Rearrangement of annelated housanes to triquinane-like hydrocarbons by electron transfer (1,3-cyclopentanediyl radical cations) and acid catalysis (cyclopentyl carbenium ions)

The electron-transfer-catalyzed rearrangement of the annelated housane 4a on treatment with tris(p-bromophenyl)aminium hexachloroantimonate (TBA(.+)SbCl(6)(-)) affords regioselectively the two isomeric olefins endo-5a and 6a by 1,2 migration of the two groups at the methano bridge. Acid-catalyzed rearrangement gives in addition to endo-5a and 6a also the regioisomer endo-7a as major product. The formation of both rearrangement products endo-5a and 6a suggests a planar conformation for the radical-cation and carbocation intermediates. The regioselectivity is rationalized in terms of electronic stabilization of the radical versus cationic sites by the substituent at the rearrangement termini in the radical-cation and carbocation intermediates. Of interest for preparative purposes. the annelated housane 4a leads under electron-transfer conditions to unusual triquinane-related olefins by means of an unprecedented synthetic pathway.