Note Regioselective synthesis of a glycomimetic trisaccharide of Sialyl Lewis (sLe x ) Sameh E. Soliman a , Rafik W. Bassily a , Ramadan I. El-Sokkary a , Joseph Banoub b , Mina A. Nashed a, * a Department of Chemistry, Faculty of Science, Alexandria University, Ibrahimia, PO Box 426, Alexandria 21321, Egypt b Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3V6 article info Article history: Received 28 September 2008 Received in revised form 11 November 2008 Accepted 27 November 2008 Available online 10 December 2008 Keywords: Sialyl Lewis (sLe x ) Oligosaccharides Sialic acid bioisosters Substituted lactoside abstract Sialyl Lewis (sLe x ) is the smallest naturally occurring carbohydrate ligand that binds to E-Selectin on the activated endothelium. We report here the total synthesis of acetic acid-sLe x analog (12), for testing as a therapeutic agent. Methoxyethyl 4-O-(3,4-O-isopropylidene-b-D-galactopyranosyl)-b-D-glucopyran- oside (3) was prepared starting from the methoxyethyl-b-D-lactoside (2), which was selectively benzoy- lated to give the methoxyethyl 2,6-di-O-benzoyl-4-O-(2,6-di-O-benzoyl-3,4-O-isopropylidene-b-D- galactopyranosyl)-b-D-glucopyranoside (4). Glycosylation of acceptor 4 with methyl 2,3,4-tri-O-ben- zyl-1-thio-b-L-fucopyranoside (5) in the presence of cupric bromide and tetrabutylammonium bromide afforded the corresponding methoxyethyl 2,6-di-O-benzyl-3-O-(2,3,4-tri-O-benzyl-a-L-fucopyranosyl)- 4-O-(2,6-di-O-benzyl-3,4-O-isopropylidene-b-D-galactopyranosyl)-b-D-glucopyranoside (6). Selective removal of the 4 00 ,6 00 -O-isopropylidene group from 6 gave the deprotected trisaccharide 7. The regiose- lective esterification of O-3 00 of trisaccharide 8 (obtained from the dibutylstannylene derivative of 7) with benzyl-2-bromoacetate and tetrabutylammonium bromide afforded the 3 00 -O-carbobenzyloxym- ethyl trisaccharide derivative 9, which on saponification and hydrogenolysis with palladium–charcoal afforded the target trisaccharide 12 glycomimetic of Sialyl Lewis (sLe x ) trisaccharide omitting the sialic acid moiety. Ó 2008 Elsevier Ltd. All rights reserved. Cell-surface glycoconjugates are known to act as cell–cell recog- nition molecules via the specific binding between carbohydrates on one cell and the protein receptors on the opposing cell. 1 The selectin biomolecules are a family of carbohydrate binding pro- teins that mediate the tethering and rolling of the leukocytes in the blood vessel endothelium at the sites of inflammation. 2,3 They are also implicated in the hematogenous metastasis of some cancer cells. 4,5 The sialyl Lewis X (sLe x ) tetrasaccharide, 6–8 a-Neup5Ac- (2?3)-b-Galp-(1?4)-a-Fucp-(1?3)-GlcpNAc, has been generally recognized as a common ligand for all selectin biomolecules. 9,10 Numerous analogs and mimetics of the Le x and sLe x have been designed and synthesized previously 11–17 to elucidate the mecha- nism of selectin recognition and to afford novel therapeutic agents for the treatment of allergy, microbial infection, and inflammatory and autoimmune diseases. 18–20 Recently, Asnani and Auzan- neau 21,22 used our strategy, which was reported for the synthesis of partially benzoylated 3 0 ,4 0 -O-isopropylidene-b-D-lactosides, to prepare key intermediates for the assembly of sialyl Lewis X (sLe x ) analogs. 23,24 The aim of our work is to synthesize the glycomimetic trisaccharide of sLe x , lacking the sialic acid moiety, for testing as a therapeutic agent. In a previous work, we have shown that 3 0 ,4 0 -O-isopropylidene- b-D-lactosides could be selectively benzoylated at positions 2,2 0 ,6,6 0 having free hydroxyl groups at O-3 and potentially free at O-3 0 , but the structure of the resulting tetrabenzoate 4 was not fully characterized. 23,24 In the initial stage of the synthesis (Scheme 1), the catalytic O- deacetylation of the hepta-acetate (1) 25 afforded methoxyethyl 4- O-(b-D-galactopyranosyl)-b-D-glucopyranoside (2), which on treat- ment with 2,2-dimethoxypropane in the presence of camphorsul- fonic acid 26 gave the methoxyethyl 4-O-(3,4-O-isopropylidene-b- D-galactopyranosyl)-b-D-glucopyranoside (3). Regioselective ben- zoylation of the 3 0 ,4 0 -O-isopropylidene derivative (3) at low tem- perature produced mainly, methoxyethyl 2,6-di-O-benzoyl-4-O- (2,6-di-O-benzoyl-3,4-O-isopropylidene-b-D-galactopyranosyl)-b- D-glucopyranoside (4) as fine needles in excellent yield. The structural elucidation of the methoxyethyl-b-D-lactoside acceptor (4) was accomplished using a two dimensional 1 H– 1 H COSY experiment beginning with resonances of H-2 0 and H-2. The chemical shifts of the other 1 H resonances were established by tracing the connectivity of the cross-peaks on the COSY contour map. Additional support for the proposed structure 4 was achieved through a close examination of the 13 C NMR spectrum, 13 C– 1 H cor- relation, and DEPT experiments. Fucosylation of the disaccharide acceptor 4 with the glycosyl donor methyl 2,3,4-tri-O-benzyl-1-thio-b-L-fucopyranoside (5) in the presence of the glycosyl promoter cupric bromide, tetra-n- butylammonium bromide, and activated powdered molecular sieves 4 Å, in methylene chloride, gave the expected trisacccharide 0008-6215/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2008.11.019 * Corresponding author. E-mail address: mina4nashed@yahoo.com (M.A. Nashed). Carbohydrate Research 344 (2009) 395–399 Contents lists available at ScienceDirect Carbohydrate Research journal homepage: www.elsevier.com/locate/carres