Furans in Synthesis. 5. Furan-Terminated Cationic Cyclizations in the Preparation of Fused, Spirocyclic and Bridged Ring Systems. An Application to the Synthesis of Nakafuran 9

Article abstract of DOI:10.1021/jo00221a008

The title compounds 36-42 and nakafuran 9 (43) were prepared by furan-terminated cationic cyclizations.The cyclization substrates, allylic alcohols 16, 17, 25, 26, 33, and 34, and the derived enone 19 were prepared by CuCN moderated S_N2' addition of Grignard reagents derived from 2-(3-furyl)-1-bromoethane (12) and 3-(3-furyl)-1-bromopropane (13) to vinyl epoxides 14 and 22 and epoxy enol ether 21.Treatment of substrate allylic alcohols with a two-phase mixture of HCO2H and cyclohexane resulted in facile cyclization when the forming ring was six or seven membered.Enone closures proceeded only when a six-membered ring was produced or in the case of enone 48 which leads to nakafuran 9 (43).