OXOAMMONIUM SALTS .5. A NEW SYNTHESIS OF HINDERED PIPERIDINES LEADING TO UNSYMMETRICAL TEMPO-TYPE NITROXIDES - SYNTHESIS AND ENANTIOSELECTIVE OXIDATIONS WITH CHIRAL NITROXIDES AND CHIRAL OXOAMMONIUM SALTS

A new synthesis of unsymmetrical 2,2,6,6-tetraalkyl-4-piperidones from acetonin (2,2,4,4,6-penta-methyl-2,3,4,5-tetrahydropyrimidine) and several ketones is described. When the ketone was a naturally occurring optically active ketone, the piperidones were optically active. The piperidones were converted to unsymmetrical TEMPO-type nitroxides and chiral nitroxides. The optically active nitroxides were used as catalysts for oxidations or converted to chiral oxoammonium salts. The structures of the chiral compounds were determined by 2D H-1 and C-13 NMR, and the cyclic voltammetric properties of the various nitroxides were measured. Several other pyrrolidine oxoammonium salts were prepared, and both types were used as oxidizing agents. Preliminary results of chiral oxidations are presented.