Thiamine-Dependent Acyloin and Benzoin Reactions

Thiamine Diphosphate = Phosphate newly generated stereocenter 

Stereocontrol in mixed acyloin and benzoin reactions is high only if the carboligation encompasses at least one (large) aromatic aldehyde, whereas with two (small) aliphatic aldehydes only moderate e.e.s are generally obtained. 

Despite its important history, it was during the early 1990s, that the reaction pathway was elucidated in detail [1466] and it turned out that the enzyme responsible for this reaction is pyruvate decarboxylase (PDC) [1467]. The C2-unit (equivalent to acetaldehyde) originates from the decarboxylation of pyruvate and is 

In vivo, pyruvate decarboxylase [EC 4.1.1.1] catalyzes the nonoxidative decarboxylation of pyruvate to acetaldehyde and is thus a key enzyme in the fermentative production of ethanol. The most well-studied PE)Cs are obtained from baker s yeast [1477, 1482, 1483] and from Zymomonas mobilis [1484]. 

Transketolases from various sources have been shown to possess a broad acceptor spectrum yielding products with complete (5)-stereospecificity for the newly formed stereocenter [1505]. Generic aldehydes are usually converted with full stereocontrol and even a,(3-unsaturated aldehydes are accepted to some degree. However, hydroxylated aldehydes show enhanced rates by mimicking the natural substrate [1499]. Transketolases can be obtained from yeast [1506] and spinach [1507] and their overexpression has opened the way for large-scale production [1508, 1509]. 

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