Synthesis and properties of 1,1,3,3,-tetramethyl-2-(2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl) uronium triflate

✍ Scribed by Klaus Bock; Jose Fernández-Bolaños Guzmán; Susanne Refn

• Publisher: Elsevier Science
• Year: 1992
• Tongue: English
• Weight: 280 KB
• Volume: 232
• Category: Article
• ISSN: 0008-6215
• DOI: 10.1016/0008-6215(92)80067-b

✦ Synopsis

An efficient method for the synthesis of 1,2-truns-glycosides, introduced by Hanessian and Banoub ', involves treatment of acetylated glycosyl halides with partially protected sugar derivatives in the presence of silver triflate (promoter) and 1,1,3,3_tetramethylurea (proton acceptor).

However, 1,1,3,3_tetramethylurea in this reaction acts not only as a base but also reacts with the glycosyl halide, particularly at higher temperatures, to form an ionic compound that can be detected by TLC. Thus, 1,1,3,3-tetramethyl-2-(2,3,4,6-tetra-O-acetyl-cw-o-glucopyranosylhironium triflate (2) was isolated crystalline (56%) after treatment of 2,3,4,6-tetra-0-acetyla-o-glucopyranosyl bromide (1) in dichloromethane with silver triflate and 1,1,3,3_tetramethylurea at room temperature for 15 min. However, if the reaction mixture was kept for 1 h at -7O", then filtered, and concentrated, 'H-NMR spectroscopy of a solution of the residue in D,O revealed 1,3,4,6-tetra-O-acetyl-&D-glucopyranose

(5) together with < 10% of 2. This finding indicates that, at low temperature, the equilibrium 3a + 3b favours the acetoxonium ion 3b, which yields 5 on quenching, whereas, at higher temperatures, 2 is formed.

Structure 2 was deduced primarily from the NMR data and its chemical reactivity. Thus, 2 was soluble in water, and the 'H-and 13C-NMR data for solutions in D,O, CDCl,, and CD,OD are presented in Tables I andII (the assignments were based on COSY and heteronuclear 2D correlated experiments). The 3JH,H values indicated a 4C,(~) conformation and ruled out the orthoester structure 4, the NMR data of which would be expected to be similar to those of