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] (<b>5</b>) on Kaiser's oxime resin is described. Conversion of <b>5</b> to the reactive α-chloroglycine derivative <b>9</b> allowed the stereoselective addition of thiols, alcohols, amines and other nucleophiles with formation of the modified cyclopeptides <b>11a−n</b>.