MECHANISM OF CYCLOADDITION TO INDOLIZINES

The peculiarities of [8 + 2] cycloaddition reactions of indolizines wi th dienophiles are reviewed. Quantum chemical SINDO1, AM1 and ab initi o calculations of transition states were performed for [8 + 2] cyclo-a ddition reactions of indolizine and 6-nitroindolizine with a series of alkenes with donor and acceptor groups. The calculations predict a di polar 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.