Stereocontrol Strategies in the Asymmetric Bioreduction of Alkenes

The asymmetric bioreduction of prochiral conjugated alkenes using ene-reductases allows powerful strategies to access both enantiomers of the product with high stereoselectivity. This may be achieved (i) by using pairs of (iso)enzymes, which bind the alkene moiety in mirror-image orientations to affect hydride attack from opposite sides, (ii) via a switch in the (E/Z)-geometry of the alkene unit, or (iii) by changing the size of the protective groups of the substrate, which enforces a flipped orientation in the active site. Modeling studies provide a rationale for the molecular basis of substrate binding and allow the prediction of the stereochemical outcome of this useful bioreduction. 1 Introduction 2 Enzyme-Based Stereocontrol: 'Enantiomeric' Ene-reductases 3 Substrate-Based Stereocontrol: Flipping Substrates 3.1 Stereocontrol via (E/Z)-Configuration of Substrate 3.2 Stereocontrol via Substituent Effects 4 Modeling of Substrate Complexes 5 Conclusions