Synthetic Approaches to Mycorrhizin A and Gilmicolin. III A Synthesis of Tricyclic Enones Epimeric with and Identical with the Key Synthetic Intermediate of Koft and Smith

Abstract

The cyclopentadiene adduct (2) derived from the benzofuranol (1) undergoes selective deprotonation with lithium diisopropylamide at C 8a, leading with allyl bromide to the 8a-allyl derivative (3) and with Me₃SiCl to the silyl ether (8). Reduction and methanolysis of (8) gives the epimeric hydroxy ketones (9) and (10) which by pyrolysis and O-silylation can be converted into the epimeric triethylsilyloxy tricyclic enones (13) and (15); the latter enone is identical with the key synthetic intermediate of Koft and Smith, but is the minor product. Attempted Mitsunobu inversion of the hydroxy enone (12) effects isomerization to the saturated 4,7-dione (18). The structure of the 8a- allyl derivative (3) is established by X-ray crystallography.