Amino-1,2,4-thiadiazoles synthesis

Thiadiazoles. - Synthesis. 5-Amino-1,2,3-thiadiazole is prepared from N CCN and I S in the presence of a tertiary amine327 Similarly, 2-diazo-2-cyanoacetic acid derivatives react with H S/... 

The structure of only one 1,2,4-thiadiazole salt has been determined. Unambiguous synthesis of the imino compound obtained on alkali treatment of the salt formed by the reactions of methyl iodide with 5-amino-1,2,4-thiadiazole established that the salt possesses structure 7 136 pjjjg quatemization exactly parallels the reaction occurring in... 

The literature synthesis of the racemic 2-azidonitrile (80) by diazotisation of the 5-amino-1,2,3-thiadiazole (78) could not be repeated the 2-chloronitrile (79) was the only product obtained from the reaction <96TA607>. 

The synthesis of the benzoimidazo[l,2- ][l,2,3]thiadiazole 61 can be explained using the same mechanistic model to that used for the Hurd-Mori reaction. The amino benzimidazole 58 when treated with thionyl chloride at reflux affords the benzoimidazo[l,2-r ][l,2,3]thiadiazole 61. If, however, the reactant 58 is treated with thionyl chloride at room temperature, the chloromethyl derivative 59 is formed. This derivative was then transformed into product 61 on reflux with thionyl chloride. The proposed mechanism for the formation of product 61 is for the initial formation of the sulfoxide 60, which then undergoes a Pummerer-like rearrangement, followed by loss of SO2 and HC1 to give the c-fused 1,2,3-thiadiazole 61 (Scheme 7) <2003TL6635>. 

Perhaps the earliest reported method for the synthesis of the 1,2,3-thiadiazole ring system was the one described by Pechmann and Nold in which diazomethane was reacted with phenyl isothiocyanate. Of the four possible isomers that could be obtained from the reaction, 5-anilino-l,2,3-thiadiazole 62 (R1 Ph, R2 = H) was the only product formed (Equation 16) <1896CB2588>. This method continues to be used as a route to 5-amino substituted 1,2,3-thiadiazoles. 4,5-Disubstituted 1,2,3-thiadiazoles have been produced in excellent yield by reaction of l,l -thiocar-bonyl diimidazole with ethyl diazoacetate <1988SUL155>. 

A variation of Wolffs synthesis involves the reaction of diazonitrile with H2S as the thionating agent to give 5-amino-l,2,3-thiadiazole in 73% yield (Equation 21) <1997H(44)197>. 

A variation using thioacylguanidines affords 5-substituted-3-amino-l,2,4-thiadiazoles. If an amidinothiourea is oxidized, 5-amino-l,2,4-thiadiazoles are obtained. A recent example of this type of synthesis has been reported the amidinothiourea 87 was oxidized to the 1,2,4-thiadiazolium salt 88 on treatment with bromine (Equation 25) <2003SC2053>. 

In another variation of a type E synthesis, thioamides or thioureas condense with /V,/V-dimcthylacylamide dimethyl acetal to give imino compounds which react with amino-transfer reagents like hydroxylamine-O-sulfonic acid and mesitylsulfonyloxyamine (MSH) to give 3,5-substituted-l,2,4-thiadiazoles in excellent yields <1996CHEC-II(4)307>. There have been no new reports of type E syntheses since the publication of CHEC-II(1996). 

The transformation of 1,2,4-thiadiazoles bearing a reactive substituent such as amino or halogen group in the 5-position is the most useful method for the synthesis of 5-substituted 1,2,4-thiadiazole derivatives. The latter compounds can be reacted with nucleophiles to afford a wide range of derivatives this is not the case for 3-halogen-substituted compounds. 

The synthesis of 1,2,5-thiadiazoles from amino acetonitrile salts 172 was reviewed in CHEC-II(1996). Owing to the ready formation of 2-amino acetonitrile salts from aldehydes via a one-pot Strecker synthesis, this synthetic pathway... 

