A SYNTHESIS OF METHYL 2-ALPHA-ACETOXY-3-DEOXO-3-ALPHA-HYDROXYANGOLENSATE

The title compound (1a) has been partially synthesised from gedunin. It is the first natural 1α,2α,3α-trihydroxy terpene molecule to have been synthesised. Attempts to introduce a 2α-hydroxy group into a preformed methyl angolensate derivative failed, and the synthesis was achieved by the prior preparation of 2α-acetoxy-7-deacetoxy-7-oxokhivorin (2a), 1 which was converted into the title compound. The key stage in the conversion is the Baeyer-Villiger oxidation of the 7-oxo group to a lactone, which was studied in some detail. The use of buffered peracetic acid effects the required oxidation in good yield without oxidising the furan ring; the use of pertrifluoroacetic acid oxidises the furan ring to an unsaturated lactone without attacking the carbonyl group. The implications of this are considered in an Appendix.