A-Haloketones

Nucleophilic reactivity of the sulfur atom has received most attention. When neutral or very acidic medium is used, the nucleophilic reactivity occurs through the exocyclic sulfur atom. Kinetic studies (110) measure this nucleophilicity- towards methyl iodide for various 3-methyl-A-4-thiazoline-2-thiones. Rate constants are 200 times greater for these compounds than for the isomeric 2-(methylthio)thiazole. Thus 3-(2-pyridyl)-A-4-thiazoline-2-thione reacts at sulfur with methyl iodide (111). Methyl substitution on the ring doubles the rate constant. This high reactivity at sulfur means that, even when an amino (112, 113) or imino group (114) occupies the 5-position of the ring, alkylation takes place on sulfiu. For the same reason, 2-acetonyi derivatives are sometimes observed as by-products in the heterocyclization reaction of dithiocarba-mates with a-haloketones (115, 116). 

The condensation of a-haloketones with monosubstituted alkyl or aryl-selenoureas (25) leads to 2-alkylamino- (26. 27) or 2-arylaminOselana-zoles (28) while disubstituted selenoureas give 2-(dialkylamino) selenazoles (26. 27) (Table X-3a). 

The only method that yields the 2-unsubstituted thiazoie derivatives directly involves the condensation of a-haloketones with thioformamide. As in the case of previously reported a-haloaldehydes, yields are better when more reactive bromoketones are used instead of a-chloroketones. Cyclization can be achieved by adding ketones dissolved in dioxane in small quantities to the thioformamide formed in situ at below 40°C. The temperature is kept below 70°C during the addition, and then the... 

By condensing a-haloketones with diacetylaminothioacetamide (13), Pyl et al. (533) obtained the corresponding 2-aminomethylthiazoles (15) after hydrolysis (Scheme 8). 

These products are used as starting material for the preparation of 2-substituted thiazol-4-ylacetic acids. a-Benzoyloxythiopropionamide and a-benzoyloxy-a-benzoylthioacetamide condensed with an equimolar amount of an a-haloketone in alcoholic solution yield the following compounds (409, 419, 569) 24, Rj = CH3, PhCO, R2 = Me... 

The mechanism of the Hantzsch s synthesis was studied at a very early stage by several authors. The intermediates were generally assumed to be open-chain a-thioketones, but in a series of papers by Murav eva and Schukina (470, 490) the isolation of hydroxythiazolines from the reaction between a-haloketones and a variety of thioureas was reported. 

The reaction of N-methyl-(p-dimethylamino)thiobenzamide (99) with a number of a-haloketones and a-bromoheptaldehyde gave stable 4-hydroxythiazolinium salts (100), which could be subsequently dehydrated by methanolic hydrogen chloride to the thiazolium salts (101), (Scheme 44) (622). 

In a similar way, the a-haloketones gives the corresponding 2-thiomethyl-3-ethylthia2olium salts (722). 

Of all the methods described for the synthesis of thiazole compounds, the most efficient involves the condensation of equimolar parts of thiourea (103) and a-haloketones or aldehydes to yield the corresponding 2-aminothiazoles (104a) or their 2-imino-A-4-thiazoline tautomers (104b) with no by-products (Method A, Scheme 46). 

The 2-aminothiazoles are extracted with ether after alkalinization of the reaction mixture. The yields are almost theoretical with a-haloketones and lower with a-haloaldehydes. 

In many cases, the a-haloketone does not appear to be an intermediate in this reaction, since reagents such as sulfur trioxide, sulfuric, or 60% nitric add lead to 2-aminothiazole but with lower yields (11 to 43%). Formamidine disulfide [-S-C(=NH)NH2]2, a product of the oxidation of thiourea, seems to be the intermediate in this reaction, since upon treatment with ketones, it gives 2-aminothiazole (604). However, the true mechanism of this reaction has not yet been completely elucidated. 

Aminolhtazoles were synthetized from thiourea by three methods Method A, from a-haloketones or aldehydes designated as (Cl) or (Br) Method B, from ketones and iodine (fj) or bromine (Btj) Method C, from iodomercuriketones. Method D consists in condensing ketones with cyanamid and sulfur. 

Compound 132 condensed with 1 or 2 moles of aliphatic or aromatic a-haloketones in acetonic or alcoholic solution yielded either the corresponding 2-thiazolythiourea (133) (559, 753, 797) or sym-substituted bis(2-thiazolyl)amine (134) (Scheme 64 and Table 11-19) (430, 553, 653). 

Hantzsch and Feist Syntheses. The Hant2sch synthesis of pyrroles iavolves condensation of an a-haloketone (10) with a p-keto ester (6) ia the presence of ammonia or an amine (22). 

FAWORSKI WALLACH Rearrangement Reanangement of a-haloketones or a,a -dihalokeiones to acids or acrylic adds (via cyclopropanones). 

MATTOX KENDALL Dehydrohaiogenation Dehydrohaiogenatton of a haloketones with 2,4 dinitrophenylhydrazine or LiCI DMF... 

Reaction ol a haloketones with tnalkylphosphite to give ketophosphonate or vinylphosphate... 

Conversion of a-haloketones to olefins using hydrazine (via enedlitnides C-C-N NH). Also reduction of o,3-epoxy ketones to allyl alcohols. 

It is believed that a reactive ground-state species, the zwitterion A, is an intermediate and that it rearranges to the observed product. To test this mechanism, generation of species A by nonphotochemical means was undertaken. a-Haloketones, when treated with strong base, ionize to such dipolar intermediates. Thus, the bromoketone 6 is a potential precursor of intermediate A ... 

The effectiveness of ylides in the field of polymer science was first described in 1966 by George et al. [11] who felt that 3- and 4-(bromo acetyl) pyridines, which contain both the a-haloketone and the pyridine nucleus in a single molecule, could be quaternized to polymeric quaternary salts and finally to polymeric ylides Schemes 9 and 10 by treating these polymeric salts with a base. 

Reaction of 621 with a-haloketones gave [77IJC(B)46] [3,2-b] isomers... 

In connection with route A, the formation of sulfones from sulfinates and a-haloketones on the one hand, and of isomeric enol sulfonates on the other (cf. Section III. A.4), should be pointed out. 

The most plausible mechanism for the interconversion of la and Ih is shown in Scheme 2. Similar mechanism has been put forward for epimerization of a-substituted ketones under basic conditions and for the equilibration via an enolate prior to nucleophilic substitution was observed by Numazawa et al. (ref. 13). The same mechanism seems to operate in the reduction of some steroid a-haloketones (ref. 14) or tra/ty-3-chloroflavanone (ref. 15) with sodium borohydride where an inversion of configuration takes place at the a carbon parallel to the reduction of the... 

Disconnection of the central bond is unhelpful neither specific enolate (34) nor a-haloketone (35) will be easy to make. 

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