Thiazolium dipolar ion

We see that the essence of the action of thiamin diphosphate as a coenzyme is to convert the substrate into a form in which electron flow can occur from the bond to be broken into the structure of the coenzyme. Because of this alteration in structure, a bond breaking reaction that would not otherwise have been possible occurs readily. To complete the catalytic cycle, the electron flow has to be reversed again. The thiamin-bound cleavage product (an enamine) from either of the adducts in Eq. 14-20 can be reconverted to the thiazolium dipolar ion and an aldehyde as shown in step b of Eq. 14-21 for decarboxylation of pyruvate to acetaldehyde. 

Thiazolium dipolar ion (ylid) 732, 733, 733s Thin layer chromatography 103... 

McNelis, 1959). This subject has been reviewed recently by Metzler (1960) and will not be discussed in detail here. In brief, Breslow postulated that thiamine pyropho.sphate ionizes at the 2-position of the thiazole ring, and that the thiazolium dipolar ion (IV) reacts with pyruvate to form an intermediate (2-laetylthiamine pyrophosphate) (V) which undergoes decarboxylation to produce 2-hydroxyethylthiamine pyrophosphate (VI, VII) (Fig. 2). Species (VI) is regarded as active acetaldehyde, and can... 

Active carbonyl compounds such as benzaldehyde attack the electron-rich double bond in DTDAFs to give a dipolar adduct, which immediately undergoes dissociation with formation of two molecules of 146 (64BSF2857 67LA155).Tlie existence of by-products such as benzoin led to the synthetic application of thiazolium salts in the acyloin condensation. For example, replacement of the classic cyanide ion by 3-benzyl-4-methyl-5(/3-hydroxyethyl) thiazolium salts allowed the benzoin-type condensation to take place in nonaqueous solvents (76AGE639) (Scheme 57). 

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