The Intramolecular Diels-Alder Reaction with Furan-Diene: A Novel Route to (±)-Gibberellin A5

Abstract

A novel potential approach to 20-norgibberellins is described and exemplified with the total synthesis of (±)-gibberellin A5 from methyl (1S,2R, 5S)-2-(2-furyl)-5-hydroxy-6-methylenebicyclo[3.2.1]octane-1-carboxylate. Key steps involve an intramolecular Diels-Alder reaction with the furan-diene moiety leading to a kaurenoid analogue and the subsequent ring contraction of the central 5-membered ring to yield the required carboxy-substituted cyclopentane moiety via a photochemical α-diazo ketone Wolff rearrangement.