Laboratorium für Organische Chemie, Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich 8093, Switzerland.
J Am Chem Soc. 2021 Oct 27;143(42):17557-17565. doi: 10.1021/jacs.1c07354. Epub 2021 Oct 14.
Potassium acyltrifluoroborates (KATs) undergo chemoselective amide-forming ligations with hydroxylamines. Under aqueous, acidic conditions these ligations can proceed rapidly, with rate constants of ∼20 M s. The requirement for lower pH to obtain the fastest rates, however, limits their use with certain biomolecules and precludes in vivo applications. By mechanistic investigations into the KAT ligation, including kinetic studies, X-ray crystallography, and DFT calculations, we have identified a key role for a proton in accelerating the ligation. We applied this knowledge to the design and synthesis of 8-quinolyl acyltrifluoroborates, a new class of KATs that ligates with hydroxylamines at pH 7.4 with rate constants >4 M s. We trace the enhanced rate at physiological pH to unexpectedly high basicity of the 8-quinoline-KATs, which leads to their protonation even under neutral conditions. This proton assists the formation of the key tetrahedral intermediate and activates the leaving groups on the hydroxylamine toward a concerted 1,2-BF shift that leads to the amide product. We demonstrate that the fast ligations at pH 7.4 can be carried out with a protein substrate at micromolar concentrations.
钾酰三氟硼酸盐 (KAT) 与羟胺发生化学选择性酰胺形成连接。在水相、酸性条件下，这些连接可以迅速进行，速率常数约为 20 M s。然而，为了获得最快的速率而需要较低的 pH 值，限制了它们与某些生物分子的使用，并排除了体内应用。通过对 KAT 连接的机制研究，包括动力学研究、X 射线晶体学和 DFT 计算，我们确定了质子在加速连接中的关键作用。我们将这一知识应用于 8-喹啉酰三氟硼酸盐的设计和合成，这是一类新的 KAT，它们在 pH 7.4 下与羟胺连接，速率常数>4 M s。我们将生理 pH 下的增强速率追溯到 8-喹啉-KAT 出人意料的高碱性，这导致即使在中性条件下它们也会质子化。这个质子有助于形成关键的四面体中间体，并激活羟胺上的离去基团，进行协同的 1,2-BF 转移，从而得到酰胺产物。我们证明，在 pH 7.4 下快速连接可以在微摩尔浓度的蛋白质底物上进行。
