Synthetic and Stereochemical Methodology

There are no special synthetic features to the conversion of 4-piperidones to corresponding reversed esters (phenyl lithium is generally preferred to a phenyl Grignard as the organometallic reagent, while dehydration products are sometimes encountered when the 4-piperidinols are esterified), but a 

6 Treat 2-butanone with acetylene and dehydrate the resultant i-alcohol c Ref 57 

The more active antipodal forms of a- and /3-prodine are neatly linked as follows pyrolysis of (+)-a-prodine (cis steric pathway) gives the (—)-tetrahydropyridine (43) with preservation of C-3 chirality, while dehydration of (+)-/3-prodinol (trans steric course also leaving C-3 geometry intact)(61) 

Finally, the absolute configuration of (-)-/3-2-methyl-4-phenyl-4-piperidinol has been established by X-ray analysis of the hydrochloride (the salt provides the heavy atom necessary to the anomalous dispersion tech-nique).(17) This physical procedure is the method of choice because it avoids problems of racemization and inversion inherent in all chemical transformations however well conceived. 

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