,
the presence of an additional electron-withdrawing group (which alone
would not be sufficient to act as activator), such as halogen, helps to boost
the degree of activation [1123] (Scheme 2.136). Consequently, di-carboxylic
acids and di-esters are accepted by OYEs, although ester hydrolysis is a
common side-reaction when using whole cells. Due to their reduced carbonyl
activity, carboxylic acids are less activated than the corresponding esters.
2.2 Reduction Reactions
163
alkenes using ene-reductases can be delineated:
• Only C¼C-bonds which are ‘activated’ by electron-withdrawing substituents
(EWG) are reduced (Scheme 2.133) [1112], electronically ‘isolated’ double
bonds are not accepted [1113]. In a rough approximation, the activating capability of an EWG goes in line with its electron-withdrawing strength. With
activated, conjugated 1,3-dienes only the α,β-bond is selectively reduced, leaving the nonactivated γ,δ-bond behind (Scheme 2.133). In a similar manner,
cumulated 1,2-dienes (allenes) mainly give the corresponding 2-alkenes. A
rearrangement of the allene to give an acetylene may be observed
occasionally [1114].
• Acetylenic triple bonds yield the corresponding (E)-alkenes [1115]. The latter
may be subject to further (slow) reduction.
The following functional groups may serve as ‘activating’ groups:
• α,β-Unsaturated carboxaldehydes (enals) are quickly reduced in a clean fashion
yielding saturated aldehydes when pure ene-reductases are used. In contrast,
whole-cell reductions are heavily plagued by competing carbonyl reduction,
which often outcompetes the ene-reductase to furnish the corresponding allylic
alcohol (thereby depleting the substrate) and/or the saturated prim-alcohol (via
over-reduction of the desired product) [1116, 1117]. These undesired sidereactions sometimes allow to use an allylic alcohol as substrate, which is
transformed via the corresponding enal by whole cells [1118] (Scheme 2.133).
• α,β-Unsaturated ketones (enones) are good substrates for ene-reductases. With
whole cells, competing carbonyl-reduction is slower as compared to enals and
the product distribution depends on the relative rates of competing carbonyl- and
ene-reductases [1119, 1120] (Scheme 2.134).
• α,β-Unsaturated nitro compounds can be readily transformed into chiral nitroalkanes. Depending on the type of OYE, reductive biodegradation may occur via
the Nef-pathway [1121]. Due to the high acidity of nitroalkanes, any chiral
center at Cα is prone to racemization, whereas Cβ-analogs are perfectly stable
[1122] (Scheme 2.135).
• Cyclic imides, such as maleimide, are readily reduced without competing side
reactions.
• α,β-Unsaturated carboxylic acids or esters have to be regarded as
‘borderline’-substrates:
Simple α,β-unsaturated mono-carboxylic acids or mono-esters are not readily
reduced by OYEs (they are substrates for ‘enoate-reductases’). However,
the presence of an additional electron-withdrawing group (which alone
would not be sufficient to act as activator), such as halogen, helps to boost
the degree of activation [1123] (Scheme 2.136). Consequently, di-carboxylic
acids and di-esters are accepted by OYEs, although ester hydrolysis is a
common side-reaction when using whole cells. Due to their reduced carbonyl
activity, carboxylic acids are less activated than the corresponding esters.
2.2 Reduction Reactions
163