Electrocyclization reactions of 1-aza- and 1-oxapentadienyl and -heptatrienyl cations:: Synthesis of pyrrole and furan derivatives

Quantum chemical DFT calculations (B3LYP/6-31+G*) have been used to gain insight into the conformational and energy properties of the 1-aza- and 1-oxapentadienyl and -heptatrienyl cations 1, 2, 3, and 4. The calculated thermodynamic and kinetic data of the ring-closure reactions giving the cyclic products 5-14 are reported and discussed with respect to the experimental results. Experimentally, synthetic routes to the alpha,beta-unsaturated carbonyl compounds 24 and 27, each with a leaving group in the gamma-position, have been developed. These compounds have been investigated with respect to their ability to undergo 1,5-electrocyclization reactions to yield 2,5-disubstituted furans 28 upon heating in the presence of acid, presumably through the intermediate formation of the 1-oxapentadienyl cations 2. From the corresponding imine 29a the pyrrole 30d was obtained after treatment with tetrakis(triphenylphosphane)palladium. In the presence of benzylamine and the Pd-0 catalyst, the corresponding pyrroles 30a-c were formed from 24 and 27. The homologous alpha,beta,gamma,delta-unsaturated carbonyl compounds 31 afforded 2-vinyl-substituted furans 32 upon heating with acid, and the 2-vinyl-substituted pyrroles 34 on treatment with benzylamine and the Pd catalyst. No seven-membered heterocyclic rings were formed. Surprisingly, the alpha,beta-unsaturated carbonyl compounds with two phenyl substituents at the gamma-position also provided pyrrole derivatives 40 through a formal dimerization. ((C) Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002).