芐基保護氨基

一、保護

芐基（Bn）是最穩定的氨基保護基之一，同PMB一樣，對大多數反應都是穩定的，但比PMB更加穩定，因而也更難脫除。酰胺的芐基，常規加氫方法不易脫除，可以通過Na/NH3脫除。

芐基（Bn）的引入

一般和PMB 一樣也采用C6H4CH2Br或C6H4CH2Cl和K2CO3、DIPEA、NaH、Et3N和n-BuLi在有機溶劑(如DMF、二氯甲烷和乙腈等)中反應來引入，或C6H4CHO/NaBH4、NaBH3CN或NaBH(OAc)3還原胺化引入。

烷基化引入芐基(Bn)示例

J. Cossy; I.Pevet et al., Eur. J. Org. Chem.,2001, 15, 2841

AnhydrousK2CO3(500 mg, 3.62 mmol, 2.0 equiv.), n-Bu4NI (66 mg, 0.18mmol, 0.1 equiv.), and benzyl bromide (0.24 mL, 1.98 mmol, 1.1 equiv.) wereadded successively to a solution of compound1(500 mg, 1.81mmol) in CH3CN (5 mL). After 2 h at 35 - 40 °Cand overnight at room temperature, the reaction mixture was diluted with ether,filtered, and concentrated under reduced pressure. The crude material waspurified by flash chromatography (cyclohexane - AcOEt, 95:5 to 90:10) to give compound2(0.54 g, 81%) as a colorless oil. [α]D20= +12.9 (c =1.99, CHCl3).

還原胺化引入芐基（Bn）示例

Rompaey, Karolien Van;Eynde, Isabelle Van den et al., Tetrahedron,2003, 59(24), 4421-4432

Compound1(0.5 g, 1.70 mmol) was dissolved in 1,2-dichloroethane(50 mL) and the aldehyde (0.122 g,1.70 mmol), Et3N (0.172 g,1.70 mmol), NaBH(OAc)3 (0.505 g,2.38 mmol) and MgSO4(30wt%) were added. The reaction was stirred at room temperature. RP-HPLCindicated reaction times between 2 and 4 h. The mixture was quenched with saturatedNaHCO3(50mL) and extracted with EtOAc (3 x 70 mL). The organiclayer was dried (MgSO4), filtered and evaporated to give compound2, which were used without furtherpurification.

二、去保護

Bn常用催化氫解脫去，如H2/20%Pd(OH)2-C 、H2/Pd-C 、H2/PdCl2 、Pd/HCOOH或Pd-C/HCOOH、Pd-C/HCOONH4、Pd-C/NH2NH2或Pd-C/環已烯作氫源轉移氫化，而用H2/Pd-C去保護通常很慢，除非添加Boc2O促進Bn的離去。另外CCl3CH2COCl/CH3CN、Li/NH3、Na/NH3、CAN和CH3CHClOCOCl也經常應用。酰氨上的芐基一般較難用氫解脫除，此時可以用AlCl3進行脫除。

一、H2/Pd-C氫化脫芐基(Bn)示例

Basso, Andrea; Banfi, Luca et al., J. Org. Chem.,2005, 70(2),575-579

A mixture of compound1(1.5 g, 2.4mmol) and 10% Pd/C (300 mg) in methanol (40 mL) was stirred overnight under H2(3bar). The reaction mixture was filtered over Celite, and the filtrate wasconcentrated in vacuo. Compound2(1.07 g) was obtained as a pale yellow oil in quantitativeyield.

二、HCOONH4/Pd-C氫化脫芐基(Bn)示例

Tilekar, Jayant N; Patil, Nitin T et al., Tetrahedron,2003, 59(11), 1873-1876

A solution of compound1(1.1 g, 2.75 mmol), ammoniumformate (0.92 g, 15.1mmol) and 10% Pd–C (0.2 g) inmethanol (10 mL) was refluxed for 40 min. The catalyst was filtered throughcelite and washed with methanol (5 mL x 2). To the filtrate,cooled to 0°C was added sodium bicarbonate (0.725 g, 8.61 mmol) and benzyloxycarbonylchloride (0.47 g, 2.70mmol) and the stirred reaction mixture warmed to room temperature. After 2 h,methanol was removed under reduced pressure and the residue was extracted withethyl acetate (5 mL x 3). Combined extract was washed with brine, dried overanhydrous sodium sulphate and concentrated on rotovapor to afford a residue whichwas purified by column chromatography (chloroform/methanol, 9/1) to givecompound2(0.81 g, 84%)as a thick liquid; Rf(20%methanol/chloroform) 0.6; [α]D =+18.02 (c = 0.20, CHCl3).

