Stereochemical memory in the regioselective and diastereoselective rearrangement of tricyclo[3.3.0.02,4]octanes (housanes) by electron transfer (1,3-cyclopentanediyl radical cations) and acid (cyclopentyl carbenium ions) and silver-ion catalysis

The electron-transfer-catalyzed rearrangement of the housanes 5 affords regioselectively only the two cyclopentenes 6 (CH3 migration) and 7 (R migration) by 1,2-migration of the two groups at the methano bridge to the methyl terminus. The 1,2-shift of the CH3 group prevails, and the rearrangement ratio is essentially insensitive to the migratory aptitude of the R substituent. This stereochemical memory effect derives from the conformational impositions on the stereoelectronic requirements during the 1,2-migration in the 1,3-radical-cation intermediates. Similar regioselectivities and diastereoselectivities are observed for the TFA-catalyzed and silver(I)-ion-promoted rearrangements, whereas the rearrangement catalyzed by HClO4 affords a complete reversal in the product selectivity and both the regioselectivity and the diastereoselectivity are much reduced. Migration to the phenyl terminus is favored to afford the 6' and 7' cyclopentenes, of which the former (CH3 migration) dominates. For the minor regioisomer, only the cyclopentene 6 is formed by an exclusive 1,2-shift of the CH3 group. This dichotomy in product selectivities is rationalized in terms of two distinct mechanisms for the various activation modes: a common one for the electron-transfer-induced, TFA-catalyzed, and silver(l)-ion-promoted rearrangements and a different one for HClO4.