Synthesis of complex carbobicyclic compounds from sugar allyltins: functionalization of the allylic position in bicyclo[4.3.0]nonene derivatives

Abstract

Two approaches for the functionalization of the allylic position in 7,8,9-tri- O-benzyl-5-substituted bicyclo[4.3.0]non-2-ene derivative 12 were examined. The first method, which involves an epoxidation of the C2–C3 double bond followed by a base induced isomerization, was found to be inappropriate. Although the epoxides were formed in good yields, the base-induced isomerization of the latter performed under the harsh conditions (LDA, HMPA, 80 °C) did not lead to the desired allylic alcohol 13, but to the tricyclic derivative 15 resulting from the opening of the oxirane ring by the anion generated from the benzyl group at the C9-position. The other, more promising approach, consisted of the cis-hydroxylation of the C2–C3 double bond followed by selective protection of one of the hydroxyl groups (at the C2-position) using the dibutyltin methodology. The free hydroxyl group located at the C3 position can be eventually eliminated to give the desired olefin with the double bond between the C3 and C4 carbon atoms of the bicyclic system.