Synthesis of the cyclopropa[c]benzofuran system of mycorrhizin A and of gilmicolin from a benzofuranol. Crystal structure of a 5,8-Methanocyclopropa-[c]naphtho[2,3-b]furan derivative

Abstract

The cyclopropa[c]benzofuran derivative (17), which contains the structural features of methyl ethers of mycorrhizin A and gilmicolin save for the C₃ side chain, has been synthesized from the benzofuranol (4) through the benzofuran-4,7-dione (5) and the 7a-methoxy derivative (8). The cyclopropane ring of (17) was introduced by ultraviolet irradiation of a 1-pyrazoline (13) at -78º; irradiation at room temperature gave mainly a profoundly rearranged acidic product for which the methanoindenone structure (18) is proposed. The stereochemistry of the penultimate cyclopropane (16) has been established by X-ray crystallography.