Donor–acceptor accelerated norbornadiene rearrangements

Abstract

The norbornadienone acetals 3 with a CO₂Me, CONMe₂ or CHO substituent at C-2 undergo rearrangement under very mild conditions; cycloheptatrienes are obtained for CO₂Me and CONMe₂ substituents and the furanone acetal 9 for the CHO substituent. The donor-acceptor acceleration is consistent with a formal 1,3-shift to a norcaradiene proceeding either via a zwitterionic intermediate or a concerted-forbidden path. Rearrangement via a biradical is not consistent with the slower rearrangement of 7-cyano-7-methoxy-2,3-bis(methoxycarbonyl)norbornadiene 16. The indene 19 racemises rapidly at a temperature 100 °C below that required for 18 establishing that a donor and an acceptor (Me₃SiO and CN) at a potential radical centre promote homolysis to a greater extent than two donor groups (two alkoxy groups).