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.
Adam
Waldemar
W
Institut für Organische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany. adam@chemie.uni-wuerzburg.de
Librera
Christian P
CP