【J. Org. Chem.】新型缩合剂，氨基酸不用保护直接缩合新方法，收率高达95%

传统多肽合成方法已沿用数十年，但需多步保护-脱保护操作、化学计量量的缩合试剂与添加剂，且消耗大量溶剂，存在原子经济性低、废弃物产生量大等缺陷。利用非保护氨基酸直接构建肽键是解决上述问题的优选方案，但该策略面临非保护氨基酸在有机溶剂中溶解度低、副反应与消旋化等难题。【】

2026年， Alex Boateng等人 开发了一种以廉价市售三甲氧基硅烷为介导的肽键直接合成方法。三甲氧基硅烷可增溶有机溶剂中的非保护氨基酸、瞬时保护氨基，并同步活化羧基用于缩合。该方法为一锅法，在 70℃下反应 20 小时，可由非保护氨基酸与氨基酸叔丁酯制备N端游离二肽，收率中等至优异（最高 95%）；反应立体选择性极高，非对映选择性 dr>20:1，底物适用范围广泛，且可用于含生物活性序列的三肽合成。【J. Org. Chem., 2026, 91, 2530-2537； doi.org/10.1021/acs.joc.5c02942 】

实验条件优化

以H-L-Phe-OH与H-L-Ala-Ot-Bu为模板，对反应条件进行筛选。

三甲氧基硅烷用量：2.0 当量收率 62%，3.0 当量收率提升至 68%，4.0 当量无进一步提升，确定3.0 当量为最优用量。

反应温度：50℃收率降至 54%，20℃无产物，80℃收率 68%，100℃收率 56%，确定70℃为最优温度。

催化剂验证：加入 CuCl、CuCl₂、B (C₆F₅)₃、Ta (OEt)₅、Ta (OMe)₅、Zn (OTf)₃等金属 / 路易斯酸催化剂，产物收率下降或无产物，证明该体系无需催化剂

溶剂筛选：单一溶剂 CPME、1,4 - 二氧六环、THF、CHCl₃收率均低于混合溶剂；CPME/1,4 - 二氧六环（1:1）分离收率 82%，CPME/MeCN（1:1）分离收率 75%，为最优混合溶剂。

最优条件确定：非保护氨基酸（1.0 当量）、氨基酸叔丁酯（2.0 当量）、三甲氧基硅烷（3.0 当量），CPME/1,4 - 二氧六环（1:1）为溶剂，70℃反应 20 小时。

底物适用范围1. 非保护氨基酸（亲电试剂）

以H-L-Ala-Ot-Bu 为亲核试剂，烷基侧链（丙氨酸、缬氨酸、亮氨酸等）、含硫醚（苄基半胱氨酸、甲硫氨酸）、醚（叔丁基丝氨酸）、酯（叔丁基谷氨酸）、酰胺、吲哚、芳环（取代苯丙氨酸、酪氨酸）、位阻型 / 易消旋苯甘氨酸等底物均适用；苄基半胱氨酸丙氨酸二肽克级合成（4.0 mmol）分离收率 92%，所有产物 dr>20:1。

2. 氨基酸叔丁酯（亲核试剂）

以H-L-Phe-OH、H-L-Cys (Bzl)-OH为亲电试剂，烷基、硫醚、醚、酯、杂环、胍基、胺基侧链，以及非天然氨基酸酯、β- 氨基酸酯、易二聚 / 易消旋酯均兼容，产物收率45%–95%，立体选择性保持 dr>20:1。

3. 三肽合成

采用两步一锅法：先按最优条件合成二肽，室温下直接加入Fmoc 保护氨基酸酰氯，无需添加剂继续反应 20 小时，成功合成含生物活性序列的三肽（如 Ala-Phe-Ala、铁氧还蛋白序列、黄嘌呤氧化酶抑制剂序列、亮氨酸脑啡肽序列），收率良好且 dr>20:1。

