... Bioorganic & Medicinal Chemistry Letters. ... 13 The carbobenzyloxy protected dipeptide 10 was prepared in 98% yield via union of commercially available N-carbobenzyloxy-l-leucine (7) with the ethyl ester of l-tryptophan (9) (prepared in 89% yield by esterification of l-tryptophan 8 in refluxing ethanol with two equivalents of dl ... A total of 18.8 g of 16 was produced by this method, and the final saponification to phosphoramidon has been reported by De Nanteuil. 11. ...