Thiamin diphosphate ylide form

In the decarboxylase- and TK-catalyzed reactions the ylide attacks the keto function of a-keto acids to form an intermediate carboxylate ion. Decarboxylation, i.e. carbon-carbon bond fragmentation leads to the same reactive carbanion as in the BAL-catalyzed reaction (Scheme 5.10). Once again, it enantiose-lectively forms the new carbon-carbon bond of the desired a-hydroxyketone. TK requires not only thiamin diphosphate but also a second co-factor, Mg2+ [28]. 

Studies on thiamine (vitamin Bi) catalyzed formation of acyloins from aliphatic aldehydes and on thiamine or thiamine diphosphate catalyzed decarboxylation of pyruvate have established the mechanism for the catalytic activity of 1,3-thiazolium salts in carbonyl condensation reactions. In the presence of bases, quaternary thiazolium salts are transformed into the ylide structure (2), the ylide being able to exert a cat ytic effect resembling that of the cyanide ion in the benzoin condensation (Scheme 2). Like cyanide, the zwitterion (2), formed by the reaction of thiazolium salts with base, is nucleophilic and reacts at the carbonyl group of aldehy s. The resultant intermediate can undergo base-catalyzed proton... 

The reaction actually is two processes a-cleavage and a-condensation via a thiamin diphosphate adduct (Scheme 32). These involve generation of a second carb-anion after the ylide has added to the carbonyl of the substrate. The carbanion that is expected to be generated in the reactions is of some interest. Sable demonstrated that such a carbanion can form nonenzymically by showing that... 

Although thiamine, a thiazolium salt, contains a pyrimine ring, it is the thiazole ring that is responsible for its biological action, thiamine dihosphate being the coenzyme of decarboxylases. The mechanism of the catalytic decarboxylation (e.g. of pyruvic acid to acetaldehyde) was interpreted by Breslow in 1958. The active species is the N-ylide 12 formed from thiamine diphosphate and basic cell components ... 

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