STEREOCHEMISTRY OF ALKYLATION OF ALPHA-LITHIOPIPERIDINES - DIFFERING EFFECTS OF FORMAMIDINE AND URETHANE ACTIVATING GROUPS

The stereochemistry of alkylation of alpha-lithio-N-(N'-tert-butylformimidoyl)-4-tert-butylpiperidines was studied. Monoalkylation proceeded to give equatorial 2-substituted piperidines, which in turn gave diequatorial 2,6-disubstituted piperidines when subjected to a second lithiation-alkylation procedure. The stereochemical result of the second alkylation is in contrast to alpha-lithio-2-substituted piperidines that are stabilized by the N-(tert-butoxycarbonyl) group; these gave the axial 6-substituted piperidine on treatment with electrophiles. A mechanistic rationalization of these results is given that invokes nonbonding interactions between the stabilizing groups and piperidine ring substituents.