Fmoc-OSu

Fmoc-OSu (Su = succinimidyl), H2O, CH3CN. The advantage of Fmoc-OSu is that little or no oligopeptides are formed when amino acid derivatives are prepared. 

H2, Pd black, EtOH, 45°, 92% yield.If the hydrogenolysis is performed in the presence of (B0C)20 or Fmoc-OSu, the released amine is converted to the BOC and Fmoc derivatives in situ ... 

The Na-Z- or 7V"-Boc-protected lipo-amino acids and their esters are obtained by standard protocols.113X In contrast, for the synthesis of the A -Fmoc derivatives intermediate bis-silylation and the use of Fmoc-Cl as acylating agent are recommended. 129 The standard procedure with Fmoc-OSu leads to low yields (25%) mainly due to low solubility of the amino acids. 133 Alternatively, the improved solubility of the 4-toluenesulfonate salt of the benzyl esters is exploited for reaction with Fmoc-OSu. The benzyl ester is then removed by catalytic hydrogenation, although the Fmoc group is known to be reduced under these conditions unless particular precautions are taken. 133 156-159 ... 

The related Fmoc derivative was obtained by cleavage of the Boc group with 50% TFA in CH2C12 and reaction with Fmoc-OSu in aq Na2C03/THF under standard conditions yield 1.9 g (100%). 

A-(tert-Butoxycarbonyl)azetidine-2-carboxylic acid is obtained by reaction of azetidine-2-carboxylic acid with Boc-N3, m 112 Boc20, 113-"4 or 2-(fert-butoxycarbonyloxyimino)-2-phe-nylacetonitrile. 115 In a similar manner, the Fmoc derivative is prepared by the standard procedure using Fmoc-OSu as acylating agent.1"6 For derivatives of azetidine-2-carboxylic acid, see Table 4. 

Boc-18-crown-6-L-DOPA (4 66.4 g, 0.133 mol) was treated for 2h with 4M HQ in dioxane. After concentration to dryness under vacuo, the amino acid hydrochloride obtained (57.96 g, 0.133 mol) was dissolved in a H20/MeCN (1 9) mixture. DIPEA (69 mL) and Fmoc-OSu (49.80 g, 0.148 mol) were then added portionwise to the soln which was stirred mechanically for 3 h. The solvents were removed under reduced pressure and the residual oil was dissolved in CH2C12. The organic phase was washed with 1 M HC1 then water. After drying (MgS04), the solvent was removed and the residue was triturated with Et20 to give 5 as a white solid yield 95% mp 110-114°C TLC Rj 0.30 (silica gel, CHCl/MeOH/AcOH 70 15 15) [a]D25 -14.8 (c 1, DMF). 

The above compound 66 (1.22 g, 2.51 mmol) was dissolved in anhyd acetone (20 mL) and cooled to 0°C. The soln was saturated with anhyd HC1 and stirred at rt for 30 min. After the solvent was removed, the dihydrochloride 67 was neutralized with 10% Na2C03 and washed with hexane. A soln of Fmoc-OSu (931 mg, 2.51 mmol) in dioxane (26 mL) was added dropwise to the amino acid soln at 5°C and the mixture was stirred at rt for 12 h. The mixture was diluted with H20 and washed with Et20. The aqueous phase was acidified to pH 3 with coned HC1 and the suspension formed extracted with EtOAc. The organic phase was dried (Na2S04) and concentrated to yield the desired Fmoc-protected amino acid 60 yield 1.11 g (91% yield for the two steps) TLC Rf 0.14 (silica gel, CHCI3/MeOH 5 1) [a]D25 +18.8 (c 0.5, CHC13) HRFAB-MS mlz 489.2014 [M + H]+. 

The first synthesis1121 of A -foi-BocHN-alky]) amino acid building units used nucleophilic substitution of a-halocarboxylic acids or their esters 30 (Scheme 18) with mono-Boc-sub-stituted diamines 29. The nucleophilic substitution product 31 was reacted with Fmoc-OSu to give the orthogonally protected building unit 32. 

