Fused thiadiazoles

Methods for the preparation of fused thiadiazoles starting from quinone 1,2-dioximes have been reviewed in CHEC(1984) and CHEC-II(1996). No recent work has been reported. 

A useful strategy for the formation of fused thiadiazoles is the annulation of suitably functionalized 1,2,5-thiadiazoles. Common routes involve the use of 3,4-difluoro-l,2,5-thiadiazole, 3,4-diamino-1,2,5-thiadiazole, l,2,5-thiadiazole-3,4-dicarbonyls, l,2,5-thiadiazole-3,4-dicarbonitrile, amino-1,2,5-thiadiazole-3-carboxamides and carboxamidines. These afford heteroarene-fused 1,2,5-thiadiazoles (which are covered in Volume 9). Below follows a brief description of fused thiadiazoles that fall within the scope of this chapter. 

In the presence of triethylamine, the reaction of 2-amino-5-methyl[l,3,4]thiadiazole 134 with 2-benzyl-5-chloro-[l,2,4]thiadiazol-2-one 135 gives 3-(benzylcarbamoylimino)-6-methyl-3//-[l,3,4]thiadiazolo[2,3-c][l,2,4]thiadiazole 138. Presumably, the first-formed intermediate 136 rearranges through the thiapentalene intermediate 137 to the fused thiadiazole product 138 (Scheme 10) <1994T7019>. 

A reductive ring opening of a linearly fused thiadiazole derivative 92 was reported by Nawrocka and Stasko <1997PJC792>. Reaction of this compound with sodium borohydride in methanol under heating afforded the iV-aminoquinazolone-thione 93. 

Three different ring systems with 3-fusion of the [l,2,4]triazine ring (a fused thiadiazole and two different triazoles) have been described during the past period. 

Scheme 44 also shows two further synthetic routes to [l,3,4]thiadiazolo[2,3-c][l,2,4]triazinones. Reaction of the 3-mercapto- or 3-methylsulfanyltriazinone 221 (R1 = H or R1 = Me) with a set of isothiocyanates was reported to give the 2-amino-substituted fused ring system 222 in medium to good yield (36-84%) <1997JHC1351>. Derivative 223 was described to undergo cyclization to a fused thiadiazole 224 by treatment with carbon disulfide in the presence of potassium hydroxide in ethanol <2001PHA376>. 

Aromatic and heteroaromatic o-diamines were also converted in high yields to fused thiadiazoles 23 and 24 under the same conditions (Scheme 13 1967JOC2823, 1975JOC2749). 

Oxidation of quinoline fused thiadiazoles occurs selectively at the quinoline nitrogen to provide the quinoline-Ai-oxide (78) in 70% yield <86IJC(B)271>. 

The cyclic voltammograms of thiadiazole fused [2,5-(l,3-dithiol-2-ylidene)-l,3,4,6-tetrathiapentalenes], BDT-TTPs 88, in benzonitrile exhibited four pairs of redox waves corresponding to one-electron transfer processes at 4-0.60, 4-0.81, 4-1.30, and 4-1.47 V (vs. saturated calomel electrode (SCE)). The El values are a little higher than that of 4,5-bis(methylthio)-BDT-TTP (4-0.49 V). The difference is attributed to the electron-withdrawing character of the fused thiadiazole ring on the bicycle <1997SM(86)1821>. 

In 7-amino[l,2,5]thiadiazolo[3,4-d]pyrimidine the N—S bond (1.622 A) is substantially shorter than the N—S single bond (1.735 A) whereas the C—N bonds in the thiadiazole ring are shorter than the corresponding bonds in 9-methyladenine. Comparisons of bond lengths indicate that double bond character of the C—N bond is about 50% in the fused thiadiazole, which indicates an efficient electron delocalization between the ring as opposed to the finding for the oxadiazole analogue when a 75% C—N double bond character is indicated. 

Amino derivatives of isoxazole, 1,3,4-oxadiazole, thiazole, 1,2,4-thiadiazole and 1,3,4-thiadiazole containing the amino group as part of a partial amidine structure react with trichloromethanesulfenyl chloride to give isolable trichloromethanesulfenamides which react with substituted anilines to yield fused thiadiazoles of general structure shown in equation (9). The method has been used to prepare 3H-isoxazolo[3,2-c]-, 3H-thiadiazolo[2,3-c]-, 3H- l,3,4-thiadiazolo[2,3-c]- and 3//-[l,2,4]thiadiazolo[4,3-d]-[l,2,4]thiadiazoles, as well as 3//-[l,2,4]thiadiazolo[3,4-6][ 1,3,4]oxadiazoles (75JOC2600). 

A sulphenamido group placed a to a ring-nitrogen reacts with an arylamine (such as a nitroaniline) to form a fused thiadiazole ring. 

I -Chloro-1 -phenyliminomethanesulphenyl chloride reacts without external heat with aminoazines to form a fused thiadiazole ring. Chlorothioformyl chloride... 

An alternative preparation (cf. p. 412) of a fused thiadiazole consists of heating a diamine and thionyl chloride in toluene rather than in pyridine-dichloro-... 

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 ... 

Novel electron donors containing 1,2,5-thiadiazole and 1, I -dihydro-4,4 -bi(pyridylidene) units were synthesised by Yamashita et al. Thus, compound (100) was prepared starting from the chloropyridine (99) <97CC1851>. The fused thiadiazole systems (101) which contain a hypervalent sulphur atom were also prepared by Yamashita and were shown to have good electron accepting properties <97T10169>. 

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