1,3,4-Thiadiazole, 2-chloro-*, synthesis

The use of the dehydrating agent 2-chloro-l,3-dimethylimidazolinium chloride (DMC) in combination with DMSO for the preparation of 1,2,4-thiadiazoles via a type A synthesis has been published. Excellent yields >90% are reported along with a mechanism for the transformation <1999JOC6989>. 

The preparation of 5-chloro-l,2,4-thiadiazol-2-ium chlorides 95 by treatment of formimidoyl isothiocyanates 94 with a twofold excess of methanesulfenyl chloride has been reported in an unusual variation of a type C synthesis. These salts show interesting chemical behavior toward several nitrogen and carbon nucleophiles. The nature of the N-substituent determines the stability of the salt 95. When the substitutent on nitrogen is /-butyl, the salt 95 decomposes readily in solution to give the 5-chloro-l,2,4-thiadiazole 96 (Scheme 10) <2003HAC95>. 

A useful method for the synthesis of 5-chloro-l,2,4-thiadiazoles 125 is the reaction of amidines with trichloromethyl-sulfenyl chloride (Equation 34). 

O-Alkylation of 4-hydroxy-3-morpholino-l,2,5-thiadiazole 132 has been achieved with the chiral cyclic chloro-methyl sulfite 133 which subsequently suffers ring opening on treatment with simple alcohols <2001RCB436> or alkylamines <2002RJ0213> to afford the timolol analogues 134 with very little racemization (Scheme 20). This indicated an almost exclusive attack of the oxy anion on the exocyclic carbon atom and is a significant improvement on the previous oxirane method, which suffers from racemization. An alternative biocatalytic asymmetric synthesis of (A)- and (R)-timolol has also appeared <2004S1625>. 

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>. 

Amino-5-ethyl[l,3,4]thiadiazole 105 upon reaction with 1-chloro-l-phenyliminomethanesulfenyl chloride yields 6-ethyl-3-phenylimino-37/-[l,3,4]thiadiazolo[2,3-7][l,2,4]thiadiazole 106 (Equation 27) <1986S1027>. For another synthesis of a compound related to compound 106, see Section 11.07.9.2 <1994T7019>. 

The last example for the synthesis of this ring system discussed in this section is somewhat different from the previous ones as it presents formation of a positively charged thiadiazolo[3,2-tf][l,3,5]triazinium salt as published by Okide 1994JHC535 the 2 amino 5 alkyl[l,3,4]thiadiazole 167 was reacted with l chloro l,3 bis(dimethylamino) 3-phenyl-2-azaprop-2-enylium perchlorate (a reagent which was synthesized by the same author earlier <1992JHC1551>) to give the quaternary salt 168 in moderate yield (45%) (Scheme 32). 

Cyanooximes are less desirable starting materials for the synthesis of 1,2,5-thiadiazoles. Yields of 3-chloro derivatives 38 were low (1967JOC2823, 1992WOP9203433 Scheme 22). 

Amidines are converted into 1,2,4-thiadiazoles by reaction with isothiocyanates, iminosulfenyl chlorides, di- and trichloromethyl sulfenyl chlorides, and carbon disulfide in the presence of sulfur <82AHC(32)285> for example, 5-mercapto-3-methyl-l,2,4-thiadiazole (205) is obtained by the treatment of acetamidine with carbon disulfide and sulfur under basic conditions (Equation (29)) <85JAP85255783>. A useful method for the synthesis of 5-chloro-l, 2,4-thiadiazole (206) (R = 6-methyl-2-pyridyl) involves the reaction of amidines with trichloromethylsulfenyl chloride (Equation (30)) <91JAP9183590>. 

A useful method for the synthesis of 5-chloro-l,2,4-thiadiazoles (206) is the reaction of amidines with trichloromethylsulfenyl chloride (see Equation (30)). 3-Halo derivatives (349) (X = Cl, Br, I) (Equation (57)) have been obtained in moderate yields from the corresponding amines (348) via the Sandmeyer-Gatterman reaction <84CHEC-I(6)463>. 3-Chloro-l,2,4-thiadiazolin-5-ones (350) and (351) can be prepared by reacting chlorocarbonylsulfenyl chloride with carbodiimides or cyanamides respectively (Scheme 79) <84CHEC-I(6)463>. 

A further general synthesis, resulting in 3-substituted 5-chloro-l,2, 4-thiadiazoles, has been found by Goerdeler88 and his co-workers in the action of halogenated methylmercaptans on compounds incorporating an amidino group. 

The replacement of the reactive 5-chloro by a sulfonamido group in 1,2,4-thiadiazoles affords a convenient synthesis of the 5-sulfonamido heterocycles.92- 3,168-172 173 Thus, 5-halogeno compounds (171), on treatment with p-acetamidobenzenesulfonamide and potassium carbonate in diphenyl ether at 200°, afford the sulfonamido derivatives (170) in 70 % yields. Nitrobenzenesulfonamides do not react. This route is valuable, because the products are obtainable with difficulty by sulfonylation of the parent amine (see Section III,D, 1,A).8-8S-86-87 The superiority of route 171 -> 170 over route 169-y 170, though unusual in the heterocyclic field, recalls similar observations made in the synthesis of sulfonamidotriazines.174... 

Amidines (282 R = H) also condense with carbon disulfide in the presence of sulfur to produce 5-mercapto-l,2,4-thiadiazoles (17) in reasonable yields, as indicated in Scheme 101 (68BRP1116198). A general method for the synthesis of 5-chloro-l,2,4-thiadiazoles (289 X = Cl) involves the reaction of amidines with trichloromethylsulfenyl chloride (288 X = Cl) in the presence of a base under mild conditions (Scheme 102) (65AHC(5)119). A variety of 3-substituted thiadiazoles (118) are obtained when dichloromethylsulfenyl chloride (288 X = H) is used in this reaction. 

Pyrazolo[4,3-d]pyrimidine and pyrazolo[3,4-d]pyrimidine sulfonamides have been synthesized as selective calcitonin inducers <02JMC2342>. A series of para-substituted 3-arylpyrazolo[3,4-d]pyrimidines have been synthesized and evaluated as inhibitors of lck kinase <02BMCL1687). The synthesis of 3-aryl-pyrazolo[4,3- f]pyrimidines as potential corticotropin-releasing factor (CRF-1) antagonists has been described <02BMCL2133>. Cyclization of 4-chloro-5-heteroimine-l,2,3-thiadiazoles furnished pyrazolo[3,4-d]pyrimidines, triazolo[4,5-d]pyrimidines and purines <02BMC449>. 

The reaction products of cyanogen with alcohols, dialkyl oximidates (44) and alkylcyanoformimidates (45), also available through the reaction of sodium cyanide with chlorine in aqueous alcohol, both react with the sulfur chlorides forming the appropriately substituted thiadiazole. Wen reported the synthesis of diethoxy-1,2,5-thia-diazole (46) from 44 and sulfur dichloride. Several analogs of 45 were converted to 3-chloro-4-alkoxy-l,2,5-thiadiazoles (46a) by treatment with sulfur monochloride. 

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