Convenient high yield and stereoselective synthesis of O-glycopeptides using N-a-Fmoc-Tyr/Ser[b-D -Glc(OAc) 4 ]OPfp generated in solution q Beechanahalli P. Gangadhar, a Seetharama D. S. Jois b and Ambikaipakan Balasubramaniam a, * a Division of GI Hormones, Department of Surgery, University of Cincinnati Medical Center, Cincinnati, OH 45267, USA b Department of Pharmacy, National University of Singapore, Singapore 117543 Received 23 September 2003; revised 23 October 2003; accepted 24 October 2003 Abstract—Fmoc-AA-OPfp (AA ¼ Tyr or Ser) (1 equiv) was reacted with b-D-Glc(OAc) 5 (6 equiv) in the presence of BF 3 Et 2 O (6 equiv) in CH 2 Cl 2 at room temperature for 2 h, and the glycosylation reaction mixture was used directly to couple to the amino group of the peptide resin without isolation and purification of the Fmoc-AA[b-D-Glc(OAc) 4 ]-OPfp. Moreover, the -OAc protecting groups of glucose was removed just prior to releasing the peptide from the resin using 6 mM NaOMe in 85% DMF-MeOH. The crude product obtained by TFA cleavage contained >90% of the target O-glycopeptide, and the 500 MHz 1 H NMR analysis re- vealed that the glycosylation reaction was nearly stereoselective (>97% b-anomer). This method is rapid and stereoselective, and can now be exploited for the routine synthesis of O-glycopeptides. Ó 2003 Elsevier Ltd. All rights reserved. Carbohydrate moieties in glycoproteins play a crucial role in a number of biological process including cell recognition, cell adhesion, infection, and tumor meta- stasis. 1 Glycosylation of peptides has also been shown to increase proteolytic stability, and promote blood brain barrier (BBB) permeability. 2;3 Moreover, glycosylation enhances solubility, and may also contribute to the stabilization of peptide structures. The latter may impart differential receptor selectivity. Thus glycopeptides have attracted much attention in recent years. It is therefore desirable that a convenient and high yield method be available for the routine synthesis of glyocopeptides. Although a significant advancement has been made in the solid and solution phase glycopeptide synthesis during the last decade, 1–3 limited availability of pro- tected glycosylated amino acid derivatives has been impeding O-glycopeptide research. This could, at least in part, be attributed to the difficulties such as multiple steps, lower degree of stereoselectivity, cumbersome purification steps, and/or low yields associated with the currently used O-glycosylation strategies including: (1) b-D-Gal(OAc) 5 and BF 3 Et 2 O, 4 (2) a-D-Ac 4 Glc-Br and AgOTfl; 5–7 (3) b-D -2-N 3 -2-deoxy-Gal-Br and Ag 2 CO 3 / AgClO 4 , 8 and (4) a-imino esters, glycosyl bromide, and AgOTfl. 9 Our interest in O-glycopeptides emanates from the demonstration that O-glycosylation could promote BBB entry of peptides. 3 This is a useful strategy for the delivery of various agonists and antagonists of neuro- peptide Y (NPY) that have been developed by us and other investigators. 10;11 This is clinically important because central NPY has been implicated in the pathophysiology of obesity, sei- zures and mental disorders. 10–12 With this in mind, we wanted to develop a convenient glycosylation strategy that would be rapid, economical and amenable under normal conditions. Fmoc-AA-OPfp (AA ¼ Tyr or Ser) has been successfully glycosylated in good yields using a-D-Ac 4 Glc-Br and Keywords: O-Glycopeptides; Solid phase synthesis; Stereoselective; NMR. q Supported in part by a NIH grant GM47122. * Corresponding author. Tel.: +1-513-558-3819; fax: +1-513-558-0750; e-mail: ambi.bala@uc.edu 0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2003.10.178 Tetrahedron Letters 45 (2004) 355–358 Tetrahedron Letters