p-ionone end a-pinene oxides 1,3 isomerixe rapidly and selectively to l-(1,2,2trimethylcyclopent-1-yl)-pent-2-en-1.4dione 2 and the industrially important 2.2.3~trimethyl-3-C~aceeeldehyde4.undatbeinfluenccofcatalyticamOuntsOflrminilrmsaltsA3.In~~ find insights into the mechaniim of our pmccdure. protic and Lewis acids-catalyxcd rcanangements havealsubeenr Recently, we reported that aminium salt initiation represents a powerful protocol for the isomerkation of oxiranes into their corresponding carbonyl compounds. Undoubtedly, the fast conversion of g-ionone oxide 1 into l-( 1,212~aimethylcyclopent-1-yl)-pent-aen-1Pdione 2, induced by catalytic amounts of @is-@ bromophenyl) aminium hexachloroantimonate (p-BrC&I&N+ SbCle-[Ed = 1.16 V. vs SCR] A, was the most intriguing result, 1 (equation 1): 1 1 2
The exceedingly high efficiency and selectivity of this new catalytic procedure were totally unexpected, if compared with those observed in the several protic-and Lewis acids induced processes.2 In the firs instance, this behaviour was ascribed to a plausible chain radical cation mechanism induced by the aminium salt, a well known one-electron oxidizing agent.3
We now report the results of a mom systematic study, showing that aminium salts can be fruitfully employed to induce tire isomerization of another suitable oxirane, i.e a-pinene oxide 3, into 2,2,3-trimethyl-3cyclopenteneacetaldehyde 4, an important intermediate for the synthesis of santalol, a component of several fragrances,* as well as of necrodane-type monoterpenes, with a remarkable insect-repellant activities5 (equation 2). However, further experimental results have also been collected for the substrate 1.