A DOUBLE HEMIKETAL FORMATION/HETERO-MICHAEL ADDITION APPROACH TO THE [6,5,5]-DISPIROKETAL SYSTEM OF SPIROLIDES

摘要

An approach to the [6,5,5]-dispiroketal ring system of spirolides is described. It was found that cisoid isomers, which suffered from a destabilizing dipole dipole interaction, were preferentially formed over transoid isomers by a double hemiketal formation/hetero-Michael addition sequence regardless of the reaction conditions used. However, the stereochemistry at C12 was controlled by the methyl group at C13, resulting in the preferential formation of undesired 12S isomers. As expected from precedents, the desired isomer could be obtained upon exposure of the 12R isomer, formed by the sequence, to TsOH in benzene, albeit in a ratio of 1:3 favoring its C15-epimer.