苄氧羰基(Cbz)保护氨基及脱保护方法

苄氧羰基（Cbz）是1932年Bergmann发现的一个很老的氨基保护基，但一直到今天还在应用。其优点在于：试剂的制备和保护基的导入都比较容易；N-苄氧羰基氨基酸和肽易于结晶而且比较稳定；苄氧羰基氨基酸在活化时不易消旋；能用多种温和的方法选择性地脱去。

苄氧羰基的导入，一般都是用Cbz-Cl。游离氨基在用NaOH 或NaHCO3 控制的碱性条件下可以很容易同Cbz-Cl反应得到N-苄氧羰基氨基化合物。α,β-二胺可用该试剂在pH= 3.5-4.5稍有选择性地被保护，其选择性随碳链地增长而减弱，如H2N(CH2)nNH2, n=2时71%被单保护;n=7时29%被单保护[G.J. Atwell, W. A. Denny., Synthesis, 1984, 1032]。氨基酸酯同Cbz-Cl的反应则是在有机溶剂中进行，并用碳酸氢盐或三乙胺来中和反应所产生的HCl。此外，Cbz-ONB（4-O2NC6H4OCOOBn）等苄氧羰基活化酯也可用来作为苄氧羰基的导入试剂，该试剂使伯胺比仲胺易被保护，但苯胺由于亲核性不足，与该试剂不反应[D.R. Kelly, M. Gingell,Chem. Ind.(London),1991, 888]。

Cbz-Cl很容易用苯甲醇同光气的反应来制备（见下式），在低温下可以保存半年以上而不发生显著的分解。

除Cbz-Leu为油状物外，绝大多数氨基酸的苄氧羰基衍生物都可以得到结晶。有的N-苄氧羰基氨基酸能同它的钠盐按一定比例形成共晶，共晶产物的熔点较高，并难溶于有机溶剂。例如，苯丙氨酸经苄氧羰基化后再加酸析出Cbz-Phe时往往得到共晶产物（熔点144℃），此共晶产物用乙酸乙酯和1M HCl一道震摇时可完全转化为Cbz-Phe而溶于乙酸乙酯中。因此。除Cbz-Gly以外，一般都是采用酸化后用有机溶剂提取的方法来得到纯的N-苄氧羰基氨基酸。

反应实例

Cbz-Cl (18.5 μl,0.155 mmol) in diethyl ether (0.2 ml) was dropped to a solution of (R)-1(36.4 mg, 0.129 mmol) in 10% aqueous Na2CO3(1.8 ml) at 0°C,and stirred for 5 h. The reaction mixture was acidified with 10% citric acid,extracted with CHCl3(10mlX3). The organic layer was washed with water, driedover Na2SO4, evaporated to give light yellow gels, which werepurified by preparative TLC (CHCl3/MeOH=5:1) to afford (R)-6 (25.7 mg, 47.1%) as yellowamorphous solid.

【Konda-Yamada,Yaeko; Okada, Chiharu et al., Tetrahedrom; 2002, 58(39),7851-7865】

A 3-L, three-necked, Morton flask equipped with an efficient mechanical stirrer,thermometer, and a dropping funnel is charged with L-methionine methyl ester hydrochloride 1 ( 117.6 g , 0.56 mol), potassium bicarbonate ( 282.3 g , 2.82 mol, 5 eq.), water (750 mL), and ether(750 mL), and the solution is cooled to 0°C .Benzyl chloroformate ( 105 g , 88.6 mL, 0.62 mol, 1.1 eq.) is added dropwise over 1 hr, thecooling bath is removed, and the solution is stirred for 5 hr. Glycine ( 8.5 g , 0.11 mol, 0.2 eq.) is added (to scavenge excess chloroformate) and the solution is stirred for an additional 18 hr. The organic layer is separated, and the aqueous layer is extracted with ether (2 × 200 mL). The combined organic layers are washed with 0.01 M hydrochloric acid (2 × 500 mL), water (2 × 500 mL), and saturated brine (500 mL), and then dried (Na2SO4), filtered, and evaporated on arotary evaporator. The resulting oil is further dried in a Kugelrohr oven ( 50°C , 0.1 mm , 12 hr) to leave product 2 as a clear oil that solidifies upon cooling: 165– 166 g (98–99%), mp 42– 43°C .