Ethyl bromoacetate (5.54 mL, 50 mmol) in THF (25 mL) was added dropwise to a cooled (ice bath) soln of amine 29 (llOmmol) in THF (25 mL). After stirring for 2.5 h at rt, the mixture was taken to dryness and the residue resuspended in E O. The predpitate was removed by filtration and washed with E O. The combined organic phase was taken to dryness and the residue purified by column chromatography (silica gel, Et20). 4M NaOH (2.5 mL) was added to the soln of /V-alkyl-Gly-OEt (10 mmol) in dioxane (35 mL) and MeOH (12.5 mL). After stirring for 30 min at rt, the mixture was taken to dryness. The residue was dissolved in H20 and the pH adjusted to 9-9.5 with coned HC1. To this mixture a soln of Fmoc-OSu (10 mmol) in MeCN (20 mL) was added in one portion and the pH maintained at 8.5-9.0 with TEA. The MeCN was removed and the residue added to 20% citric acid soln (60 mL). The soln was extracted with EtOAc (3 x 75 mL) and the combined organic layers were washed with H20 and brine, dried (Na2S04), and taken to dryness. The residual oil was recrystallized (EtOAc/hexane). 

H-m-Amb-OH (2 0.75 g, 5 mmol) was dissolved in H20 (5mL) (the pH was adjusted to 8 by the addition of TEA). To this soln at 0 °C was added Fmoc-OSu (1.5 g, 4.5 mmol) in MeCN (5 mL). The pH of the soln was monitored and kept at -8 by addition of TEA. With the pH constant, the mixture was allowed to stir at rt for 3 h. The mixture was then filtered, and the solvent was removed under reduced pressure. The isolated residue was added to vigorously stirred 1 M HCI. The resulting precipitate was collected by filtration, dissolved in EtOAc, and washed with several portions of 1M HCI. After removal of solvent under reduced pressure, the product was purified by recrystallization (EtOAc/hexane) to give Fmoc-m-Amb-OH yield 1.27 g (68%) mp 200-201 °C. 

A soln of 4-(bromomethyl)phenylacetic acid (6.41 g, 28.0 mmol) in acetone (200 mL) was added drop-wise to a stirred soln of 2 M NH3/EtOH (800 mL) at rt. This mixture was stirred for a further 1 h and the solvents were then removed. To the residual solid 1M NaOH (28.0 mmol) was added followed by the removal of the H20. The residue was dissolved in further H20 (200 mL) and the concentration was repeated to remove any remaining ammonia. The solid was dried in vacuo and dissolved in H20 (150 mL). A soln of DIPEA (5.0 mL, 28.6 mmol) and Fmoc-OSu (9.45 g, 28.03 mmol) in MeCN (150 mL) was added and the mixture was stirred at rt for 2 h. Most of the MeCN was removed, and the residue was then poured into 1.5 M HC1 (200 mL). The solid precipitate was collected by filtration, washed with H20, and dried in vacuo. The crude product was purified by flash chromatography (silica gel, hexanes/CHCl3/ MeOH/AcOH), and then recrystallized (EtOH/Et20) yield 5.3 g (49%) mp 191-193°C Anal. Calcd for C24H21N04 C, 74.4 H, 5.5 N, 3.6. Found C, 73.9 H, 5.2 N, 3.5. 

As in the case of Boc protection, the Fmoc group is not usually introduced on solid phase, but rather in solution, by the use of an activated Fmoc derivative (e.g. the chloroformate Fmoc-Cl or O-Fmoc-.V-hydroxysuccinimide, Fmoc-OSu) and aqueous base (Experimental Procedure 10.3)., V-/ lkylamino acids bound to cross-linked polystyrene have been Fmoc-protected by treatment with Fmoc-Cl (4 equiv.) and DIPEA (6 equiv.) in DCM for 2 h [132,259], Primary amines on insoluble supports can also be converted into Fmoc derivatives under these conditions [260]. 

Fig. 4. Preparation of the AmAbz scaffold and of its protected derivatives 15 and 16. (i) MeOCOCl, Na2S04, dioxane (ii) A-hydroxymethylphthalimide, 96% H2S04-H20 (9 1, v/v) (iii) NaOH, H20 (iv) Boc20 or Fmoc-OSu, NaOH, dioxane-H20.

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