反应机理

三甲氧基硅烷在加热条件下双重作用：

1、与非保护氨基酸的氨基形成瞬时保护结构，避免氨基副反应；

2、活化羧基，与氨基酸形成稳定五元环中间体，实现选择性缩合；

3、无需额外催化剂，加热即可完成活化与脱保护，全程一锅完成。

实验操作

General Procedure: Trimethoxysilane-Mediated Peptide Bond Formation

Unprotected amino acid 1 (0.5 mmol, 1 equiv), dry CPME (0.75mL), dry 1,4-dioxane (0.75 mL), and trimethoxysilane (0.191 mL, 1.5mmol, 3 equiv) were charged into a flame-dried 5 mL screw-cap vial equipped with a magnetic stirring bar inside a glovebox. Subsequently,amino acid tert-butyl ester 2 (1.0 mmol, 2 equiv) was added to the mixture via a syringe under glovebox conditions. The reaction vial was then sealed under argon and removed from the glovebox. The mixture was stirred at 70 °C in a preheated oil bath for 20 h. After completion, the reaction was diluted with chloroform (1.5 mL), and approximately 0.3 g of silica gel (SiO2) was added. The mixture was stirred vigorously at room temperature (rt) for 5 min, then filtered through a G4 sintered funnel containing a Celite pad. The Celite pad was washed thoroughly with chloroform (150 mL). Volatiles were removed under reduced pressure, and the crude reaction mixture was transferred onto a silica gel column using a pipet. Purification was carried out using methanol in chloroform (0.2−1% MeOH in CHCl3) by flash column chromatography to afford the desired product 3.

General Procedure: Tripeptide Synthesis

Unprotected amino acid 1 (0.5 mmol, 1 equiv), dry CPME (0.75mL), dry 1,4-dioxane (0.75 mL), and trimethoxysilane (0.191 mL, 1.5mmol, 3 equiv) were charged into a flame-dried 5 mL screw-cap vial equipped with a magnetic stirring bar inside a glovebox. Subsequently, amino acid tert-butyl ester 2 (1.0 mmol, 2 equiv) was added to the mixture via a syringe under glovebox conditions. The reaction vial was then sealed under argon and removed from the glovebox. The mixture was stirred at 70 °C in a preheated oil bath for 20 h. The reaction vial was removed from the oil bath and allowed to cool to ambient temperature. Fmoc-protected amino acid halide (Fmoc-AA3-Cl 4, 0.75 mmol, 1.5 equiv) (prepared from its respective Fmoc-AA3-OH according to the literature12) was then added, and the reaction was stirred at rt for another 20 h. After completion, the reaction was diluted with chloroform (1.5 mL), and approximately 0.3 g of silica gel (SiO2) was added. The mixture was stirred vigorously at room temperature for 5 min, then filtered through a G4 sintered funnel containing a Celite pad. The Celite pad was washed thoroughly with chloroform (150 mL). Volatiles were removed under reduced pressure, and the crude reaction mixture was transferred onto a silica gel column using a pipet. Purification was carried out using AcOEt in hexane (20−80% AcOEt in hexane) by flash column chromatography to afford the desired product 5.

本文建立了一种无需金属催化剂、由非保护氨基酸与氨基酸叔丁酯合成N 端游离多肽的方法，核心试剂为廉价、市售易得的三甲氧基硅烷。该策略适用于含烷基、硫醚、醚、酚、酯、胺、酰胺、胍基、杂环等多样侧链的氨基酸，包括 β- 氨基酸、易消旋、易自二聚氨基酸，对应二肽收率45%–95%；通过两步一锅法可高效合成含生物活性基序的三肽，产物光学纯度优异（dr>20:1）。

参考资料：Trimethoxysilane-Mediated Peptide Bond Formation from Unprotected Amino Acids and Amino Acid t‑Butyl Esters； Alex Boateng,* Isai Ramakrishna, Tomohiro Hattori,* and Hisashi Yamamoto*；J. Org. Chem., 2026, 91, 2530-2537； doi.org/10.1021/acs.joc.5c02942