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-methylacetylene (IIa) giving 1:1 cycloadduct IIIa and not the expected product of dipolar [8+2] cycloaddition cyclazine IVa. According to NMR data the structure 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 nitroindolizine. The mechanism of model reactions between nitroindolizine We and aminoacetylenes IIb,c is studied by an ab initio and AM1 method. Results of calculations indicate that the initially formed zwitterionic intermediates may undergo further ring closure either to cycl[3.2.2]azines IVb,c or 1,2-oxazine cycloadducts IIIb,c. Although the structures TV are lower in energy than III, the activation barrier for the formation of III is smaller than the barrier leading to IV.