Optically pure 2,3-cis-disubstituted piperidines 7a and 7b have been prepared from chiral lactams 2. The first step involves C-2 functionalization with Grigard reagent, then a diastereoselective reduction of resulting oxazolidines 5 furishes the desired piperidines 7.
Preparation of (R)-(+)-3-phenyl-2,3,5,6,7,8-hexahydro-oxazolo[3,2-a]pyridin-4-ylium bromide: synthesis of (S)-(+)-coniine, (R)-(-)-coniceine and (R)-(+)-anabasine
We describe the transformation of (R)-(-)-1-(2'-hydroxy-1'-phenylethyl)piperidin-2-one 1 into (R)-(-)-3-phenyl-2,3,5,6,7,8-hexahydro-oxazolo[3,2-a]pyridin-4-ylium bromide 2 using POBr3, Reduction of 2 with Red-Al at -78 degrees C gave (3R,8aR)-(-)-3-phenylbexahydro-2H-oxazolo[3,8-a]-pyridine 3 as a single diastereoisomer. The synthetic potential of these transformation is illustrated by the enantiopure synthesis of (S)-(+)-coniine, (R)-(-)-coniceine and (R)-(+)-anabasine.