1,2,5-Thiadiazole 1,1-dioxides structure

The condensation of sulphamide and diethyl oxalate produces 3,4-dihydroxy-1,2,5-thiadiazole 1,1-dioxide as the dipotassium salt in high yield. The free acid (177) is an excellent precursor for the preparation of other functional derivatives, and for building up fused ring systems incorporating the 1,2,5-thiadiazole 1,1-dioxide structure (see below). The two isomeric benzothiadiazole acetoximes (178) and (179) undergo the Beckmann reaction, resulting in the fission of their carbocyclic ring. This provides an effective synthesis of novel ajS-unsaturated 1,2,5-thiadiazoles (180 X = COaH, Y = CR=CHCN X =... 

While 1,2,5-thiadiazole 1,1-dioxide has not yet been prepared, extrapolation of data on the known 3,4-dimethyl-l,2,5-thiadiazole 1,1-dioxide 23 <1998JP091> indicated that the nonaromatic or antiaromatic 1,2,5-thiadiazole 1,1-dioxide has a more delocalized structure than its isomeric thiadiazole 1,1-dioxide anologues <1997JMT119, 2001JMT285>. 

In earlier accounts of the permanganate oxidation of 7 by Khaletskii et aU the principal product was reported as l,2,5-thiadiazole-3,4-dicarboxylic acid-1,1-dioxide. This structure was later corrected to... 

Although 3,4-dichloro-l,2,5-thiadiazole-l,1-dioxide (76) is related structurally to 3,4-dichlorothiophene-1,1-dioxide their chemical properties are quite different. The thiophene compound is a reactive diene and readily dimerizes via an auto Diels-Alder reaction. Furthermore, its chlorine atoms are unreactive toward weak nucleophilic reagents like water and alcohol. The thiadiazole compound, on the other hand, shows no tendency to dimerize and is highly reactive toward water and alcohol. 

It is significant that these reactions are restricted exclusively to the 5-methyl-group in 3,5-dimethyl-1,2,4-thiadiazole, even when an excess of the appropriate reagent is employed,287 reflecting the superior reactivity of 5- compared with identical 3-substituents in the 1,2,4-thiadiazole structure.3 The point is further illustrated in the metallation of the 3,5-dimethyl compound (392, R = Me) with butyllithium after further treatment with carbon dioxide, the 5-heteroarylacetic add (396) is the sole product (62%).291... 

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