Application of carboxylic-phosphinic mixed anhydrides in the fragment peptide synthesis of protected analogues of substance P

Mixed carboxylic‐phosphinic anhydrides derived from peptide acids and 1‐oxo‐1‐chlorophospholane have been applied in the synthesis of the protected [Leu11]‐SP1‐11 by the fragment coupling strategy. The yields from fragment couplings were ca. 75%, the products were of high purity while the conditions of formation and coupling of the corresponding mixed phosphinic anhydrides, for optimum yields, have been evaluated.