Studies of heterocyclic compounds. Part 31. 4-Alkyl-5-alkylimino-Δ2-1,2,4-thiadiazolines: synthesis and cycloaddition reactions with nitriles in attempts to prepare 3aλ4-thia-1,3,4,6-tetraazapentalenes

Abstract

Several 4-alkyl-5-alkylimino-Δ²-1,2,4-thiadiazolines have been synthesized from 1,2,4-thiadiazoles, namely 3,4-dimethyl-5-methylimino-Δ²-1.2,4-thiadiazoline 9, 3-methyl-6,7-dihydro-5H-1,2,4-thiadiazolo[4,5-a]pyrimidine 24, 6,7-dihydro-5H-1,2,4-thiadiazolo[4,5-a]pyrimidine 26, and 3-methy-5,10-dihydrobenzo[e]-1,2,4-thiadiazolo[4,5-a][1,3]diazepine 34. Cycloaddition reactions of compounds 9, 24, 26 and 34 were carried out in order to synthesize 1H,6H-3aλ⁴-thia-1,3,4,6-tetraazapentalenes 6, whose system is a hitherto unknown variation of the 1,6,6aλ⁴-triheterapentalene structure. Compound 9 failed to react with nitriles and decomposed under forcing conditions. Compounds 24, 26 and 34 underwent cycloaddition with nitiriles, with elimination of acetonitrile from 24 and 34 and of hydrogen cyanide from 26, and incorporation of the nitrile into the product 4-alkyl-5-alkylimino-Δ²-1,2,4-thiadiazoline. 1H,6H-3aλ⁴-Thia-1,3,4,6-tetraazapentalenes 6 were not isolated or detected spectroscopically but it is proposed that they are intermediates in the cycloaddition of 4-alkyl-5-alkylimino-Δ²-1,2,4-thiadiazolines with nitriles. The heterocyclic system 34 is a new one.