【M. Carrasco, R. J. Jones, S. Kamel et a1., Org.Syn., 70, 29】

To a mixture oftoluene (3.85 L),water (3.85 L), andK2CO3(470 g, 3.40 mol) were successively added 1a (770 g, 2.72 mol) and CbzCl (488 g, 2.72 mol) with vigorous stirring at atemperature below 25 °C. After stirringat room temperature for 3 h, triethylamine (27.5 g, 270 mmol) and NaCl (578 g) were successively added, and the mixture wasstirred for a further 30 min. The organic layer was separated and concentratedto give the desired product as oil, which was used for the next reactionwithout purification. The analytical sample was prepared by columnchromatography。

【Inaba, Takashi; Yamada, Yasuki et al J. Org. Chem.,2000, 65(6), 1623-1628】

N-苄氧羰基（Cbz）的去保护

苄氧羰基的脱除主要有以下几种方法：1). 催化氢解；2). 酸解裂解；3). Na/NH3（液）还原。一般而言目前实验室常用简洁的方法就是催化氢解，但当分子中存在对催化氢解敏感或钝化的基团时，我们就必须采用化学方法如酸解裂解或Na/NH3（液）还原等。

催化氢解如下式所示。催化氢解的供氢体可以是H2、环己二烯、1，4-环己二烯、甲酸铵和甲酸等，以后面四个试剂为供氢体的反应又叫催化转氢反应，通常这比催化氢化反应更迅速。

催化剂主要用5-10%的钯-碳、10-20%的氢氧化钯-碳或钯-聚乙烯亚胺，钯-聚乙烯亚胺/甲酸对于除去Cbz要比前两者要好。当HBr/HOAc脱去Cbz保护基时，产物往往带又一点颜色，而且分解产生的溴化苄会产生一些副反应并难以除尽，而催化氢解多数能得到无色得产物。由于硫能使催化剂中毒，因此，含有胱氨酸、半胱氨酸等含硫的肽等N-苄氧羰基氨基衍生物一般不用催化氢解法脱除。一般溶剂可以用甲醇，乙醇，乙酸乙酯，四氢呋喃等，在醇类质子溶剂中反应速度要快的多。

如果在Boc2O存在下用Pd/C进行氢化，则释放出的胺直接转变成Boc衍生物[ M. Sakaitani, K. Hori, Y. Ohfune., TetrahedronLett., 1988, 29, 2983]。而且这类反应往往要比不加Boc2O来的快，其主要由于氢解出来的胺往往会与贵金属有一定的络合，使催化剂的活性降低，和Boc2O反应为酰胺后则去除了这一效果。另外有时在氢解时加入适当的酸促进反应也是一样的道理，避免了生成的胺降低反应的活性。

另外当分子中有卤原子(Cl, Br, I)存在时，一般直接用Pd/C会造成脱卤的发生，一般这种情况下，使用PdCl2为催化剂，以乙酸乙酯或二氯甲烷为溶剂可较好的避免脱卤的发生。

用MeOH/DMF为溶剂时，在Cbz-赖氨酸衍生物氢化的过程中会生成N-甲基化的赖氨酸。使用氨为溶剂时，H2/Pd-C在-33℃下氢化，肽中的半胱氨酸或蛋氨酸单元不使催化剂毒化，此外，氨还会阻止BnO醚的还原，所以对Cbz可得到一些选择性。

反应实例

A solution of (R)-8 (0.170 g, 0.52 mmol) in absolute methanol (3ml) was hydrogenated in the presence of 15% Pd/C (0.026 g)at room temperature for 12 h. The mixture was filtered (Celite) and washed with methanol. Then, perchloric acid (0.050ml, 0.83 mmol) was added and the mixture was stirred for 5 min. The solvent wasevaporated to afford (R)-7·HClO4, mp 233–235°C; [a]D23=−15.6 (c=0.68, methanol).

