Stereoselective enzymatic reduction of keto-salinosporamide to (−)-salinosporamide A (NPI-0052)

Salinosporamide A (NPI-0052, 1), a highly potent 20S proteasome inhibitor, has been prepared from its ketone precursor (2) by asymmetric enzymatic reduction. The yields are quantitative with complete stereoselective conversion to the desired product, with no evidence for the undesired diastereomer. This process should lead to new synthetic strategies for the total synthesis of 1.