Synthesis of peptides containing various functions was efficiently accomplished by treatment of the tetrabutylammonium salts of N-protected amino acids (or peptides) with amino acid esters in the presence of bis (o-or p-nitrophenyl) phenylphosphonate. Synthesis of leucine- enkephalin was successfully achieved by the above method employing a (3+ 2) or (4+ 1) fragment coupling approach.
![benzyl 2-[(3-benzylsulfanyl-2-phenylmethoxycarbonylamino-propanoyl)amino]-4-methyl-pentanoate Structure](https://image.chemsrc.com/caspic/142/77511-09-2.png)