Synthesis of 5-isopropenyl/vinyl substituted pyrbvddinones via [4+2] cycloaddition reactions of 1,3-diaza-1,3-butadienes with isopropenyl/vinylketenes and their further transformations: [4+2] and unusual [3+2] cycloadditionsw α-nitrosostyrenes

1.3-Diaza-1.3-butadienes (1 and 5) underwent [4+2] cycloaddition reactions with isopropenyl/vinylketenes (2), acting as 2π components, to result in pyrimidinones (4 and 7). The treatment of 5-isopropenyl substituted pyrimidinones with phosphorus pentasulfide in the presence of sodium carbonate in dry tetrahydrofuran gave pyrimidinones (8). The reactions of nitrosoalkenes with 5-vinyl substituted pyrimidinones resulted in [4+2] cycloadditions leading to 1,2-oxazines (15), whereas 5-isopropenyl substituted pyrimidinones resulted in unusual [3+2] cycloaddition reactions yielding the nitrones (14).