三、ClCOOCH2CCl3脫芐基(Bn)示例

V. H. Rawal, R. J.Jones et al., J. Org. Chem.,1987, 52, 19

To compound1(2.30 g, 6.8 mmol) inacetonitrile (25 mL) was added trichloroethyl chloroformate (0.100 mL, 6.8mmol). The mixture was stirred for 30 min and concentrated. The crude productwas chromatographed (hexanes:methylene chloride = 1:l) to yield compound 2 (2.68 g, 93%) as a white needles, whichcrystallized from absolute ethanol: mp 162.5 °C.

To a solution of compound 2(1.40 g, 3.3 mmol) in acetic acid (30 mL) wasadded powdered zinc (0.5 g) inportions over a period of 2 h. The reaction was filtered, and the precipitate waswashed thoroughly with methylene chloride. The filtrate was concentrated andextracted with ether. The organic layer was neutralized with saturated sodiumbicarbonate solution and dried (Na2SO4). The crudeproduct was concentrated and chromatographed (methylene chloride:ether = 10:1) to yield compound3(0.72 g, 88%) asa yellow oil, which gradually crystallized: mp 103-104°C.

四、 Na/NH3脫芐基(Bn)示例

Wang, Xiaodong J; Hart, Scott A et al., J. Org. Chem.,2003,68(6), 2343-2349

NH3(ca. 160 mL) was distilled into 40 mL of THF at -78 °C and allowed to warm to reflux (-33 °C). Na (ca. 2.0 g, 87mmol) was added until a deep blue solution was sustained. A solution of acid1(2.0 g, 4.3 mmol) in THF (10mL) was added directly to the Na/NH3 solution slowly via cannula overca. 5 min. After being stirred for 45 min at reflux, the reaction was quenchedwith NH4Cl (10 mL) and then allowed to warm to rt with concentration to ca. 30 mL(caution! NH3evolved). The mixture was diluted with NH4Cl (50 mL),acidified with 1 N HCl to pH 7, and extracted with CHCl3(10 x 50 mL), dried on MgSO4, and concentrated to give 810 mg(66%) of the alcohol2as a pale yellow oil (further purification can beachieved by chromatography on silica with 3% MeOH in CHCl3if desired).

4.4.2.5CAN脫芐基(Bn)示例

Bull, Steven D;Davies, Stephen G et al., J. Chem. Soc.Perkin Trans. 1,2001, 23,3106-3111

CAN (3.9 g, 7.1 mmol) was added portionwise to a stirredsolution of24(1.0 g,2.54 mmol) in MeCN-H2O (30 mL, 5:1) and stirred at RT. After sixteen hours,the reaction was quenched by the addition of saturated aqueous sodiumbicarbonate solution and stirred vigorously for ten minutes before extractionwith Et2O. The combined organic extracts were dried (MgSO4), filtered and concentrated in vacuo before purification by column chromatography on silica gel (hexane-Et2O = 5 : 1 and 1%Et3N) gave25(562 mg, 73%) as a colourless oil; [α]24D= - 58.6 ( c=1.05, CHCl3).

CAN選擇脫芐基(Bn)示例2

Bull, Steven D; Davies, Stephen G et al., J.Chem. Soc. Perkin Trans. 1,2000,22, 3765-3774

CAN (190 mg, 0.35 mmol) was addedportionwise to a stirred solution of compound1(90 mg, 0.17 mmol) inMeCN-H2O (5 : 1, 5 mL) and stirred at RT. The reaction was quenched by the additionof saturated aqueous sodium bicarbonate solution and stirred vigorously for tenminutes before extracting with Et2O. The combined organic extracts were dried (MgSO4), filtered and concentrated in vacuo efore purification by column chromatography on silica gel (gradientelution, 30-40 petrol-Et2O 7: 3 to 1 : 1) to give compound2(68 mg, 91%) asa gum. [α]D24= - 8.3 (c = 1.2, CHCl3).

六、CH3CHClOCOCl脫芐基(Bn)示例

US6410592：該方法也可用于脫甲基

To a solution of compound1(727 mg) in dichloromethane (75ml) which was being maintained at 0 °C under nitrogen was added 1-chloroethylchloroformate (0.208 ml) dropwise.The mixture was then allowed to warm to room temperature, before being heatedto reflux. After approximately 2 h analysis of the reaction mixture indicatedcomplete consumption of the starting material. The dichloromethane wasevaporated and the residue was then taken up into methyl alcohol and heated toreflux for 1 h. The solvent was evaporated to afford the compound2(481 mg, 85%), whichwas used in the next reaction with out further purification.

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