Studies on the synthesis of polyquinanes from alkylidene carbenes via 1,3-Diyls

Abstract

Cycloaddition reactions of 1,3-diyls generated from alkylidene carbenes have been studied. Intramolecular cycloaddition of alkylidene carbene to an olefin affords 1,3-diyl by homolytic cleavage of adduct. This 1,3-diyl is trapped by the other olefin, diylophile in a proper position to produce triquinane. Both of linear and angular triquinanes can be assembled by this new methodology. Among many methods for the generation of alkylidene carbene, thermolysis of a,b-epoxy aziridinylimine is chosen. α, β-Epoxy aziridinylimines from trienyl alcohols with a carbon or an ether linkage were heated to reflux in toluene to afford linearly fused tricyclopentanoids with a good yield and high stereoselectivity. Total synthesis of hirsutene, linear triquinane natural product, was successful through 14 steps from methallyl alcohol. Triquinane core structure with dimethyl substituents was assembled by thermolysis of an a,b-epoxy aziridinylimine. The triquinane allylic alcohol was converted to hirsutene by only 2 steps, e.g., trimethylsilylation of alcohol and Ni catalyzed allylation of methyl magnesium bromide. α, β-Epoxy aziridinylimines with an exo olefin to which alkylidene carbene would add and a diylophile were heated to reflux in toluene to afford mixtures of products. The mixtures contained an alkynol which was generated by 1,2-shift and angular triquinane. In some case, (8-ethylidene-7-phenyl-hexahydro-3a,6-methano-inden-6-yl)-methanol or enal were also formed. The ratio of products was dependent on the substituents of diylophiles. The more efficient electron-withdrawing group was, the higher the yield of angular triquinane was. The possibility for the synthesis of tetraquinane structure was found. The thermal reaction of α, β-epoxy aziridinylimine with a cyclopentane ring in it afforded an alkynol and tetraquinane structure.