Reactions of terpenoids in strong acids. Part I. Novel rearrangements of geraniol and nerol to form 3β,6α,6aα-trimethyl-cis-perhydrocyclopenta[b]furan

Abstract

Treatment of geraniol or nerol with fluorosulphonic acid at low temperatures gave good (78 and 57%) yields of a novel iridoid ether identified as 3β,6α,6aα-trimethyl-cis-perhydrocyclopenta[b]furan. α-Cyclogeraniol (2,6,6-trimethylcyclohex-2-enylmethanol) and 9-hydroxyirid-1-ene [2-(2,3-dimethylcyclopentyl)propan-1-ol] were converted into the same product (87 and 98% yield, respectively) under the same conditions or (for irid-1-en-9-ol) by treatment with toluene-p-sulphonic acid at 70°(44% yield).

¹ H N.m.r. spectra of solutions of all four substrates in fluorosulphonic acid at –78° revealed the almost exclusive formation of the oxonium ion of the iridoid which was cleaved above 20° to form a carbonium-oxonium dication derived from irid-1-en-9-ol. These two ions were formed on treatment of geraniol or the iridoid with fluorosulphonic acid–antimony pentafluoride at ca.–70°, but in this medium further rearrangement occurred at higher temperatures to give (exclusively at 27°) a carbonium ion tentatively assigned a bicyclo[2.2.1]heptane skeleton.