Intermolecular and intramolecular cycloaddition reactions of 1-ethyl-1,2,4-triazinium salts with alkynes

Azomethyne ylides generated from 3-alkylthio substituted l-alkyl-5-aryl-1,2 ,4-triazinium salts 4a-c on treatment with triethylamine undergo 1,3-dipola r cycloaddition with dimethyl acetylenedicarboxylate to give pyrrolo[2,1-f] [1,2,4]triazines, while 1-ethyl-5-phenyl-1,2,4-triazinium salts 1a,b beari ng the C=C bond in the side-chain 3-alkynylthio substituent react with acet ylenes to undergo either intermolecular 1,3-dipolar cycloaddition or intram olecular inverse electron demand Diels-Alder reactions.