【C. Jaume; G. G.Santiago et al., Tetrahedron: Asymmetry, 2000, 11(22),4549-4458】

A solution of N-Cbz arylglycinol (17) (1.02 mmol) in MeOH (10 mL) wasstirred for 15 min in the presence of an excess of Pd(OH)2/C under a dihydrogenatmosphere. The solution was then filtered on a Celite pad and the solvent removedin vaccuo. Purification of the crude afforded the desired free 2-arylglycinols(S)-21 in 87% yield, white solid; [a]D20=+47.0 (c=0.78,CHCl3); mp 94-96°C(AcOEt)。

【B.Pierfrancesco; C. silvia et al., Tetrahedron, 1999, 55(10),3025】

576.6mg of compound 1 (1 mmol) was dissolved in 20 ml of methanol. Then 150 mg ofammonium formate (3 mmol) and 75 mg of 10% Pd-C was added and the reactionmixture was stirred at room temperature 10 min and then heated to reflux for 45min. The mixture was filtered through celite and the filtrate was evaporate todryness to give 430 mg of compound 2 (98%). This compound was used without further purification in the subsequentstep.

【Alargov, D. K; Naydenova, Z; Monatsh. Chem., 1997, 128(6-7),725-732】

Compound 1 (0.6 g, 0.8 mmol) was dissolved in 1:1 formicacid/methanol (60 mL) and added to a round-bottom flask (100 mL) containing 1equiv of palladium catalyst (10% Pd/C, 1.0 g, 0.9 mmol). The mixture was continuously stirred under refluxtemperature for 24 h. The catalyst was removed by filtration and washed with anadditional 10 mL of methanol. The combined solvents were removed by evaporationunder reduced pressure to give Compound 2 (0.34 g, 81%, a white solid, mp 96-98 °C). This compound was used withoutfurther purification in the subsequent step.

【Fyles, T. M.; Zeng, B.; J. Org. Chem., 1998, 63(23), 8337-8345】

10%Pd-C wasaddede to a solution of compound 1 (596 mg , 1.77 mmol) and (Boc)2O(773 mg, 3.54 mmol) in etnyl acetate (30 ml). The reation vessel was evacuatedand back-filled with nitrogen (three times), then back-filled with hydrogen (1atm). After 2 h, the mixture was filtered and concentrated. Purification bysilica gel chromatography (30% ethyl acetate/ hexanes - 50% ethyl acetate/hexanes) gave compound 2 (289 mg, 54%).

【WO2004092166】

To a solution ocompound 1 (900 mg) in methylene chloride (16.5 ml) was addede PdCl2 (30mg) and triethylamine (0.229 ml). Triethyl silane was added (2 x 0.395 ml) over2 h. The reaction mixture stirred 1 h and 2 ml of trifluoroacetic acid wasadded. After 30 min the reaction was basified with 2 N NaOH, extracted with methylene chloride, dried over MgSO4,filtered and concentrated. Chromatography was run on a biotage 40S column with3-5% MeOH/CH2Cl2 with 0.5% NH4OH to provide compound 2 as a oil (501mg, 74%).

