Reactivity of Intermediates in Benzoylformate Decarboxylase: Avoiding the Path to Destruction
作者:Qingyan Hu、Ronald Kluger
DOI:10.1021/ja027976r
日期:2002.12.1
environment that facilitates the unimolecular decarboxylation process. However, the resulting nonenzymic carbanion reacts very rapidly to give products that lead to the irreversible destruction of the cofactor. This contrasts with the normal process on the enzyme. Brønsted acids on the enzyme may divert the reaction to the benzaldehyde precursor, or the enzyme may block access to the pathway that leads
The Need for an Alternative to Radicals as the Cause of Fragmentation of a Thiamin-Derived Breslow Intermediate
作者:Michael Bielecki、Ronald Kluger
DOI:10.1002/anie.201702240
日期:2017.5.22
(1) is a conjugate of benzoylformate and thiamin that loses CO2 to form the classic Breslow intermediate (2), whose expected fate is formation of the thiamin conjugate of benzaldehyde (3). Surprisingly, it was observed that 2 decomposes to 4 and 5 and rearranges to 6 in competition with the expected protonation to give 3. Recent reports propose that the alternatives to protonation arise from homolysis
Substituent Effects in Carbon−Nitrogen Cleavage of Thiamin Derivatives. Fragmentation Pathways and Enzymic Avoidance of Cofactor Destruction
作者:Ian F. Moore、Ronald Kluger
DOI:10.1021/ja0173627
日期:2002.2.1
respectively, leading to the conclusion that C-N cleavage is insensitive to substituenteffects. On the basis of these results, we conclude that cleavage occurs by a facile process that resembles the outcome of a [1,5] sigmatropic rearrangement. An enzyme may avoid the fragmentation by holding the intermediate in a conformation that prevents such a process, allowing the normal catalytic process to proceed.
Decomposition of 2-(1-Hydroxybenzyl)thiamin. Ruling Out Stepwise Cationic Fragmentation
作者:Ian F. Moore、Ronald Kluger
DOI:10.1021/ol005927i
日期:2000.7.1
[reaction: see text] The rapid fragmentation of 2-(1-hydroxybenzyl)thiamin (1) is initiated by transfer of a proton from C2alpha to give an enamine. The subsequent irreversible process can be written as a concerted (or stepwise) rearrangement involving migration of the hydroxyl hydrogen to the methylene bridge. An attractive alternative is internal addition of C2alpha to the pyrimidine, generating
Fast fragmentation and slow protonation: a buffer-dependent isotope effect in reactions ofN-methyl hydroxy(benzylthiamine) analyzed by the Keeffe–Jencks equations
作者:Glenn Ikeda、Ronald Kluger
DOI:10.1002/poc.764
日期:2004.6
the subsequent fragmentation step (cleaving the pyrimidine–thiazolium bridge derived from thiamine) at higher buffer concentrations. Comparison of the observed rates of reaction of protio and C2α-deutero MHBnT reveals a non-linear variation of the kinetic isotopeeffect that fits precisely to a ratio derived from the Keeffe–Jencks rate law formulation for E1CB reactions. The fragmentation step is clearly