Solid-phase carbohydrate synthesis via on-bead protecting group chemistry Hilbert M. Branderhorst, Rob M. J. Liskamp and Roland J. Pieters * Department of Medicinal Chemistry and Chemical Biology, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, PO Box 80082, 3508 TB, Utrecht, The Netherlands Received 13 November 2006; revised 22 February 2007; accepted 8 March 2007 Available online 13 March 2007 Abstract—Di- and tri-saccharides were synthesized on a solid phase. The procedure started with a non-protected sugar linked via either cys- teine or glutamine to a polystyrene resin. Selective dimethoxytritylation chemistry and subsequent steps yielded a resin-bound acceptor that could be glycosylated to yield b1,6-linked disaccharides. Reiteration of the procedure produced the trisaccharide. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction The difficulties in synthesizing complex synthetic carbo- hydrates limit our understanding of their roles in biological systems and also limit carbohydrate-based drug develop- ment. Carbohydrates are nevertheless emerging as a class of compounds, which play crucial roles in major diseases such as (avian) flu, 1 AIDS, 2 and cancer. 3 The shift of the syn- thesis of oligomeric nucleotides and peptides to the solid phase, and subsequent automation, has led to major steps forward in their ease of preparation and application in bio- logy. For carbohydrates, a universal solid-phase synthetic method is more difficult but would have a great scientific impact. Solid-phase synthesis of carbohydrates was first re- ported in 1971 4 and since then much progress was made. 5 The recent automation of the process represents a milestone. 6 All published strategies in common are based on a two-step protocol, which includes an on-bead glycosylation step and an on-bead deprotection step. This means that strategically protected carbohydrate donors are used for Glycosylation, which, after coupling, can selectively be deprotected at the desired position to afford a new glycosyl acceptor for further elaboration. Although connecting the building blocks one by one is a rapid procedure, the synthesis of all the desired building blocks is a major effort to be customized for each desired target carbohydrate. In response to this, we opted to explore the use of resin-bound unprotected sugars to be se- lectively converted to acceptors by on-bead selective chem- istry commonly used in solution (Scheme 1). In principle, a single carbohydrate can thus be elaborated into several di- saccharides depending on the protecting chemistry used. Such selective chemistry would have to be sequence inde- pendent, and based on an excess of reagent to push reactions to completion. If these procedures prove to be simple and compatible with automation, many complex sugars, includ- ing branched ones, can be made from a few building blocks by reiteration of the steps. To take the first step toward this distant goal, we started with the well-studied selective C-6 chemistry, based on the bulky 4,4-dimethoxytrityl (DMT) protecting group that selectively reacts with primary hy- droxyl groups even when applied in excess. The DMT group was chosen for the mild cleavage conditions (1% dichloro acetic acid in CH 2 Cl 2 ) and the visible color release by the DMT-cation that can be used for verification. O OH HO HO HO O OH PGO PGO PGO O LG OAc AcO AcO AcO O O PGO PGO PGO O OH HO HO HO selective protection chemistry selective protection chemistry C6 C3 C4 C6 C3 C4 glycosylate, then deacetylate glycosylate, then deacetylate etc. Scheme 1. General solid-phase synthesis of carbohydrates including a selec- tive on-bead protection step followed by glycosylation and deprotection and reiteration. A selective C6 protection step is exemplarily shown as used in this paper. As a linker, the trifunctional amino acid cysteine (1) was chosen. Its sulfur atom can be linked to the sugar anomeric center. 7 After all synthetic procedures, this thioglycosidic Keywords: Carbohydrates; Solid-phase synthesis; Glycosylation. * Corresponding author. Tel.: +31 30 2536944; fax: +31 30 2536655; e-mail: r.j.pieters@uu.nl 0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2007.03.051 Tetrahedron 63 (2007) 4290–4296