Several five-membered carbocyclic compounds having two alkyl chains at the vicinal positions are known as biologically active natural products. Although the most simple and reactive five membered cyclic compound is cyclopentadiene, there is no report on the general method for the introduction of two kinds of carbon chains into the vicinal positions of the diene. A method for the selective preparation of cyclopentadienes with two kinds of carbon chains at the vicinal positions was investigated by treating esters of cyclopentadienylethanols, prepared from sodium cyclopentadienide and epoxides, with a base. The present method was successfully applied to the synthesis of C_9-terpene lactones, i. e. oniku- and mitsugashiwa-lactones (16 & 17) isolated as the physiologically active principles of Boshniakia rossica Hult and Menyanthes trifoliata L, from the intermediate anion (10) of cyclopentadienolactone prepared from cyclopenta-dienylethyl methyl carbonate (7a). Arethrolone (26a) was prepared from 2-methylcyclopentadienylethanol (18) obtained by the reduction of 10. The method is expected to be generally applicable to the syntheses of the natural products with vicinally dialkylated five-membered carbocyclic structure. As an example, routes to two prostanoids will be also discussed.
