Stereoselective Modification of a Cyclopentapeptide via an α-(Ethylthio)glycine Residue^†

The solid-phase synthesis of cyclo[Val-D-Gly(SEt)-Pro-Phe-D-Ala] (5) on Kaiser's oxime resin is described. Conversion of 5 to the reactive α-chloroglycine derivative 9 allowed the stereoselective addition of thiols, alcohols, amines and other nucleophiles with formation of the modified cyclopeptides 11a−n.