Ring cleavage

Ring Oeavage.— The ring cleavage of thiazoles in alkaline media, studied with model thiazolium ions by a combination of kinetic and spectral methods, appears to be initiated by the following stages  

The thiazole nucleus can also be cleaved by photosensitized oxidation 2,4,5-triphenylthiazole consumes 0.45 mole of oxygen in methanol in the presence of a sensitizer, to form benzil and benzamide (11 and 18%), presumably via the cyclic peroxide (92) and the Schiff s base (93). In 

Because of the easy and versatile synthesis of thiazoles (cf. Chapter II), this reaction could have interesting synthetic applications (481). 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10 mole sec ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e 

In a study aimed at identifying thiazole derivatives, Travagli (485) obtained the ring cleavage of 2-aminothiazoIes (205) by their phenyl-hydrazinolysis in acetic acid. 

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The high regioselectivity ( stereoelectronic control ) in the ring cleavage by chlorination of sulfur was anticipated. It had been found before that in corresponding bicyclic systems such as in the scheme below oxidation of the sulfur atom always led to the undesired cleavage of the S—Cg bond. This was rationalized through the observation on molecular models that... 

In the presence of [PtC ] and a base. 2-aminothiazole undergoes ring cleavage of the C-S bond to give PtLCF (L=HSCH=CHNHCN) (699). The Pd and Pt complexes of 2-aminothiazoles show biological activity (1596i. 

Thiazole is relatively resistant to oxidation, and very few reactions of this type have been recorded. Under photosensitized oxygenation, triphenylthiazole affords various products of ring cleavage depending on... 

Schatzmann, in 1891, tried to prepare 2-thiazolines by hydrogenation of thiazoles and by the action of sodium and ethanol on 2,4-dimethyl-thiazole, 2-methylthiazole, and 2-methyl-4-phenylthiazole (476). None of these substrates was reduced to thiazoline the second gave no reaction and the first underwent ring cleavage, leading to a mixture of n-propylmercaptan and ethylamine (Scheme 90). Three years later the same... 

Active Raney nickel induces desulfurization of many sulfur-containing heterocycles thiazoles are fairly labile toward this ring cleavage agent. The reaction occurs apparently by two competing mechanisms (481) in the first, favored by alkaline conditions, ring fission occurs before desul-, furization, whereas in the second, favored by the use of neutral catalyst, the initial desulfurization is followed by fission of a C-N bond and formation of carbonyl derivatives by hydrolysis (Scheme 95). 

The hydroisoquinolines are more susceptible to ring cleavage than the isoquinolines. Ring cleavage occurs with nitrogen elimination when... 

Unlike the 2-aLkyl-2-imida2olines, this stmcture is stable and resistant to hydrolysis. After ring cleavage, reaction with sodium chloroacetate yields linear products ... 

Dimethylisoxazol-5-amine is easily acylated to its formyl derivative (697) which, on catalytic hydrogenation, undergoes ring cleavage and recyclization to yield 5,6-dimethyl-pyrimidin-4(3H)-one (698) other acyl derivatives give analogous 2-substituted pyrimidines... 

AP298). A similar result may be achieved by ring cleavage followed by acylation and final reclosure 54JCS665). 

There is also the possibility of removing the 2-oxo group by ring cleavage and subsequent recyclization. Lumazine can be hydrolyzed by strong alkali to 2-aminopyrazine-3-carboxylic acid (153) which is converted first into the amide (154) and then cyclized by ethyl orthoformate into pteridine-4-one (155 equation 47) (51JCS474). 

Having its pyrazolic 4-position substituted, electrophilic attack on indazoles takes place in the 3-position and in the homocycle (the 5- and 7-positions). The condensation of a benzene ring results in a decrease of the aromaticity of the pyrazole moiety, as in naphthalene compared to benzene, and therefore basic ring cleavage is easier in indazoles than in pyrazoles (Section 4.04.2.1.7(v)). 

Irradiation of 4-hydroxy- and 4-alkoxy-3-pyrazolin-5-one derivatives (163 R = OH, OR) leads to ring cleavage with the formation of /3-diamides (165) 69TL271). The methylene blue sensitized rearrangement of the same pyrazolinone (R = H) to the oxindole (166) also... 

The cyclohexylpyrazole (376) and the azlrlne (377) are formed by irradiation of 3-dlazo-4-methyl-5-phenylpyrazolenine (378) in cyclohexane (Scheme 35) (77JA633). The former is the result of carbene insertion into cyclohexane followed by a [1,5] hydrogen shift, whereas the latter arises by ring cleavage of nltrene (379) or by a concerted pathway. 

Irradiation of 3,5-disubstituted isoxazoles in alcoholic solvents gave reaction products such as acetals incorporating the reaction solvent. The use of triethylamine in acetonitrile media produced ketene-aminals by reductive ring cleavage. The reductive ring cleavage product was also obtained by irradiation of the isoxazole in alcohol in the presence of copper(II) salts (Scheme 3) (76JCS(P1)783). 

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