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

Azomethyne ylides generated from 3-alkylthio substituted 1-alkyl-5-aryl-1,2,4-triazinium salts 4a–c on treatment with triethylamine undergo 1,3-dipolar 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 bearing the C≡C bond in the side-chain 3-alkynylthio substituent react with acetylenes to undergo either intermolecular 1,3-dipolar cycloaddition or intramolecular inverse electron demand Diels–Alder reactions.