Mechanistic insight into benzenethiol catalyzed amide bond formations from thioesters and primary amines
作者:Nicolai Stuhr-Hansen、Nicolai Bork、Kristian Strømgaard
DOI:10.1039/c4ob00073k
日期:——
The influence of arylthiols on cysteine-free ligation, i.e. the reaction between an alkyl thioester and a primary amine forming an amide bond, was studied in a polar aprotic solvent. We reacted the ethylthioester of hippuric acid with cyclohexylamine in the absence or presence of various quantities of thiophenol (PhSH) in a slurry of disodium hydrogen phosphate in dry DMF. Quantitative conversions into the resulting amide were observed within a few hours in the presence of equimolar amounts of thiophenol. Ab initio calculations showed that the reaction mechanism in DMF is similar to the well-known aqueous reaction mechanism. The energy barrier of the catalyzed amidation reaction is approximately 40 kJ mol−1 lower than the non-catalyzed amidation reaction. At least partially this can be explained by a hydrogen bond from the amine to the π-electrons of the thiophenol, stabilizing the transition state in the aromatic thioester amidation reaction. Under similar conditions, cysteine-free ligation was achieved by coupling a fully side-chain protected 15 amino acid phosphopeptide thioester to the free N-terminal of a side-chain protected 9 amino acid peptide producing the corresponding 24 amino acid phosphopeptide.
在极性非质子溶剂中研究了芳硫醇对无半胱氨酸连接(即烷基硫酯和伯胺之间形成酰胺键的反应)的影响。我们在存在或不存在不同量的苯硫酚 (PhSH) 的情况下,在磷酸氢二钠在干燥 DMF 中的浆液中,使马尿酸乙硫酯与环己胺反应。在等摩尔量的苯硫酚存在下,在几个小时内观察到定量转化为所得酰胺。从头计算表明,DMF 中的反应机理与众所周知的水溶液反应机理相似。催化酰胺化反应的能垒比非催化酰胺化反应低约40 kJ mol−1。这至少部分可以通过从胺到苯硫酚的π电子的氢键来解释,从而稳定了芳香族硫酯酰胺化反应中的过渡态。在类似条件下,通过将完全侧链保护的 15 个氨基酸磷酸肽硫酯偶联到侧链保护的 9 个氨基酸肽的游离 N 端,产生相应的 24 个氨基酸磷酸肽,实现无半胱氨酸连接。