Domino carbocationic cyclization of functionalized cyclopropyl ketones: facile one-pot access to peri- and angularly...

Conjugate adducts obtained by base-induced 1,4-addition-elimination of various aryl/heteroaryl acetonitriles with 1-(2-arylcyclopropyl)-3,3-(bismethylthio)-2-propen-1-ones have been shown to undergo facile acid-induced domino carbocationic rearrangement yielding a variety of substituted tricyclic aromatic and heteroaromatic frameworks in high yields in a one-pot operation. The methodology provides efficient, high-yield routes for synthesis of novel substituted dihydrophenalenes, dihydrobenzo[d,e]anthracene, cyclopenta[a]naphthalene, and fused heteroaromatics such as substituted 4,5-dihydrobenzo[c,d]indole, dihydronaphtho[1,8-b,c]thiophene, dihydroindeno[5,4-b]- and -[4,5-b]-thiophenes, cyclopenta[a]carbazole, and dihydrocyclopenta[e]indazol-3-one derivatives. The probable mechanism of this interesting domino process appears to involve stepwise or concomitant acid-induced ring opening and intramolecular cyclocondensation of cyclopropyl ketones to give benzo-fused arene (or heteroarene) intermediates bearing a reactive benzylic carbocation that is captured intramolecularly either by a preexisting aromatic (or heteroaromatic) ring or by a newly formed benzene ring to give either peri-fused or angularly fused products, respectively. Thus, the overall domino process entails formation of two C-C bonds, a substituted benzene ring along with a peri-fused cyclohexane or angularly fused cyclopentane ring in a single operation.