Thiazolium group

The first step of this reaction, decarboxylation of pyruvate and transfer of the acetyl group to lipoic acid, depends on accumulation of negative charge on the carbonyl carbon of pyruvate. This is facilitated by the quaternary nitrogen on the thiazolium group of thiamine pyrophosphate. As shown in (c), this cationic... 

Even more impressive is Diederich s pyruvate oxidase mimic based on a cyclophane (Fig. 4) in methanol this host can bind an aromatic aldehyde within its cavity, create a covalent intermediate by reaction with its attached thiazolium group, oxidise this tethered intermediate by intramolecular transfer of a hydride-equivalent to the appended flavin, and then release the methyl ester product by solvolysis. Catalytic... 

The amino group is expected to have decreased reactivity in thiazolium salts. 2-Amino-4,5-trimethylene thiazole (224) heated in diluted HQ at 80°C, however, gives the product 227a (463). The probable mechanism is shown in Scheme 139. This mechanism suggests a retro-Hantzsch ... 

In styryl compounds, a part of the methine chain is replaced by a phenyl group. Their name is based on the nuclei from which they are issued 3-ethyl-5-methyl-2-(p-dimethylaminostyryl)thiazolium iodide (Scheme 7). 

At first, the dimeric nature of the base isolated from 3-ethyl-2-methyl-4-phenylthiazolium was postulated via a chemical route. Indeed the adduct of ICH, on a similar 2-ethylidene base is a 2-isopropylthiazolium salt in the case of methylene base it is an anilinovinyl compound identified by its absorption spectrum and chemical reactivity (45-47). This dimeric structure of the molecule has been definitively established by its NMR spectrum. It is very similar to the base issued from 2.3-dimethyl-benzo thiazolium (48). It corresponds to 2-(3 -ethyl-4 -phenyl-2 -methylenethiazolinilydene)2-methyl-3-ethyl-4-phenylthiazoline (13). There is only one methyl signal (62 = 2.59), and two series of signals (63= 1.36-3.90, 63= 1.12-3.78) correspond to ethyl groups. Three protons attributed to positions T,5,5 are shifted to a lower field 5.93, 6.58, and 8.36 ppm. The bulk of the ten phenyl protons is at 7.3 ppm (Scheme 22). 

The monomethine cyanines with a methyl group on the chain (Table 2113) are prepared in a basic medium from a 2-alkyl-substituted thiazolium by condensation of an electrophilic reagent. 

Anilino vinyl derivatives of thiazolium (30, R = H) or acetanilido (30, R = C0CH3), as well as formyl methylene 30b (methods E-G), give asymmetrical dyes when condensed with a methyl reactive group of another species (Scheme 42). Mesosubstituted symmetrical or unsymmet-rical thiazolocyanines are obtainable via /S-alkylmercaptovinyl thiazolium derivatives (32) (methods H and I) (Scheme 43). a or /S carbon atoms of the trimethine chain can be substituted by acetyl when a dye is treated with acetic anhydride (method L). The hydrolysis of neocyanines lead to trimethine cyanine by fractional elimination of a composant chain (method K). 

Two different access routes are used, whether the leaving group is carried on thiazolium derivatives such as anilinovinyl (method A), acetanilidovinyl (method B), formyl methylene, or thioformylmethylene or on the ketomethylene compound (method C). The use of acid anhydride together with pyridine has been patented (method E). 

The reaction corresponds to a proton transfer and not to a net formation of ions, and thus the AS is of minor importance in the whole series, especially for the two t-Bu derivatives. This last effect is believed to be due to a structure-promoting effect of the bulky alkyl groups in the disordered region outside the primary hydration sphere of the thiazolium ion (322). 

For the methyl-substituted compounds (322) the increase in AG and AHf values relative to the unsubstituted thiazole is interpreted as being mainly due to polar effects. Electron-donating methyl groups are expected to stabilize the thiazolium ion, that is to decrease its acid strength. From Table 1-51 it may be seen that there is an increase in AG and AH by about 1 kcal mole for each methyl group. Similar effects have been observed for picolines and lutidines (325). 

Takamizawa et al. developed a general ring-expansion reaction of heterocycles that, applied to thiazolium salts, yields 1,4-thiazines (496, 497) thiamine (220) reacts with dialkyl acylphosphonates (221) to give the tricyclic 1,4-thiazine (222) (498), which is easily hydrolyzed to dihydro-1,4-thiazinone (223) (499) (Scheme 106). In the case of thiazolium slats containing no functional groups (224), 1,4-thiazine derivatives (226) were directly obtained in fairly good yields (Scheme 107). 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The C-coordinated thiazolium complexes are the result of the proton-induced cyclization reactions (980M513). Thus, complex 1 on protonation with tetrafiuoroboric acid yields the C-coordinated thiazolium structure 2. In turn, the nitrile complex 3 under these conditions is transformed to the thiazolium cationic species 4. Protonation of the amido complex 5 with tetrafiuoroboric acid also results in a cyclization but it proceeds differently. The amino group of the CONH2 moiety is lost and BF3-framework is coordinated via the carbonyl oxygen in an overall neutral complex 6. 

Cyanine Dyes Derived from Thiazolium Salts C. Nitrogen Atoms Replacing Methine Groups of the Chain... 

Electron-donating groups, effect on thiazolium ring cleavage, 33 Electrophilic reaction, of thiazolium salts, in basic medium, 34 Energetic transfer yield, of thiazoiocyanines, 78... 

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