三氟乙酰基（Tfa）是Weygand最先引入到多肽合成中的。三氟乙酰基（Tfa）可用三氟醋酐导入，在稀碱液中很容易脱去。Tfa保护的氨基酸或多肽在高真空下易于气化，因而能用于气相层析以检测消旋的程度和测定天然肽的排列顺序，而且由于含有F，也可用19F NMR来检测合成肽的纯度、消旋程度以及类似物的鉴定等。由于N-Tfa-氨基酸在接肽时易于消旋，也是采用此保护基时应该注意的地方。
三氟乙酰基的引入
由于三氟醋酐同氨基酸反应时易生成恶唑烷酮而发生消旋，因此，同甲酰基的引入一样，在低温下于三氟醋酸溶液中用三氟醋酐酰化为好。一般而言，CF3COOEt/Et3N/MeOH是较好的方法，可在仲胺存在下，选择性地保护伯胺。并且该方法地聚合物方法也已得到发展。在TFAA/18-crown-6/Et3N中，伯胺与18-crown-6形成络合物，可选择性地酰化仲胺。而在仲胺存在下，CF3COO-邻苯二甲酰亚胺也可选择性地将TFA基团引入到伯胺。
一、TFAA引入三氟乙酰基示例
Chambers, JamesJ; Parrish, Jeasen C et al., J. Med.Chem., 2003, 46(16),3526-3535
To a stirred suspension of thehydrobromide salt of compound1 (1.3 g,3.6 mmol) and 4-N,N- (dimethylamino) pyridine (0.04 g, 0.3 mmol) in CH2Cl2(40 mL) was added Et3N (16.0 mL, 12.0 mmol), and themixture was cooled to 0 °C.Trifluoroacetic anhydride (2.5 mL, 17 mmol) was then added to the reaction dropwise. Themixture was allowed to warm to room temperature and stirred for 8 h. The mixturewas then diluted with CH2Cl2(50 mL) andwashed with 2 N HCl (50 mL), saturated NaHCO3(50 mL), and brine (50 mL). The organic phase was thendried (MgSO4), filtered, and evaporated to leave compound2as a white solid that was recrystallized fromEt2O (1.2 g, 92%):mp 197-198 °C.
二、 三氟乙酸乙酯引入三氟乙酰基示例
Knoops, Niele; Derioover, Geert ret al., Tetrahedron, 1997, 53(37), 12699-12716
Ethyl trifluoroacetate(9.2 mmol) was dissolved in 10 ml dry diethylether and stirred at 0 °Cfor 10 minutes. Compound1 (8.8mmol) was added and the reaction mixture was stirred at the same temperaturefor one hour. After removal of the solvent, the residues were purified by fastcolumn chromatography (SiO2; CHCl3) to give compound 2(yield98 %) as a yellow crystalline m.p.: 68-69 °C(CH2Cl2/hexanes).
三、三氟乙酸乙酯选择性保护伯胺示例
Whitlock, GavinA; Carreira, Erick M et al., Helv. Chim.Acta., 2000, 83(8), 2007-2022
Compound 1 (0.39 mmol) was dissolved in THF (10 mL), cooled to 0°C, and CF3COOEt (0.01 mL, 0.39 mmol) wasadded. The mixture was stirred at 0°Cfor 1 h and then at 23°Cfor 1 h. The solvent was then evaporated under reduced pressure to give a paleyellow oil. Purification by FC (silica gel; MeOH) afforded 2(119 mg, 85%) as colorless oil. TLC(MeOH): Rf = 0.29. [α]24D=+4.8 (c =0.48, CHCl3).
三氟乙酰基的脱去
三氟乙酰胺也是较易去保护地酰胺之一。Tfa基可以在水或乙醇水溶液中用0.1-0.2 N NaOH处理或者用1 M哌啶溶液处理很容易地脱去。由于脱去地条件温和，也适用于一些长链肽中的Tfa基的脱去，例如，Anfisen用上述条件于8 M尿素中5℃处理8小时脱去42肽中的Lys侧链的Tfa基，不过考虑到溶解度以及断链副反应等不利因素，长链肽的碱水解脱除保护基时要综合考虑各种因素。在K2CO3或Na2CO3/MeOH/H2O条件下，Tfa可在甲基酯存在下于室温去保护。也可在NH3/MeOH，HCl/MeOH或通过相转移水解(KOH/Et3Bn+Br-/H2O/CH2Cl2或乙醚)脱去。
一、KOH脱去三氟乙酰基示例
Chambers, James J; Parrish, Jason C etal., J. Med. Chem., 2003, 46(16), 3526-3535
A solution of compound 1 (1.7 g, 5.4 mmol) in MeOH (250 mL) was cooledto 0°C, and then 5 N KOH solution(30 mL) was added slowly. The reaction mixture was allowed to warm to roomtemperature and stirred overnight, and then the MeOH was removed by rotaryevaporation. The residue was diluted with H2O (25 mL) and extracted with Et2O (4 x 100 mL),dried (Na2SO4), filtered, and evaporated to afford clear oil. This oil was dissolved inEt2O (100 mL), filtered through a plug of glass wool, and neutralized by theslow addition of oxalic acid (54 mL, 0.1 Min MeOH). The solvents were removed, and the resulting white residue wasrecrystallized from MeOH to afford compound 2(0.9 g, 59%) asthe hemioxalate salt, m.p 243 °C.
二、 K2CO3脱去三氟乙酰基示例
Whitlock, GavinA; Carreira, Erick M et al., Helv. Chim.Acta., 2000, 83(8), 2007-2022
Compound 1(45 mg, 0.18 mmol) wasdissolved in 5% K2CO3 in MeOH/H2O (15 mL), and the soln. was stirred at 23°C for 4 h. H2O (3 mL) wasadded, the soln. was saturated with NaCl, and then extracted with CH2Cl2(5×15 mL). The combined org. extracts were dried(Na2SO4) and concentrated under reduced pressure toafford compound 2 (23.5 mg, 85%).
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