1.2.4- Triazoles semicarbazones

Semicarbazones 2 were converted into their corresponding mono-, di- and trisubstituted 2,4-dihydro-l,2,4-triazol-3-ones 3 (Scheme 3 1986JHC881). 

The proposed mechanism includes the addition of sulfur monochloride to the N-H bond of semicarbazone or thiosemicarbazone with the formation of an N-S-S-Cl intermediate followed by its cyclization into the triazole or thiadiazole ring with the extrusion of two sulfur atoms and HCl. 

Triazine formation, 377, 282 ac3ti cyanide, 283 semicarbazone, 282 Triazole formation... 

Examples adduced in Scheme 87 indicate the caution required when a nitrilimine intermediate favours the formation of a heterocycle other than triazole. The bromo derivative (179) of the semicarbazone (180) is formally analogous to (172) the nitrilimine (181) is formed even in the presence of such weak bases as sodium acetate or water and undergoes ring closure to the 1,3,4-oxadiazole (182), In anhydrous acetic acid the triazolinone (183 R = H) is formed (72JCS(P1)1918) a similar reaction occurs on treating the iV-methyl homologue of (180) with bromine in acetic acid to obtain (183 R = Me). 

Anodic oxidation of azomethine, hydrazone, oxime, formazane, and semicarbazone structures has been used to initiate the intramolecular cyclization [119] under formation of heterocycles like triazoles [126,127], oxadiazoles [128,129], triazolinones [129], benzoxa-zoles [130,131], benzimidazoles [130,131], pyrazoles [132], indazoles [133], furoxanes [134], and tetrazolium salts [135] (see Chapter 18). Some of these reactions can be performed advantageously by indirect electrolysis using tris(4-bromophenyl)amin or 2,3-dihydro-2,2-dimethylphenothiazine-6(l/7)-one as mediators [119,136]. Two examples are given in Eqs. (19) and (20). 

The preparation and reactions of the azine, phenylhydrazone, and phenyl-semicarbazone of 4-amino-5-formyl-2-methyltriazole proceeded normally. One acetal has been prepared as follows. 4-Amino-3-benzyl-5-formyltriazole and boron trifluoride (as diethyl ether complex), stirred in methanol, gave 4-amino-3-benzyl-5-dimethoxymethyltriazole (20°C, 6 hr, 53%) and a dimeric by-product, 4-amino-3-benzyl-5-(3-benzyl-5-dimethoxytriazol-4-ylimino-methyl)triazole [73JCS(P1)2037]. The amino group in 4-aminotriazole-5-carbaldehydes lends itself readily to the formation of amidines and imidates (Section III,B,1), but it resists acylation. 

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