Glycosylating enkephalins: Design, glycosylation using sugar acetates in the preparation of glycosyl amino acids for glycopeptide syntheses, binding at the opioid receptors and analgesic effects

Improved procedures for the glycosylation of serine and threonine utilizing Schiff base activation are reported. The procedures are less expensive and more efficient alternatives to previously published methods. The Schiff bases exhibited ring-chain tautomerism in CDCl3 as shown by 1 H NMR. Acting as glycosyl acceptors, the Schiff bases reacted at RT with simple sugar peracetate donors with BF3•OEt2 promotion to provide the corresponding protected amino acid glycosides in good yields. With microwave irradiation, the reactions were complete in 2-5 minutes. Glycosylation with the dipeptide Schiff base shows the potential of this method in the preparation of peptide building blocks.; To investigate this reaction further, direct glycosylation of sugar acetates with FMOC-Ser-OH/OBZl under BF3•OEt2 promotion in a microwave provided glycosides in high yield. In addition to the expected glycoside products acetylated side products resulting from acetate migration were isolated, suggesting that activation of the anomeric sugar acetates with a Lewis acid such BF3•OEt2 led to an oxocarbenium ion, which rearranged to a 1,2-dioxocarbenium ion because of the acetate participating group at C-2. Solvent participation was also illustrated with acetate migration being more pronounced when CH3CN was used as a solvent and resulted in less product yield and higher amounts of the acetylated product. The acyl transfer products in these reactions where sugar acetates serve as glycosyl donors is reported for the first time, which also implies that ortho-ester like intermediates are important in the reaction mechanism.; Keeping the message segment constant in the sequence H-Tyr-DThr-Gly-Phe-Leu-Ser-CO-NH 2 and modification of the address segment with different carbohydrate moieties had little effect on selectivity for binding at the mu, delta or kappa-opiod receptors. However, substitution of D-threonine with D-serine or the less polar D-alanine in the message segment resulted in a loss of kappa-receptor affinity. Further replacement of D-threonine with the more hydrophobic D-valine resulted in complete loss of kappa-binding affinity generating pure mu-delta agonists. These data suggests that changes in the message segment of the pharmacophore results in the glycopeptide adopting a conformation that is less favorable for kappa-binding receptor activity. Finally, the peripheral administration and i.c.v. tests of the drugs suggest that modifications in the message segment of the pharmacophore influences the potency of these compounds.