Dynamic stereochemistry of imines and derivatives. Part VII. Thermal epimerization of chiral oxaziridines

An enantiomeric preference in both cis- and trans-oxaziridine isomers is produced by asymmetric oxygen atom transfer from (+)-peroxycamphoric acid to imines. A favourable selection of reaction conditions followed by fractional crystallization and thermal epimerization is used to obtain enantiomerically homogeneous cis- and trans-oxaziridines. Optical activity was totally retained in the thermal isomerization cycle (cis→trans→cis or trans→cis→trans) indicating that oxaziridines isomerize by nitrogen inversion rather than by bond cleavage. The barriers to pyramidal nitrogen inversion (ΔG‡) in thermally stable cis- and trans-oxaziridines were measured by both n.m.r. and polarimetric methods.