Total Synthesis of Natural Disaccharide Sambubiose (2-O-β-D-XYLOPYRANOSYL-β-D-GLUCOPYRANOSE)

Total Synthesis of Natural Disaccharide Sambubiose Chemopreventive phytochemicals (CPs) occur naturally in some plants and have shown to inhibit all levels of carcinogenesis, both in vitro and in vivo models. Among CPs, flavonoid 2-O-xylosylvitexin, belonging to Beta vulgaris var. Cycla, is the most abundant and effective1 but its extraction and separation are very difficult. This work is finalized to synthesize its disaccharidic precursor component, sambubiose, and improve its synthetic procedure reported by Erbing and Linberg in 1969. The study of the best combination of glycoside donor and glycoside acceptor, an efficient stereoselective control of O-glycosylation, and protection/deprotection strategies of functional groups are explored. To achieve that goal, it was envisioned a series of parameters that influence the output of the coupling reaction, such as protecting groups of the glycoside acceptor and glycoside donor, leaving group of the glycoside donor, and catalysts-promotors. Moreover, we studied the removal of protecting groups of the disaccharide. Regarding the coupling reaction, activating xylose as trichloroacetimidate and allyl 3,5,6-tri-O-benzyl-β-D-glucofuranoside by using trimethylsilyl triflate (TMSOTf) as catalyst gave good results in terms of yield, and the product was suitable to continue the synthesis. The best glycoside acceptor and donor were allyl 3,5,6-tri-O-benzyl-β-D-glucofuranoside and 2,3,4-tri-O-acetyl-D-xylopyranosyl-α-trichloroacetimidate, respectively. The solvent of choice in the coupling reaction was methylene chloride, which does not participate during the reaction and only 1,2-trans glycoside product is the result of 2-O-acyl vicinal group assistance (anchimeric effect). The fully protected disaccharide was gradually deprotected following the preferred scheme of deprotection that provides the deacetylation, deallylation, and debenzylation to give sambubiose. Different methods to remove the O-allyl group were explored and palladium chloride was found to be the most efficient reagent to isomerize the allyl group to the corresponding enol ether (yield 42%). Lastly, during the deprotection of the benzyl groups a spontaneous rearrangement of the 5 members glucosyl ring (glucofuranose) in the one 6 members glucosyl ring (glucopyranose) takes place to afford the natural disaccharide. Moreover, the correct synthon to couple with agliconic apigenin and to afford 2-O-xylosylvitexin was found. Design and synthetic plan for 2-O-xylosylvitexin Following the expertise obtained by the synthesis of sambubiose, we planned to synthesize the C-glycosylflavonoid. First of all, a new synthon to couple with apigenin was prepared, and the glycosylation product was obtained between 2,3,4-tri-O-acetyl-D-xylopyranosyl-α-trichloroacetimidate and β-allyl 3,5,6-tri-O-benzyl-β-D-glucofuranoside. The first attempts of activation the disaccharide ready to react with phenolic -OH aglycon were not achieved, and the obtaining of α- or β-trichloroacetimidate derivative was unsuccessful. Therefore, we thought to prepare the corresponding monosaccharide, but also in this case the treatment with trichloroacetonitrile and 1,5-diazabiciclo[5.4.0]undec-5-ene led to decomposition during purification. In an attempt to change this approach, we envisioned the reaction proposed by Field et al. for per-O-acetylated sugars. Iodine can serve as a promoter for sugar per-O-acetylation, that sugar per-O-acetates can be converted to the corresponding glycosyl iodides by TMS-I, generated in situ from iodine and hexamethyldisilane (HMDS) with complete anomeric stereoselectivity. We obtained the product with very low yields and in mixture of anomers. Moreover, after few days it was found to be unstable and decomposing. A different exploration provided the synthesis of an intermediate via glycosylation in the presence of boron trifluoride etherate (0.5–1.2 mol) or p-toluenesulfonic acid (0.1 mol). The conversions were inefficient, unlike reported in literature, where but fully peresters (i.e. altrofuranose per-O-acetylated). An additional glycosylation reaction was performed between by using trimethylsilyl triflate or p-toluenesulfonic acid. In both cases only decomposition products were obtained, with loss of benzylic and tert-butyldimethylsilyl groups. Further investigations to obtain 2-O-xylosylvitexin are now ongoing in our laboratories. Design and Synthesis of Novel Cannabionid Receptor Ligands Endocannabinoids are lipidic mediators, which include amides, esters and ethers of long-chain polyunsaturated fatty acids, and active specific membrane receptors, such as CB1, mainly localized in the central nervous system (CNS), and CB2, predominantly present in the periphery. The therapeutic use of CB1 agonists is limited by side effects in the CNS, which can be restrict by increasing CB2 receptor selectivity with ligands potential able to exert desired effects, such as those analgesic and antinflammatory. Two series of 3-aroyl-2,5-dimethylpyrrole derivatives with a good affinity for the CB1 and CB2 receptors behaved as agonists, partial agonists or antagonists were previously studied by us and structure-activity relationships (SARs) were carried out. Therefore, with the aim of extending the SARs and to obtain selective ligands we designed and synthesized new 1-alkyl-2,5-dimethyl-3-ketopyrrole substituted analogues. In particular, we considered that the variation of alkyl groups in the 1-position of the heterocycle could increase the affinity and selectivity towards CB receptors, and that the arylalkyl group on the 3-position could allow to get information on electronic characteristics, planarity and steric hindrance. Globally, our optimization process was led by modification on the pyrrole nucleus to hinder penetration of the blood brain barrier and favor per os administration, that could be achieved by improving the value of topological polar surface area and decreasing the value of clogP. By preliminar in vitro tests it appears that some compounds can be considered as selective ligands of the CB2 receptors