New preparation of (3aR*,6S*,7aR*)-6,7,7-trimethylhexahydro-2-benzofuran-1(3H)-one: formal synthesis of (±)-γ-irone

A four-step synthesis of (3aR*,6S*,7aR*)-6,7,7-trimethylhexahydro-2-benzofuran-1(3H)-one 2 has been developed. Lactone 2 is an intermediate in a synthesis of γ-irone. Opening of the Diels–Alder adduct 5 with ethanol afforded hemiester 6b with 87:13 regioselectivity. Subsequent chemoselective reduction of the ester group in 6b yielded hydroxyacid 14 which was lactonized to the cis-fused bicyclic lactone 15. Finally, hydrogenation of 15 into 2 occured with complete diastereoselectivity.