2.2 叔丁氧羰基（Boc）
除Cbz保护基外，叔丁氧羰基（Boc）也是目前多肽合成中广为采用的氨基保护基，特别是在固相合成中，氨基的保护用Boc而多不用Cbz。Boc具有以下的于的优点：Boc-氨基酸除个别外都能得到结晶；易于酸解除去，但有具有一定的稳定性，Boc-氨基酸能较长期的保存而不分解；酸解时产生的是叔丁基阳离子再分解为异丁烯，它一般不会带来副反应；对碱水解、肼解和许多亲核试剂稳定；Boc对催化氢解稳定，但比Cbz对酸要敏感得多。当Boc和Cbz同时存在时，可以用催化氢解脱去Cbz，Boc保持不变，或用酸解脱去Boc而Cbz不受影响，因而两者能很好地搭配。 2.1.1叔丁氧羰基的导入
游离氨基在用NaOH 或NaHCO3 控制的碱性条件下用二氧六环和水的混合溶剂中很容易同Boc2O反应得到N-叔丁氧羰基氨基化合物[1]。这是引入Boc常用方法之一，它的优点是其副产物无多大干扰并容易除去。有时对一些亲核性较大的胺，一般可在甲醇中和Boc酸酐直接反应即可，无须其他的碱，其处理也方便。
对水较为敏感的氨基衍生物，采用Boc2O/TEA/MeOH or DMF 在40-50℃下进行较好，因为这些无水条件下用于保护O17标记的氨基酸而不会由于与水交换使O17丢失[2]。有空间位阻的氨基酸而言，用Boc2O/Me4NOH.5H2O/CH3CN是十分有利的。
1. D. S. Tarbell, Y. Yamamoto et al., Proc. Natl. Acad. Sci., USA, 1972, 69, 730 2. E. Ponnusamy, U. Fotadar et al., Synthesis., 1986, 48
芳香胺由于其亲核性较弱，一般反应需要加入催化剂，另外对于伯胺，通过DMAP的使用可以上两个Boc.
Boc2O, DMAPBocArNBoc
ArNH2ArNHBoc对于有酚羟基存在的胺，酚羟基上接Boc的速度也是相当快的，因而一般没太大的选择性。 对于有醇羟基存在的，若用DMAP做 催化剂，时间长了以后醇羟基也能上Boc， 因此反应尽量不要过夜。
由于氰酸酯的生成，有位阻的胺往往会与Boc2O生成脲[1]。这个问题可通过该胺NaH或NaHMDS反应，然后再与Boc2O反应来加以避免[2]。
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NH2BrBrBoc2ODMAP, THFHNO17%HNBr+BrHNO56%O
1. H. J. knolker, T. Braxmeier et al., Angew. Chem., Int. Ed. Engl., 1995, 34, 2497; H. J.
knolker, T. Braxmeier et al., Synlett., 1996, 502; Kessier,A.; Coleman, C. M., et al J. Org. Chem.,
2004, 69(23), 7836-7846
2. T. A. Kelly, D. W. McNeil., Tetrahedron Lett., 1994, 35, 9003
有时在反应中有可能多加了Boc酸酐，当分子中无游离酸碱时很难出去，若一定要除去，一般在体系中加入一些N,N-二甲基乙二胺或N,N-二甲基丙二胺，而后将上了Boc的N,N-二甲基乙二胺或N,N-二甲基丙二胺用稀酸除去。
RNH2Boc2OexcessRNHBoc+Boc2O (excess)H2NNBocHNN稀酸除去
由于Boc对酸敏感，因此在合成过程中用到酸洗或酸溶解等操作时，为了保险起见，尽量不用盐酸而用10%柠檬酸（0.5M）或在低温条件进行。
2.2.1.1 氨基酸Boc保护示例
NH2COOHBoc2ONaOH, t-BuOH, H2ONHBocCOOH Oskar Keller, Walter E. Keller, Gert van Look et al., Org. Syn., 63, 160 A 4-L, four-necked, round-bottomed flask, equipped with an efficient stirrer, a dropping funnel, reflux condenser, and thermometer is charged with a solution of 44 g (1.1 mol) of sodium hydroxide in 1.1 L of water. Stirring is initiated and 165.2 g (1 mol) of L-phenylalanine is added at ambient temperature, and then diluted with 750 mL of tert-butyl alcohol. To the well-stirred, clear solution is added dropwise within 1 hr, 223 g (1 mol) of di-tert-butyl dicarbonate. A white precipitate appears during addition of the di-tert-butyl dicarbonate. After a short induction period, the temperature rises to about 30–35°C. The reaction is brought to completion by further stirring overnight at room temperature. At this
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time, the clear solution will have reached a pH of 7.5–8.5. The reaction mixture is extracted two times with 250 mL of pentane, and the organic phase is extracted three times with 100 mL of saturated aqueous sodium bicarbonate solution. The combined aqueous layers are acidified to pH 1–1.5 by careful addition of a solution of 224 g (1.65 mol) of potassium hydrogen sulfate in 1.5 L of water. The acidification is accompanied by copious evolution of carbon dioxide. The turbid reaction mixture is then extracted with four 400-mL portions of ethyl ether. The combined organic layers are washed two times with 200 mL of water, dried over anhydrous sodium sulfate or magnesium sulfate, and filtered. The solvent is removed under reduced pressure using a rotary evaporator at a bath temperature not exceeding 30°C. The yellowish oil that remains is treated with 150 mL of hexane and