Imines and derivatives. Part 21. A study of structural and mechanistic aspects of the synthesis of imine, imine oxide, and oxime derivatives of 2,2,4,4-tetramethylcyclobutane-1,3-dione by X-ray crystallography and nuclear magnetic resonance and ultraviolet spectroscopy

The dinitrone derivatives (5a, b) of 2,2,4,4-tetramethylcyclobutane-1,3-dione were synthesised by peroxyacid oxidation of the corresponding di-imines (3a, b). X-Ray crystallographic analysis of di-imine (3a) and the hydroxyimino nitrone isomers (6a, b) indicate a marked buttressing effect leading to an exclusive preference for the trans geometry in di-imine (3a) and dinitrone (5a). Thermal elimination reactions of the N-But substituted nitrones (5a), (6a), and (8a) to yield oximes and 2-methylpropene have been investigated by u.v. kinetic studies. The results are consistent with a concerted mechanism involving a five-membered cyclic transition state.