ORTHO-QUINONOID COMPOUNDS .15. RAPID FORMYL MIGRATION IN 1-FORMYLINDENES - EVIDENCE FOR CONCERTEDNESS AND EXCLUSIVE 1,5-SIGMATROPY

1-Formyl-3-methoxymethyl-1-methylindene (6) and 1-deuterioformyl-2-formyl- 1,3-dimethylindene (15) have been prepared in optically active form. Interconversion of (6) and 1-formyl-1-methoxymethyl-3-methylindene (8) at 80°C proceeds with ≥ 95% retention of optical activity over 100 half-lives. Racemisation of (15) is twice as fast as its conversion to 2-deuterioformyl-1-formyl-1,3-dimethylindene (17) showing that each act of racemisation involves formation of the symmetric 2H-indene intermediate (16) in which there is a 50% chance of CHO-CDO exchange.