SYNTHESIS OF 2S,5S-AND 2R,5S-5-HYDROXYPIPECOLIC ACID VIA AMIDE-METHYLENATION OF S-5-HYDROXY-2-PIPERIDONE WITH DIMETHYLTITANOCENE

A route to 2S,5S-and 2R,5S-hydroxypipecolic acid is presented, starting with the enantiopure 5S-5-hydroxy-piperidone 7. The key step of this reaction sequence is the chemoselective methylenation of the amide carbonyl group of 8 with dimethyltitanocene 9 to 10. The transformation of the exocyclic enecarbamate double bond to the carboxylic acid group is best accomplished via hydroboration/oxidation to the alcohol 11a,b. Separation and oxidation of the diastereomers 11a,b, to 14a, and 14b, and hydrolysis furnishes the diastereomeric pipecolic acids 15a and 15b in enantiopure form.