Mechanism of cycloaddition to indolizines

The peculiarities of [8 + 2] cycloaddition reactions of indolizines with dienophiles are reviewed. Quantum chemical SINDO1, AM1 and ab initio calculations of transition states were performed for [8 + 2] cyclo-addition reactions of indolizine and 6-nitroindolizine with a series of alkenes with donor and acceptor groups. The calculations predict a dipolar cycloaddition mechanism (electrophilic addition and ring closure) for reactions of indolizine and 6-nitroindolizine with nitroethylene. For the reaction of 6-nitroindolizine with N,N-dimethylaminoethylene, the predicted mechanism corresponds to a previously unknown 'inverse' dipolar cycloaddition (nucleophilic addition and ring closure). (C) 1998 John Wiley & Sons, Ltd.