Synthesis of 6,7-dihydro-5H-1,2,4-triazolo[3,4-b][1,3,4]thiadiazines by a C–C ring cyclization under mild conditions

Abstract

The reaction between Schiff bases derived from 4-amino-2-methyl-5-methylthio-2H-1,2,4-triazole-3(4H)-thione and phenacyl bromides yields 7-aroyl-6-aryl-6,7-dihydro-1-methyl-3-methylthio-5H-1,2,4-triazolo[3,4-b][1,3,4]thiadiazinium bromides [(6)–(11)].The structure of one of these compounds, (6)(in which both aryl groups are phenyl), has been determined by X-ray crystallography. It incorporates dioxane molecules and the bromide ion is sandwiched between two units. In the 1,3,4-thiadiazinium ring the consecutive hydrogen atoms –N(5)H–C(6)H–C(7)H– are trans to each other, the ring showing an E^C6 conformation. The crystalline trans derivatives equilibrate to a 60:40 mixture of trans and cis 7-aroyl-6-aryl isomers, which have been analysed by ¹H and ¹³C n.m.r. spectroscopy.