β-hydroxypiperidinecarboxylates:: additions to the chiral pool from bakers' yeast reductions of β-ketopiperidinecarboxylates

Reduction of the piperidine keto esters 16-19 using fermenting bakers' yeast provides high yields of the corresponding hydroxy esters 20, 26, 32 and 37 respectively, exclusively as the cts-diastereoisomers and with good levels (greater than or equal to 80%) of enantiomeric enrichment. The relative stereochemistries of the products were deduced from NMR data while the absolute configurations were determined by degradation to known piperidinemethanol derivatives or, in the case of hydroxy ester 37, by homologation to (R)-3-quinuclidinol 41b.