C[O:2][C:3](=[O:21])[C@H:4]([CH2:14][C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1)[NH:5][C:6]([O:8][C:9]1([CH3:13])[CH2:12][CH2:11][CH2:10]1)=[O:7].CO.[OH-].[Na+].S(=O)(=O)(O)O>O>[CH3:13][C:9]1([O:8][C:6]([NH:5][C@H:4]([C:3]([OH:21])=[O:2])[CH2:14][C:15]2[CH:16]=[CH:17][CH:18]=[CH:19][CH:20]=2)=[O:7])[CH2:10][CH2:11][CH2:12]1A suspension of 171 mg. (0.74 mM) of phenylalanine methyl ester hydrochloride in 10 ml. of chloroform is cooled to 0°C. and the pH adjusted to 8 by addition of triethylamine. 1-Methylcyclobutyl chloroformate 1.5 mM is added portion-wise at 0°C. and the pH of the reaction is maintained at 8 by addition of triethylamine. Upon completion of the addition, the reaction is stirred overnight at 20°-25°C. The reaction solution is washed successively with 10 ml. portions of a saturated sodium bicarbonate solution, 50% saturated solution of sodium chloride, 0.2 N sulfuric acid saturated with sodium sulfate, 50% saturated solution of sodium chloride (twice), and the chloroform solution is dried over sodium acetate. Removal of the chloroform affords N-(1-methylcyclobutyloxycarbonyl)-phenylalanine methyl ester. Saponification of the above ester is carried out by dissolving the residue in 10 ml. of methanol, water (10 ml.) is added and the pH is adjusted to 12 by addition of 2 N sodium hydroxide. After stirring overnight at 25°C., the pH is adjusted to 7 with 2 N sulfuric acid. The methanol is removed in vacuo and saturated sodium bicarbonate solution is added to the water solution. The aqueous bicarbonate solution is washed two times with ether and then the pH is adjusted to 3 by addition of concentrated hydrochloric acid. The acidified solution is extracted three times with ethyl acetate and the organic extracts combined, washed twice with sodium chloride solution and dried over sodium sulfate. The solvent is removed in vacuo and trituration with pet ether affords N-(1-methylcyclobutyl-oxycarbonyl)-phenylalanine, which is isolated by filtration.
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