Synthesis of functionalised cyclic nitrones via regioselective and unusual [3 + 2] cycloadditions of α-nitrosostyrenes with 1,3-diazabuta-1,3-dienes and imines

The α-nitrosostyrenes 2, generated in situ from α-halogeno oximes, undergo regioselective [3 + 2] cycloaddition with 1,3-diazabuta-1,3-dienes 1 and 5 leading to the cyclic nitrones 3 and 6, respectively. Similarly, the cyclic nitrones 12 are also formed in reactions of 2 with the trisubstituted amidines 11. Thermolysis of the nitrones 3 and 12d-f gives imidazole derivatives 13. The nitrones 6, on the other hand, on thermolysis under similar conditions, give the amidine derivatives 17. Interestingly, the treatment of both 3 and 6 with NaBH₄ in methanol and the reactions of 2 with N-arylbenzamidines also yield the imidazole derivatives 13.