A Stereoselective Approach for the Synthesis of r-Sialosides Cristina De Meo, Alexei V. Demchenko, and Geert-Jan Boons* Complex Carbohydrate Research Center, University of Georgia, 220 Riverbend Road, Athens, Georgia 30602 Received April 3, 2001 A highly efficient synthesis of the human melanoma associated antigen GD 3 derivative has been described. A key feature of the synthetic approach was the use of sialyl donors that were protected with a C-5 trifluoroacetamide moiety. These sialyl donors gave high yields and excellent R-anomeric selectivities in direct glycosylations with a wide variety of glycosyl acceptors ranging from C-8 hydroxyls of sialic acids and C-3 hydroxyls of galactosides to reactive primary alcohols. Introduction Sialic acids are a diverse family of naturally occurring 2-keto-3-deoxy-nononic acids that are involved in a wide range of biological processes. 1,2 The most abundant sialic acid derivative is N-acetylneuraminic acid (Neu5Ac); however, compounds that have a glycolyl moiety at the C-5 amino group (Neu5Gc) or acetyl esters at one or more hydroxyls are also frequently encountered in nature. 1,3 Sialic acids normally appear at terminal positions of oligosaccharides of glycoproteins and glycolipids where they are R(2,3) or R(2,6) linked to galactosides or R(2,6) linked to 2-acetamido-2-deoxy-galactosides. The disialosyl structures Neu5AcR(2-8)Neu5Ac and Neu5AcR(2-9)- Neu5Ac have also been found as constituents of oligosac- charides of glycoproteins and lipids. These substructures are receptors for viruses and bacteria and constitute the immunodominant epitope of tumor-associated antigens. Neu5Ac or Neu5Gc also occur in linear homopolymers where they are usually linked internally by R(2,8), R(2,9), or alternating R(2,8)/R(2,9) glycosidic linkages. These polysialic acids play important roles as neural cell adhesion molecules. 4 While relatively efficient methods have been developed for the introduction of Neu5AcR(2-3)Gal and Neu5AcR- (2-6)Gal glycosidic linkages, the synthesis of oligosac- charides that contain R(2 f 8)-linked fragments is complicated by the low reactivity of the C-8 hydroxyl of Neu5Ac. 5-7 The latter glycosides have been successfully synthesized by indirect chemical approaches whereby modified sialyl donors are employed that have a partici- pating auxiliary at C-3. 8-11 These highly elaborated donors require, however, laborious procedures for their preparation, and after a glycosylation, additional steps are needed for the removal of the auxiliary. Alternative direct glycosylation approaches have been reported, but these lead to either low yielding glycosylations or forma- tion of unnatural -sialosides or mixtures of anomers. 12-15 It is obvious that a versatile sialyl donor needs to be developed that gives excellent yields and high R-anomeric selectivities in direct glycosylations with a wide range of acceptors of different reactivities. 7 Such a donor would allow efficient synthesis of oligosaccharides of biological or medical importance that contain multiple sialic acids of different linkage type. Here, we report that the readily available sialyl donor methyl(methyl 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-2-thio- 5-trifluoroacetamido-D-glycero--D-galacto-nonulopyra- nosid)onate (2a) gives good yields and excellent R-ano- meric selectivities in direct sialylations with a wide variety of glycosyl acceptors ranging from sterically hindered C-8 hydroxyls of a sialic acid and C-3 hydroxyls of galactosides to reactive primary alcohols. The versatil- ity of the donor allowed a highly efficient synthesis of the human melanoma associated antigen GD 3 derivative, which has multiple Neu5Ac residues (Figure 1). Results and Discussion The synthesis of the human melanoma associated antigen GD 3 derivative requires the introduction of Neu5AcR(2-8)Neu5Ac and Neu5AcR(2-3)Gal glycosidic linkages. In addition, the anomeric center of a sialyl acceptor needs temporary protection, which required gly- cosylation with a primary alcohol. Previous syntheses of the carbohydrate part of this biologically important gly- cosphingolipid could only be achieved by indirect sialy- lation protocols 16-18 or a strategy whereby an R(2-8)- linked fragment was obtained by controlled degradation of colominic acid. 19-21 It was anticipated that sialyl donor 2a (Scheme 1), which is protected with a 5-trifluoroacetamido (N-TFA) (1) Sialic Acids: Chemistry, Methabolism and Function; Schauer, R., Ed.; Springer-Verlag: Wien-New York, 1982; Vol. 10. (2) Varki, A. Glycobiology 1993, 3, 97-130. (3) Inoue, Y.; Inoue, S. Pure Appl. Chem. 1999, 71, 789-800. (4) Reglero, A.; Rodriguez-Aparicio, L. B.; Luengo, J. M. Int. J. Biochem. 1993, 25, 1517-1527. (5) Okamoto, K.; Goto, T. Tetrahedron 1990, 46, 5835-5857. (6) DeNinno, M. P. Synthesis 1991, 583-593. (7) Boons, G. J.; Demchenko, A. V. Chem. Rev. 2000, 100, 4539- 4565 and references therein. (8) Ercegovic, T.; Magnusson, G. J. Org. Chem. 1995, 60, 3378- 2284. (9) Ercegovic, T.; Magnusson, G. J. Org. Chem. 1996, 61, 179-184. (10) Castro-Palomino, J. C.; Tsvetkov, Y. E.; Schmidt, R. R. J. Am. Chem. Soc. 1998, 120, 5434-5440. (11) Hossain, N.; Magnusson, G. Tetrahedron Lett. 1999, 40, 2217- 2220. (12) Tsvetkov, Y. E.; Schmidt, R. R. Tetrahedron Lett. 1994, 35, 8583-8586. (13) Castro-Palomino, J. C.; Tsvetkov, Y. E.; Schneider, R.; Schmidt, R. R. Tetrahedron Lett. 1997, 38, 6837-6840. (14) Demchenko, A. V.; Boons, G. J. Chem. Eur. J. 1999, 5, 1278- 1283. (15) Our attempts to synthesize (2-8)-linked dimers by direct sialylation employing conventional glycosyl donors {2-thiomethyl, 2-thiophenyl, 2-xanthate, and 2-phosphite of methyl 5-acetamido- 4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero--D-galacto-nonulopyranosid- (yl)onate} failed, and only traces of the desired products were obtained. 5490 J. Org. Chem. 2001, 66, 5490-5497 10.1021/jo010345f CCC: $20.00 © 2001 American Chemical Society Published on Web 07/19/2001