Dynamic stereochemistry of imines and derivatives. Part 18. Photosynthesis and photoracemization of optically active oxaziridines

Optically active oxaziridines have been synthesized with a maximum optical yield of 31% by photoisomerization (λ > 300 nm) of achiral aldo- and keto-nitrones in the presence of the chiral solvent (+)-(S) or (–)-(R)-2,2,2-trifluoro-1-phenylethanol. Photorearrangement of chiral nitrones to optically active oxaziridines was found to occur in achiral solvents with an optimum diastereoisomeric excess of 20%. Temperature, solvent and substituent effects upon the degree of asymmetric synthesis of chiral oxaziridines from the corresponding nitrone isomers have been examined.Photoracemization and photoepimerization (λ 254 nm) of oxaziridines bearing a p-nitrophenyl substituent have been observed. Mechanisms involving a nitrone intermediate, and reversible N–O bond cleavage or photoinduced pyramidal nitrogen inversion are proposed.