Stereoselective Umpolung Reactions with MetalatedP‐Chiral Cyanohydrin Phosphates—Enantioselective Synthesis of Tertiary Cyanohydrins

Abstract

AbstractWe present the first cyanohydrin derivative4that allows diastereoselective umpolung reactions. In4the OH group of the cyanohydrin is linked with a chiral phosphate, which can be removed hydrolytically after asymmetric synthesis. Cyclization of pseudoephedrine1dwith POCl3gave2d. This was followed by addition of racemic benzaldehyde cyanohydrin3to give the key intermediate4dwith complete retention of the configuration at phosphorus. Deprotonation of4d, followed by addition of a wide variety of electrophiles afforded the crystalline products5with high diastereomeric excesses (deup to 94%). High asymmetric induction was also achieved for the reaction of4−Li+with acyl halides, α‐bromoacetates, 2‐cycloalkenones, α,β‐unsaturated esters, and γ‐bromoacetates. Lewis acid assisted hydrolysis proceeded without racemization and gave high yields of ketone cyanohydrins6. From the ring‐opened chiral auxiliary7, optically pure pseudoephedrine1was readily recovered by acid hydrolysis. Optically pure (R) and (S) ketone cyanohydrins are now accessible in a very general strategy, which circumvents the substrate limitations of enzymatic synthesis.