【US20030144297】

酸解脱除

氨基甲酸苄酯在强酸性条件下容易去保护。HBr/HOAc 是酸解脱除苄氧羰基的最常用的试剂。脱除反应主要按下式进行。反应需要消耗2分子的HBr，Cbz的脱除速度随HBr浓度的增大而增大，因此实际上都是采用高浓度的过量HBr/HOAc溶液（1.2M-3.3M）以保证反应的完全。

含有丝氨酸和苏氨酸的肽或其它含羟基的氨基衍生物用HBr/HOAc脱除Cbz时会发生羟基的O-乙酰化反应。虽然O-乙酰基能用碱皂化或氨解脱去，但为了避免这个副反应，可以改用HBr/二氧六环或HBr/三氟乙酸来代替HBr/HOAc。由于HBr在三氟乙酸中的溶解度较小，因此不能预先制成HBr/三氟乙酸溶液，而只能将保护的肽或氨基衍生物溶于无水三氟乙酸中，先于0℃下通入干燥的HBr，待Cbz大部分脱除后，再室温通短时间以求完全脱除变化基。Cbz被HBr分解产生的溴化苄能同肽中的某种氨基酸反应，也是需要加以注意的。如，甲硫氨酸的硫原子能同溴化苄反应生成S-苄基甲硫氨酸，防止的办法是加入硫醚（CH3SC2H5）为捕捉剂。色氨酸被HBr/HOAc分解产生有色物质，防止的办法是加入亚磷酸二乙酯。硝基精氨酸会发生硝基的部分脱落，改用液体HBr于-67℃处理可以避免。

用液体HF在0℃处理10-30分钟即可将Cbz完全脱去。FSO3H、CH3SO3H、CF3SO3H和C6H5SCH3-TFA也是较好的试剂。Me3SiI在氯仿、乙腈中能于几分钟内选择性脱去Cbz和Boc保护基。对于BBr3/CH2Cl2而言，较大分子的肽的Cbz衍生物可在TFA中去除，因为肽在酸中的溶解度比在CH2Cl2中大。从肽中脱去Cbz，可在TFA中添加0.5 M 4-（甲硫基）苯酚或使用HF/Me2S/对甲苯酚（25:65:10,v/v）来抑制Bn+对芳香氨基酸的加成。

此外，已经报道过的还有以下的一些不常用的方法。如HCl/CHCl3、HCl/HOAc、HBr/SO2、液体HBr、TosOH、HI/HOAc、碘化磷、Et3SiH、沸腾的TFA、8M HCl的乙醇液或6 M HCl回流1小时或浓盐酸于25-75℃加热处理1-1.5小时等。

A solution of the amine Cbz compund (208 mg, 0.44mmol) in 33 % hydrobromic acid in acetic acid (1 mL) and glacial acetic acid(0.6 mL) was stirred at rt for 3 h under an atmosphere of nitrogen. Thevolatiles were removed in vacuo to leave the free amine hydrobromide (168mg, 91 %) as a brown, highly hygroscopic powder; [α]D=-18.0° (c = 0.4, EtOH)

【B.Anna; P. Gerald., Heterocycles, 2002, 58, 521】

Me3SiI (0.73 ml,0.73 mmol) was added to a soluton of compound 1 (146 mg, 0.33 mmol) inacetonitrile (10 ml) at room temperature, and the resulting mixture was stirredat room temperature for 2 h. Et3N (0.12 ml) was added and themixture was stirred at room temperature for 15 min. The solvents were removedin vacuo, and the residue was extracted with ethyl acetate. The combinedorganics were washed with sodium bicarbonate and brine, dried over sodiumsulfate and filtered. Solvents were removed and the residue was used directlyin the next step.

【US20040204397】

2024年，Vinayagam等人报道了一种利用AlCl3/HFIP(1,1,1,3,3,3-六氟异丙醇）温和脱Cbz的方法。该方法具有良好的官能团耐受性（包括其他可还原基团）、成本效益高、易于操作、安全性好、适合规模化生产、高收率以及可在常温下进行反应等优点。可以作为一种优秀的替代方案，避免了使用易燃的氢气和金属催化剂试剂，从而减少了严重的安全和环境问题。该方法最显著的特点是在存在O-和N-苄基保护基的情况下，能够选择性脱保护N-Cbz基团，从而扩展了设计合成路线的范围，适用于需要多种官能团转化的目标化合物。

【 J. Org. Chem., 2024, 89, 5665. DOI: 10.1021/acs.joc.4c00177 】

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