Diisopropylethylamine → 4-(Phenylmethyl) hydrogen L-aspartate

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A modified procedure of example I was used to assemble the protected tetrapeptide (SEQ ID NO:5) PhCH2CH2CO-N-Me-Val-Ile-Asn-Asp(OBzl)-OH wherein Boc-Asp(OBzl)-OH was linked to the photo-resin and HOBt/DCC was used for coupling Boc-Asn-OH. A solution of the protected tetrapeptide (375 mg, 0.54 mmol) and 3-methyl-1(S)-(5-1H-tetrazolyl)butylamine (375 mg, 1.25 mmol) in DMF (15 ml) was cooled to 0. DIPEA (284 mg) and then diphenylphosphoryl azide (760 mg) were added to the solution. The reaction mixture was stirred for 16 h at 0 and then partitioned between Et2O and H2O containing of few drops of concentrated NH4OH (pH=9.5). The aqueous phase was separated, washed with fresh Et2O and filtered through a 45 m membrane. The pH of the filtrate was adjusted to pH 6.5 by the addition of AcOH and again filtered through a 45 m membrane. The filtrate was loaded on a HPLC preparatory column. The column was eluted with a continuous gradient of 0.06% aqueous TFA in acetonitrile. The pure fractions as indicated by analytical HPLC were combined and lyophilized to give the corresponding Asp(OBzl) protected peptide (132 mg) of the title compound.

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