GENERAL-METHODS FOR ALPHA-O-SER/THR OR BETA-O-SER/THR GLYCOSIDES AND GLYCOPEPTIDES - SOLID-PHASE SYNTHESIS OF O-GLYCOSYL CYCLIC ENKEPHALIN ANALOGS

O-Linked glycopeptides have been efficiently synthesized using the highly nucleophilic alpha-imino esters (O'Donnell's Schiff bases) derived from L-serine (3a-c), L-threonine (4a,b), and a dipeptide ester (5). General methodology has been developed which can provide beta-glycosides of beta-hydroxy-alpha-amino acid derivatives 6-16 in excellent yield (63-94%) and excellent selectivity (>20:1) using Hanessian's modification or Helferich's modification of the Koenigs-Knorr reaction. Likewise, selective alpha-glycosylation has been achieved using the in situ anomerization methodology of Lemieux (28, 30). The increased nucleophilicity of the serine/threonine hydroxyl has been shown to be due to intramolecular hydrogen bonding to the N=CPh2 moiety. Deprotection of the intermediate Schiff bases has been demonstrated, and the glycosides have been incorporated into fully deprotected O-linked glycopeptides in high yield using either solution-phase peptide methods or solid-phase Fmoc-based technology. The potent glycosylenkephalin analogue 38 has been prepared using the solid-phase methodology.