Minor Enantiomer Recycling : Application to Enantioselective Syntheses of Beta Blockers

Continuous recycling of the minor product enantiomer obtained from the acetylcyanation of prochiral aldehydes provided access to highly enantiomerically enriched products. Cyanohydrin derivatives, which under normal conditions are obtained with modest or poor enantiomeric ratios, were formed with high enantiomeric purity by using a reinforcing combination of a chiral Lewis acid catalyst and a biocatalyst. The primarily obtained products were transformed into -adrenergic antagonists (S)-propanolol, (R)-dichloroisoproterenol, and (R)-pronethalol by means of a two-step procedure.
QC 20140505