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). 

Examples of the Dimroth rearrangement (Section IV, F) include several s3mtheses of monocyclic triazoles from other heterocyclic systems (cf. Scheme 25). Triazole-5-thiols can be prepared by treatment of 5-amino-l,2,3-thiadiazoles with bases.A similar base-induced rearrangement of sydnoneimines provides a synthesis of 4-hydroxy-triazoles. ... 

In another pubhcation [81], the treatments of 3-formyl chromones with 2-amino-5-methyl-l,3,4-thiadiazoles and mercaptoacitic acids leading to compounds 120 were carried out both with the application of microwave techniques and by the conventional parallel synthesis. 

Acetazolamide Acetazolamide is 5-acetamido-l,3,4-thiadiazole-2-sulfonamide (9.7.5). The synthesis of acetazolamide is based on the production of 2-amino-5-mercapto-l,3, 4-thiadiazole (9.7.2), which is synthesized by the reaction of ammonium thiocyanate and hydrazine, forming hydrazino-N,N -( ji-(thiourea) (9.7.1), which cycles into thiazole (9.7.2) upon reaction with phosgene. Acylation of (9.7.2) with acetic anhydride gives 2-acetylamino-5-mercapto-l,3,4-thiadiazol (9.7.3). The obtained product is chlorinated to give 2-acetylamino-5-mercapto-l,3,4-thiadiazol-5-sulfonylchloride (9.7.4), which is transformed into acetazolamide upon reaction with ammonia (9.7.5) [24,25]. 

The pronounced electron-withdrawing nature of the 1,2,5-thiadiazole system is also evidenced by strong carbonyl electrophilic activation and by enhancement of carboxy acidity. The acid dissociation constants of thiadiazole acids, discussed in Section 4.09.4.1, fall in the range 1.5-2.5. The 1,2,3-thiadiazole carboxylic acids are easily decarboxylated at 160-200 °C. This reaction has been used for the synthesis of monosubstituted derivatives as well as the parent ring and deuterated derivatives <68AHC(9)107>. An efficient bromo-decarboxylation of 3-amino-1,2,5-thiadiazole-carboxylic acid has also been reported <70BRP1190359>. 

Due to the lack of suitable aliphatic starting materials, the synthesis of aromatic forms of monocyclic 1,2,5-thiadiazoles was not accomplished for more than 70 years after the first report of the benzo analogues. General synthetic methodology for monocyclic 1,2,5-thiadiazoles was devized which readily leads to a variety of substituted derivatives, alkyl, aryl, halo, hydroxy, alkoxy, cyano, and amino as well as the parent system. A general model for substrates applicable in these syntheses... 

The synthesis of numerous thiadiazoles substituted in the 5-position with carbamoyl or heteroaryl moieties and hydroxyl or amino groups in the 2-position (Scheme 20), was achieved by reacting dithioesters with semicarbazide or thiosemicarbazide and cyclizing the resulting acylated sem-... 

Similarly, 4-(furan-2-ylmethylene)-2-phenyl-1 -substituted-5(477)-imidazolones 820, evaluated as antibacterial and antifungal agents, have been synthesized from the 4-(furan-2-yhnethylene)-2-phenyl-5(4//)-oxazolone 819 (Scheme 7.253). ° Using 4-arylmethylene-2-phenyl-5(4//)-oxazolones as substrates and 2-amino-5-methyl-l,3,4-thiadiazoles as nucleophiles the synthesis of the corresponding... 

