Formal Synthesis of Solanoeclepin A: Enantioselective Allene Diboration and Intramolecular 2+2 Photocycloaddition for the Construction of the Tricyclic Core

An enantioselective synthesis of an intermediate in the Tanino total synthesis of solanoeclepin A has been developed. The key step was an intramolecular 2+2 photocycloaddition, which led to the tricyclo5.2.1.0(1, 6) decane core in six steps. The first photosubstrate, prepared through an indium-mediated Barbier-type reaction, gave an excellent 2+2 cycloaddition, but it could not be obtained in sufficient enantiopurity. The second photosubstrate, prepared through an asymmetric allene diborylation in high enantiomeric excess, gave the 2+2 cycloaddition product in high yield on irradiation at 365 nm on 20 g scale in a flow system. Other important steps were the replacement of a boronate group at the quaternary carbon by a vinyl group and diastereoselective cyclopropanation of an allylic alcohol.