SYNTHESES AND REARRANGEMENTS OF SPIROCYCLIC OXAZIRIDINES DERIVED FROM UNSYMMETRICAL KETONES

Oxaziridines provide useful alternatives to the Beckmann rearrangement and Schmidt reaction for ring enlargement of cyclic ketones. The procedure involves the condensation of the ketone in question with optically active alpha-methylbenzylamine, oxidation of the resultant imine, and photolysis to afford ring-expanded lactams. The alpha-phenylethyl substituent can be removed after photolysis to yield the N-unsubstituted lactam. When a distal ketone substituent is present, the oxaziridines can be synthesized stereoselectively. Thus, optically active ketones can be converted to either ring-expanded lactam by choice of either enantiomer of optically active alpha-methylbenzylamine. Ketones bearing adjacent substitution are generally not amenable to such regiocontrol because the resident substituent is the key stereocontrol element for the oxaziridine synthesis, although a notable exception is 2-methoxycyclohexanone. Stereogenic centers present in such compounds undergo epimerization during the couse of the reaction sequence; in addition, substrates containing substantial amounts of enamine give rise to novel doubly oxygenated products upon oxidation. Finally, the conformational behavior of the side chains in both oxaziridines and their product lactams permits some key stereochemical assignments to be made, on the basis of chemical shift trends in the NMR spectra of these materials.