開催日: 2017/09/20 - 2017/09/22
Introduction Phenolic glycolipids 1 and 2 (PGLs) are abundant in the cell envelope of Mycobacteria fungi, and are composed of a partially O-methylated oligosaccharide and a polyketide-derived diacylated lipid1) (Figure 1). In a previous investigation, the sugar part of PGLs appeared to possess immunomodulatory effect via inhibition of proinflammatory cytokine release2). However, mechanism of their biological effects and precise structure-activity relationships are not still clear. Therefore, we aimed to develop an effective method for the synthesis of partially O-methylated glycosides varying at the lipid part and evaluate their biological activity for understanding the mode of action. Results Scheme 1 shows our strategy for synthesis of the chemical probes 3. The chemical probe 3 was synthesized in one-pot from the sugar part 4, the cyclic borane 5 and the functional part 6 by a sequential Suzuki-Miyaura coupling.3) The naphthyl group 6a would be a mimic of the lipid part. The photoaffinity labeling unit 6b and the fluorescent dye 6c would assist to identify the target receptor. Formation of 2-O-methyl-α-rhamnosides and fucosides would be a key step in the synthesis of PGL sugar parts. O-methylation of the C2 hydroxy group makes neighboring group participation promoting 1,2-trans glycosidic linakge not to be available. We found that glycosidation of glycosyl imidate 7b with I2 and Bu4N·OTf provided disaccharide 9 in good yield with excellent α-selectivity. The glycosidation method allowed one to synthesize the PGL-1 sugar part (4a) and the PGL-tb1 sugar part (4b) in a straight forward manner. We next developed the aforesaid Suzuki-Miyaura coupling (Scheme 5). The glycosides 4a were converted to the chemical probes 3a-c via a sequential Suzuki-Miyaura coupling using the cyclic boranes. The generated borinic acid 28 was inert at 60 °C and underwent next coupling reaction at higher reaction temperature to provide the asymmetrical functionalized alkanes in one-pot. Finally, immunomodulatory effect of PGL analogues on bone-marrow-derived macrophages stimulated with trehalose-6,6’-dimycolate (TDM) was evaluated (Figure 2). The PGL-tb1 disaccharide sugar part-derived glycolipid Ea possessed a strongest activity among the PGL-tb1 related glycosides. Fluorescent chemical probes Ac-Cc also showed comparable activity with glycolipid Aa-Ac. Conclusion In this study, we developed direct and stereoselective glycosidation and sequential Suzuki-Miyaura coupling using cyclic boranes. We successfully synthesized the PGL sugar parts 4 and the chemical probes 3 by a sequential Suzuki-Miyaura coupling. We expected that the synthesized chemical probes would strongly asset to elucidate precise biological information of the PGL-promoting biological activity.