Reactions of some cyclic ethers in superacids

Abstract

The reactions of some epoxides and tetrahydrofuran derivatives in superacidic media have been studied. The tetrahydrofurans decompose only at 0 °C or above, yielding, in some cases, unsaturated carbocations which react to give carbocyclic products, though many yield only tar. Cyclohexene oxides decompose more readily; unsubstituted, they slowly form an allylic ion; with one carbon at the epoxide link substituted they yield the ketone, and with both carbons substituted they give the ring-contracted aldehyde. Limonene 1,2-oxide behaves in a similar manner, though yielding small amounts of the ring-contracted protonated aldehyde (10). Reaction of geraniol 2,3-oxide is initially similar but the intermediate is intercepted intramolecularly to yield the hydroxy-iridoid ethers, 3,3,6β-trimethyl-cis-perhydrocyclopenta[c]furan and 3,3,6α-trimethyl-cis-perhydrocyclopenta[c]furan.

Protonation of cyclohexene oxide or norbornene oxide yields onium salts, stable at –70 °C, which show the addition to be either unsymmetrical (i.e. edge protonation) or to take place in two different positions.