1.2.4- Triazolo thiadiazoles formation

Heating the 5-isocyano-l,3,4-thiadiazolo[3,2- ]pyrimidin-5-one 115 with 10% hydrochloric acid gave a mixture of the 5-imino-l,3,4-thiadiazolo[3,2- ]pyrimidin-7-one 116 (10%) and the l,2,4-triazolo[l,5-c]pyrimidine-5,7-dione 117 (35%) (91JHC489). Formation of 117 probably occurred through thiadiazole ring rupture of 116 and recyclizatioii with its imino function together with desulfurization (Scheme 43). 

Bakulev et al. reported the synthesis of 5//-[l,2,3]triazolo[5,l-i>] [l,3,4]thiadiazines starting from 5-N-nitrosylamino-l,2,3-thiadiazole 68. Reduction of 68 with SnCh and 1A/HC1 and then subsequent reaction with a ketone gave the imine 69. Treatment of 69 with thionyl chloride at -80 °C led to the formation of the isolable triazolothiazine 70 which on further reaction with thionyl chloride at room temperature gave the corresponding chloro derivative 71 <00MC19>. 

Formation of the Second Ring by Insertion of a One-Atom Ring Member between Two Adjacent Substituents at the First Ring, Each Providing One Atom for the Second Ring (5)1,1 +1 — (5,5) NN/NS [l,2,4]Triazolo[3,4-b]-[l,3,4]thiadiazole... 

Table 3 Formation of [l,2,4]triazolo[3,4-b][l,3,4]thiadiazoles 85 from carbonic acid derivatives...

Contrasting with the reported formation of fused [l,3,4]thiadiazole rings in the course of the reaction of 3-substituted-4-amino-5-thio-47/-[l,2,4]triazoles 83 with various isothiocyanates (cf. Section 11.07.8.3, Table 3), the reactions with methyl isothiocyanate and with phenylisocyanate afford 3,7-disubstituted-6,7-dihydro-57/-[l,2,4]triazolo[4,3-f] [l,2,4]triazole-6-thiones 110 and -triazole-6-ones 111, respectively (Equation 29) <1986MI607, 1992IJB167>.The same reaction of 4-amino-l-methyl-3,5-bis(methylthio)[l,2,4]triazolium iodide 112 with aryl isothiocyanates yields the mesoionic compounds 113 (Equation 30) <1984TL5427, 1986T2121>. 

The reaction of a fourfold excess of aryldiazomethanes 130 with dichlorosulfine leads to 3,5-diaryl[l,2,4]triazolo[5,l+]-[l,3,4]thiadiazole-4-oxides 131. The formation of the fused heterocycles 131 is rationalized on the basis of two consecutive cycloaddition steps, each followed by elimination of hydrogen chloride promoted by the excess of aryldiazomethane (Scheme 9) <1984JCM175>. 

The thermolysis of l-([l,3,4]thiadiazol-2-yl)tetrazoles 133 gives rise to the loss of dinitrogen from the tetrazole ring and the formation of 2,6-diaryl[l,2,4]triazolo[5,l+][l,3,4]thiadiazoles 71 (Equation 39) <1985IJB908, 1990FA953>. (For another synthesis of compounds 71, see Section 11.07.8.1.5.)... 

Reaction of 5-halo-l,2,3-thiadiazoles with 1,3-diaminopropane leads to bis(l,2,3-triazolyl-l,2,3-thiadiazolyl)sulfide 105. Further intramolecular cyclization affords bis-[l,2,3]triazolo[l,3,7]thiadiazocine ring system 106 in 79% yield (Scheme 25 <20030BG4030>). The role of the ester groups on both the 1,2,3-triazole and 1,2,3-thiadiazole rings in the formation of the final product is essential. 

The structure of 3-(2-aminophenyl)-6-phenyl[l,2,4]triazolo[3,4-Z)][l,3,4]thiadiazole (3) is based on its determination by x-ray crystallography it indicates that in the course of its formation (cf. 8.07.8.3.1) a thioaroyl group has migrated and has been incorporated into the thiadiazole ring of the product <89LA1055>. 

Table 10 Formation of 6-heterosubstituted [l,2,4]triazolo[3,4-6][l,3,4]thiadiazoles (106), [l,2,4]triazolo[3,4-6][l,3,4]thiadiazol-(5//)6-ones and corresponding 6-hetero derivatives (107) from the reaction of 4-amino-5-sulfonyl-4/f-[l,2,4]triazole (104) with carbonic acid derivatives.

The 7-imino[l,2,4]triazolo[3,2-Z)][l,3]triazine compounds (71) when deprotonated, were found to form a ring-chain equilibrium with (72) (Equation (1)) <88KGS992>. Opening of the thiadiazole ring of (73) and formation of (74) was observed in the reaction with hydrazine hydrate (Equation (2))... 

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