allowed to stand overnight. Within 1 day the following portions of hexane are added with stirring to the partially crystallized product: 2 × 50 mL, 4 × 100 mL, and 1 × 200 mL. The solution is placed in a refrigerator overnight; the white precipitate is collected on a Büchner funnel and washed with cold pentane. The solid is dried under reduced pressure at ambient temperature to constant weight to give a first crop. The mother liquor is evaporated to dryness leaving a yellowish oil, which is treated in the same manner as described above, giving a second crop. The total yield of pure white N-tert-butoxycarbonyl-L-phenylalanine is 207–230 g (78–87%), mp 86–88°C, [α]D20 + 25.5° (ethanol c 1.0).
2.2.1.2 氨基酸酯Boc保护示例
NH2.HClHOCOOMeBoc2OHOEt3NNHBocCOOMe
Alessandro Dondoni, Daniela Perrone., Org. Syn., 77, 64
A 500-mL, three-necked, round-bottomed flask, is equipped with a magnetic stirring bar, thermometer, reflux condenser protected from moisture by a calcium chloride-filled drying tube, and a pressure-equalizing dropping funnel that is connected to a nitrogen flow line and is charged with a solution of 97% di-tert-butyl dicarbonate (14.3 g, 63.6 mmol) in tetrahydrofuran (100 mL), Methyl serinate hydrochloride (10.0 g, 64.3 mmol) is placed in the flask and suspended in tetrahydrofuran (200 mL) and 99% triethylamine (14.0 g, 138 mmol). The resulting white suspension is cooled with an ice-water bath and the solution of
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di-tert-butyl dicarbonate is added dropwise over a period of 1 hr. After 10 min of additional stirring, the ice-water bath is removed and the suspension is stirred overnight (14 hr) at room temperature, then warmed at 50°C for a further 3 hr. The solvent is removed under reduced pressure and the residue is partitioned between diethyl ether (200 mL) and saturated aqueous bicarbonate solution (250 mL). The aqueous phase is extracted with three 150-mL portions of diethyl ether. The combined organic phases are dried with anhydrous sodium sulfate and concentrated under reduced pressure to give 13.4-14.0 g (95-99% crude yield) of N-Boc-L-serine methyl ester as a colorless oil that is used without further purification. [α] D 23 17.0° (MeOH, c 4.41).
2.2.1.3 Boc酸酐在甲醇中与胺直接反应
COOEtBoc2ONH1MeOHNBoc2COOEt Boc2O (262 g, 1.2 mol) in MeOH (250 ml) was added to a soluton of compound 1 (157.2 g, 1.0 mol) in MeOH (350 ml) at 10°C, and the resulting mixture was stirred at room temperature for 2 h. N1, N1-dimethylethane-1,2-diamine (26 g, 0.3 mol) was added and the mixture was stirred at room temperature for 15 min. The solvent was removed in vacuo, and the residue was dissolved with ethyl acetate (750 ml). The combined organics were washed with 1 N HCl (2 x 250 ml) and brine (2 x 250 ml), dried over sodium sulfate and filtered. The solvent was removed to give compound 2 (250 g, 96%), which was used directly in the next step. 2.2.1.4 芳胺的单Boc保护示例
NH2NCOOHBoc2OEt3N, DMFNNHBocCOOH
Luo, Qun-Li; Liu, Zhi-Ying et al., J. Med. Chem., 2003, 46(13), 2631-2640
3-Aminopyridine-2-carboxylic acid (5.02 g, 36 mmol) was suspended in 60 mL of dry DMF, and Et3N (15.2 mL, 108 mmol) was added dropwise at room temperature. To the resulting brown solution was added Boc2O (11.80 g, 54 mmol). After being stirred for 10 min, the
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