Synthesis of quercetin 3-O-β-d-apiofuranosyl-(1→2)-[α-l-rhamnopyranosyl-(1→6)]-β-d-glucopyranoside

Regiospecific Synthesis of Three Quercetin O-β-Glucosides of N-Acetylglucosamine

The regiospecific synthesis of three quercetin O-β-glucosides of N-acetylglucosamine has been achieved in good yield. Selective di-and tri- O-benzylation of quercetin followed by O-glycosylation with 2-acetamido-3,4,6-tri- O-acetyl-2-deoxy-α-D-glucopyranosyl chloride under phase-transfer catalysis conditions yielded, after deacetylation and debenzylation, 3-, 3′- and 4′-glycosylated quercetin.

Synthesis of flavonoid O-pentosides by Escherichia coli through engineering of nucleotide sugar pathways and glycosyltransferase

Plants produce two flavonoid O-pentoses, flavonoid O-xyloside and flavonoid O-arabinoside. However, analyzing their biological properties is difficult because flavonoids are not naturally produced in sufficient quantities. In this study, Escherichia coli was used to synthesize the plant-specific flavonoid O-pentosides quercetin 3-O-xyloside and quercetin 3-O-arabinoside. Two strategies were used. First, E. coli was engineered to express components of the biosynthetic pathways for UDP-xylose and UDP-arabinose. For UDP-xylose biosynthesis, two genes, UXS (UDP-xylose synthase) from Arabidopsis thaliana and ugd (UDP-glucose dehydrogenase) from E. coli, were overexpressed. In addition, the gene encoding ArnA (UDP-l-Ara4N formyltransferase/UDP-GlcA C-4″-decarboxylase), which competes with UXS for UDP-glucuronic acid, was deleted. For UDP-arabinose biosynthesis, UXE (UDP-xylose epimerase) was overexpressed. Next, we engineered UDP-dependent glycosyltransferases (UGTs) to ensure specificity for UDP-xylose and UDP-arabinose. The E. coli strains thus obtained synthesized approximately 160 mg/liter of quercetin 3-O-xyloside and quercetin 3-O-arabinoside.

Semisynthesis of apigenin and acacetin-7-O-β-D-glycosides from naringin and their cytotoxic activities

Apigenin-7-O-β-D-glycosides 1-8 and acacetin-7-O-β-D-glycosides 9-16 were semisynthesized from 4'-O-benzyl apigenin 17 and acacetin 18 by glycosidation and deprotection with the corresponding α-acetylglycosyl bromide, respectively. Compounds 17 and 18 were prepared by iodination followed by base-induced elimination, 4'-O-benzylation, or 4'-O-methylation and acid hydrolysis using naringin as starting material which is readily available and cheap. Their cytotoxic potential against five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480) was evaluated by standard MTT method. The results show that compounds 2, 9, and 19 exhibit moderate cytotoxicity against HL-60, SMMC-7721, A-549, MCF-7, and SW480, while compound 3 exhibits potent cytotoxicity against MCF-7 selectively. Among the synthesized target compounds, 3, 4, 7, 11, 12, 15, and 16 were new compounds, the natural product 8 was the first synthesized and the synthesis of natural products 5, 6, 13, and 14 was efficiently improved by the new synthetic routes.

Synthesis of apiose-containing oligosaccharide fragments of the plant cell wall: fragments of rhamnogalacturonan-II side chains A and B, and apiogalacturonan

Fragments of pectic polysaccharides rhamnogalacturonan-II (RG-II) and apiogalacturonan were synthesised using p-tolylthio apiofuranoside derivatives as key building blocks. Apiofuranose thioglycosides can be conveniently prepared by cyclization of the corresponding dithioacetals possessing a 2,3-O-isopropylidene group, which is required for preservation of the correct (3R) configuration of the apiofuranose ring. The remarkable stability of this protecting group in apiofuranose derivatives requires its replacement with a more reactive protecting group, such as a benzylidene acetal which was used in the synthesis of trisaccharide β-Rhap-(1→3')-β-Apif-(1→2)-α-GalAp-OMe. The X-ray crystal structure of the protected precursor of this trisaccharide has been elucidated.