2-Bromothiazoles

Halothiazoles are usually obtained from 2-aminothiazoles through the Sandmeyer reaction. Nevertheless, ammonolysis has sometimes proved useful for the preparation of 2-aminothiazole derivatives. Detweiler et al. (18) obtained 2-(u-pyridinylamino)thiazole (1) from 2-bromothiazole (Scheme 1). The reaction is easier if a nitro group occupies the 5-position of the thiazole ring (19-21). Ethylene diamine derivatives undergo this reaction with 2-haiothiazoles (22-24). 

During the course of biochemical studies (138). the mass spectrum of 2-acetamidothiazole was recorded its main peaks are the molecular ion (m/e= 142, relative intensity = 26%) and fragments 100 (100), 58 (2. 5), and 43 (39). For 2-acetamido-5-bromothiazole the main peak results again from the loss of C2H2O by the molecular ion. 2-AcetyIacet-amido-4-methylthiazole (2S) exhibits significant loss of from the... 

Tirouflet et al. measured polarographic half-waves for a series of substituted thiazoles (143) in order to determine the ability of polarog-raphy for the selective determination of thiazoles mixtures. The values found for 2-amino-5-bromothiazole agree with the +M effect ot the NHj... 

Benzoyl chloride and derivatives acylate 2-amino-4-aryithiazoles in dioxane in yields of 80 to 90% (249, 250). The location of the acyl group on the exocyclic N has been demonstrated by the fact that the benzoyla-tion product is identical to the benzamidothiazole synthesized from benzamide and 2-bromothiazole (251). 3-Indolyl acetic acid chloride (89) acylates 2-aminothiazole in pyridine (Scheme 62) (81). 

Acetamido-4-methy)thiazole when enzymatically brominated is converted to a mixture of 2-acetamido-4-methyd 5-bromothiazole (200) and dibromacetamido-4-methyl-5-bro mo thiazole (201) (Scheme 127) (138, 436). 

This set of compounds is the most thoroughly studied among derivatives of 2-aminothiazole, thanks to the Beyer group. Heterocychzation methods are most commonly used for their preparation (see Chapter II and Refs. 37, 341, and 513-520). Reaction of 2-diazothiazoles (277) with organometallics provides another synthetic method (Scheme 1711 (521). 2-Hydrazinothiazoles may also be obtained by the action of hydrazine on 2-bromothiazoles (388) or on 2-mercaptothiazoles (522). 

At elevated temperatures (250-400°C) bromine reacts with thiazole in the vapor phase on pumice to afford 2-bromothiazole when equimolecu-lar quantities of reactants are mixed, and a low yield of a dibromothiazole (the 2,5-isomer) when 2 moles of bromine are used (388-390). This preferential orientation to the 2-position has been interpreted as an indication of the free-radical nature of the reaction (343), a conclusion that is in agreement with the free-valence distribution calculated in the early application of the HMO method to thiazole (Scheme 67) (6,117). 

The same general procedure was applied satisfactorily to the synthesis of 2-bromothiazoles using hydrogen bromide below 0°C (465, 507) (Table 11-29). 

BROMOTHIAZOLE DERIVATIVES FROM a-THIO-CYANATOKETONES AND DRY HYDROBROMIC ACID... 

The bromination with N-bromOsuccinimide in CCU can be accomplished yielding to 2-brOmothiazole (12.5% yield) (27). 

The nucleophiles used are OH (32) [the 2-hydroxythiazole can also be obtained by acidic hydrolysis with strong mineral acids (33)], OR" (5, 8, 30, 34), SR" (8, 9, 12), ArSH (35), and amines (4, 7, 14, 33). Benzamide also reacts with 2-bromothiazole, yielding 2-benzamidothiazole (36). Sulfonamide also reacts with 2-halogenothiazoles in presence of a base and copper powder, yielding 2-sulfonamidothiazoles (37, 38). 

The 2-bromothiazole can also be reduced to thiazole by zinc (18) in CH3COOH, or by electrolytic reduction (68). The deuterated thiazoles can also be obtained starting from the corresponding halogenothiazoles, by Zn/Cu in CH3COOD (18). 

Treating 2-bromothiazole with copper at 170°C in cumene as solvent affords the 2.2 -bisthiazole (Scheme 9) (69). The 2-halogenothiazoles can... 

Bromination of 2-brOmothia2ole leads to 2,5-dibromothiazole (5). 2-Bromothiazole can be used as a substrate in a malonic synthesis (72) starting from phenylacetonitrile the a phenyl-(2-thiazoiyl)-acetonitrile is obtained in high yields (84%) (Scheme 11). 

All possible dichloro- or dibromothiazoles are known. The 2.5-dihalogeno derivatives can be prepared from the 5-halogeno-2-aminothiazoles by diazotization/decomposition with CuCl or CuBr (3, 12, 13, 18, 75). The 5-halogeno-2-aminothiazoles can be easily prepared by halogenation of 2-aminothiazole (65, 76-79) 2,5-dibromothiazole can also be prepared by direct bromination of 2-bromothiazole (5). 

The mixed 2-chloro-5-bromothiazole has been obtained from 2-amino-5-bromothiazole, as previously indicated (75). 

Amongst the more unusual reactions, 2,3-thiazolo fused pyrido[3,2-d]pyrimidines have been prepared from 3-aminopicolinic acid and 2-bromothiazoles, whilst a similar derivative resulted with allyl isothiocyanate (221 222) <72IJC602). Similar products are also produced in [3 + 3] reactions of 2-aminothiazoles (Section 2.15.5.7.1). 

The synthesis of S-phosphonothiazolin-2-one 133 started with 2-bromothiazole 129. Nucleophilic displacement of the 2-bromide proceeded cleanly with hot anhydrous sodium methoxide to give 2-methoxythiazole 130. Low-temperature metalation of 130 with n-butyl lithium occurred selectively at the 5-position (76), and subsequent electrophilic trapping with diethyl chlorophosphate produced the 5-phosphonate 131. Deprotection of 131 was accomplished either stepwise with mild acid to pn uce the thiazolin-2-one intermediate 132, or directly with trimethylsilyl bromide to give the free phosphonic acid 133, which was isolated as its cyclohexylammonium salt. 

Ethyl thiazole-4-carboxylate has been prepared by hy-drogenolysis of ethyl 2-bromothiazole-4-carboxylate with Raney nickel in ethanol, by desulfurization of ethyl 2-mercaptothiazole-... 

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