Coenzyme models. 33. Evidence for retro-acyloin condensation as catalyzed by thiazolium ion and cationic micelle. Oxidative trapping of the "active aldehyde" intermediates by flavin.

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N-Hexadecylthiazolium bromide(HxdT) in the CTAB micelle, which is known as an excellent catalytic system for acyloin condensation of aldehydes, catalyzes the reverse reaction (i.e., retro-acyloin condensation) to give aldehydes from α-ketols via the active aldehyde intermediates. The existence of the novel, HxdT-mediated process was proposed on the basis of an experimental discovery that flavin (3-methyltetra-O-acetylriboflavin: MeFl), which is capable of oxidatively trapping the active aldehyde intermediates, is reduced by α-ketols such as acetoin and 3-hydroxy-3-methyl-2-butanone in the micellized HxdT solution. It was further substantiated by detection of acetaldehyde in the final reaction mixture. Based on the disappearance rate of the absorbance of MeFl, we spectrophotometrically estimated the rate constants for the retro-acyloin condensation. Similarly, biacetyl, the monohydrated species of which is analogous to α-ketol, afforded acetaldehyde and acetic acid in the micellized HxdT solution, the rate constant being greater by factors of 102–103 than those for α-ketols. The relevance of the retro-acyloin condensation to biological systems (e.g., the mechanism of transketolase catalysis) is discussed.
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