Evidence for Increased Steric Compression in Anti Compared to Syn Lignin Model Quinone Methides

Two new lignin model quinone methides (QMs) have been characterised by 1 H and 13 C NMR techniques. It was shown that the QMs existed in solution as non-interconverting syn and anti isomers, in a ratio of 2:1. Evaluation of chemical shift data and nuclear Overhauser effects from these and previously synthesised QMs showed that the anti isomer was under greater steric strain than the syn isomer. Additional evidence for the increased steric strain was obtained from a relative kinetic study of the addition of primary amines to the QMs.