1.2.4- Thiadiazoles ring interconversions

A somewhat similar type of ring interconversion involving attack on sulfur has been postulated in the 1,2,4-thiadiazole series, e.g. (337) (338). Such reactions are common in the 1,2,4-dithiazolium series, e.g. (339) —> (340). 

The effect of fusion of another ring to benzofuroxan is rather variable. Naphtho[l,2-c]furoxan (16) and some of its derivatives show AG 83 kJ mol-1.100 Accurate data are available for two fused benzofuroxans, 10 and 11 (see Table V),72 and qualitative Tc measurements have been made on a number of related compounds.100,101,344,345 The slowest interconversions were noted for the furoxano- and furazano-fused compounds (83,84).101 The (heavily biased) isomeric JV-methyltriazole derivatives 85 and 86 were, qualitatively, rather more rapidly isomerized than were compounds 10 or 16,344 while the fused thiadiazole 87344 (AG1 72 kJ mol-1), thiophene 88345 (69 kJ), and thiazole 89344 (64 kJ mol - ) had still lower barriers. The fastest furoxan yet to be measured is the linear-fused example 90 (AG ... 

Synthesis.—The majority of 1,3,4-thiadiazole syntheses are based on cyclizations of thiosemicarbazidcs or compounds incorporating this basic structural pattern. Others involve ring-closures of dithiocarbazates, acyl hydrazines, or bithioureas, or interconversions of other ring systems, particularly oxadiazoles and dithiolethiones. Recent contributions to this subject have been numerous and are presented, as far as possible, in accordance with this classification. 

Within the huge field of heterocyclic rearrangements [53], the Dimroth rearrangement gives 5-mercapto-l,2,3-triazoles from 5-amino-l,2,3-thiadiazoles and allows interconversion of 5-amino-l,2,3-triazoles into a mixture of isomers under basic condition [54]. Thus, this reaction, discovered in 1909, involves a linear intermediate that can cyclize into two isomeric compounds (Scheme 15) [55]. This ring-chain tautomerization is reversible, but the equilibrium can be pushed toward an end if one of the two isomers is stabilized. 

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