The Balz-Schiemann synthesis can be applied not only to substituted anilines but also to aminobiphcnyls1,131 or amino-substituted fused polyaromatic compounds, such as naphthalene,1114,119,129 anthracene,136 phenanthrene,1135 acenaphthene,133 fluorene,1,131,134 benzanthracene,130 136 pyrene,136 chrysene,136 fluoranthene,131 fluorenone,1,131 anthra-quinone,1,137,139,140 benzanthrone,1,117,118 phenanthraquinone,138 or xanthone.132 Fluorinated pyridines,1,141"146 methylpyridincs,126,147 149 pyridinecarboxylic acids,150 quinolines,1,151 isoquinolines,152 quinazolone,1 thiazoles,153,154 isothiazoles,156 benzothiazoles,157 thiadiazoles,155 and thiophenes154 can also be obtained from the corresponding aminated heterocycles. Modified Balz-Schiemann methods are recommended for amino nitrogen-containing heterocycles, the diazonium salts of which are rather water-soluble and unstable (a violent explosion was reported for pyridine-3-diazonium tetrafluoroborate).159 These new techniques have also been specially adapted for pyrazol-, imidazol-, or triazolamines which fail to react under classical conditions.158... 

In 1954, Goerdeler6 introduced a general synthesis of 1,2,4-thia-diazoles from amidines. This versatile method has since been widely extended and has made a great variety of 1,2,4-thiadiazole derivatives readily accessible. Basically, an amidine is converted into its i T-thio-cyanato derivative, which cyclizes spontaneously to the 5-amino-1,2,4-thiadiazole. 

The scope of the synthesis is greatly widened by the preliminary preparation of the N-halogenoamidines (56), preferably in situ subsequent displacement of halogen by thiocyanate results in the formation of the desired 5-amino-1,2,4-thiadiazoles (58) in good yields (50-80%).6,79, 80 The use of formamidine affords the parent, 5-amino-1,2,4-thiadiazole (58 R = H) while use of AT-methylformamidine (59) leads to 5-imino-4-methyl-Zl4-1,2,4-thiadiazolines (60).81... 

Further extension of this synthesis to guanidines leads to 3,5-diamino-l,2,4-thiadiazoles. Examples are so far confined to the conversion, by the usual procedure, of 2V,2V-disubstituted guanidines into 5-amino-3-dialkyl(or diphenyl)amino-l,2,4-thiadiazoles (66).84,87 In general, this variant of the reaction proceeds less uniformly than with amidines or iso(thio)ureas, and is performed without isolation of the intermediate AT-halogenoguanidines. In some cases (e.g. dimethyl and cyclopentamethylene homologs) the yields are low, and guanidine is formed as a by-product. The 3-dimethylamino homolog (66 R = R ... 

Proof for the formulation of the resulting derivatives as 5-sulfona-mido-l,2,4-thiadiazoles is provided by the series of reactions outlined in the appended scheme.81 The benzenesulfonyl derivative (211) of 5-amino-l,2,4-thiadiazole yields, on methylation, two monomethyl-ated products (212 and 213). Since the presence of the sulfonyl group (in 213) on the 5-imino group is shown by the alternative synthesis 214- 213, it follows that the sulfonyl group (in 211) is also attached to the exocyclic 5-amino group.81... 

Amino groups, on the other hand, are sulfonylated with less difficulty,126, 126 although here yields may again tend to be unsatisfactory.92 The synthesis of the sulfanilamido derivative from 3-amino-1,2,4-thiadiazole is described in the parent literature.184... 

The largest proportion of the dyes of this type, which can range from yellow to blue in colour, is of the homocyclic type. Examples such as those derived from 5-amino-l,2,4-thiadiazoles (32), 2-aminobenzothiazoles and 3-amino-2,l-benzisothiazoles (74MI11201) are in the minority. Synthesis is usually via diazotization of the appropriate heteroaromatic amino group and coupling with quaternary aniline derivatives. 

Amino-5-alkyl- or 5-phenyI-l,3,4-thiadiazoles are prepared most conveniently from thiosemicarbazide. For example, benzalthiosemicarbazone by oxidation with iron(III) chloride gives the 5-phenyl derivative. Thiosemicarbazide is also used in the synthesis of 2-amino-5-mercapto-l,3,4-thiadiazole by reaction with carbon disulfide. The product may be alkylated to yield the 5-alkylthio derivatives using a variety of alkylating agents (58MI11201). 

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