Quantum chemical analysis and experimental study of the cycloaddition reaction between aminoacetylenes and 6-nitroindolizines. NMR and ab initio evidence for the [4+2] adduct formation

7-Methyl-6-nitro-2-phenylindolizine (Ia) reacts with 1-(diethylamino)-2-met hylacetylene (IIa) giving 1:1 cycloadduct IIIa and not the expected product of dipolar [8+2] cycloaddition cyclazine IVa. According to NMR data the st ructure of IIIa consists of 1,2-oxazine fragment corresponding to the [4+2] cycloaddition of the acetylene to the sequence C(5)-C(6)-N-O of nitroindol izine. The mechanism of model reactions between nitroindolizine We and amin oacetylenes IIb,c is studied by an ab initio and AM1 method. Results of cal culations indicate that the initially formed zwitterionic intermediates may undergo further ring closure either to cycl[3.2.2]azines IVb,c or 1,2-oxaz ine cycloadducts IIIb,c. Although the structures TV are lower in energy tha n III, the activation barrier for the formation of III is smaller than the barrier leading to IV.