Components from South African apple cider: structure and synthesis

[摘要] Apple cider, fermented from assorted apple cultivars, contains a variety of polyphenolsincluding flavonoids. Polyphenolic compounds are critical in the design of juice products.These compounds play an important role in taste, flavor and coloration of juices and theirproducts.The cider under investigation is artificially sweetened after fermentation, using sugarcane by-products. In our investigation both commercially available sweetened, andpartially processed unsweetened products were investigated. Extraction with ethyl acetatefollowed by chromatographic separation (column Sephadex and preparative thin-layer)afforded flavonols, dihydrochalcones, flavan-3-ols, dihydroflavonols and one C6C3-typephenol. The structures of these compounds were characterized, mainly through highresolution(300 MHz) Nuclear Magnetic Resonance spectroscopy (including NOESY,COSY, DEPT and 13CNMR experiments). All the compounds isolated from the cider areknown to occur in the Malus genus (apples). One compound, taxifolin (dihydroflavonol),have not been previously isolated from apple cider.Characteristically, the phloretin glycoside, phloridzin, were isolated along with thediglycoside, 2',4',6',4-tetrahydroxy dihydrochalcone-2'-O-β-D-(6-β-D-xylopyranosyl)-β-D-glucopyranoside. Quercetin and two analogue glycosides, quereetrin and 3',4',5,7-tetrahydroxy flavonol-3-β-D-arabinopyranosyl were also isolated.Synthesis of phloridzin, which could serve as a model reaction for synthesis ofdihydrochalcones with more complex glycosides, was attempted. A glycoside wasattached to an appropriate acetophenone and used in a base-catalyzed aldol condensationwith a benzaldehyde to yield the precursor chalcone to dihydrochalcones. In a secondprocedure, the chalcone was synthesized first, usmg the same aldol-typereaction, followed by attachment of the glycoside. Although the synthesis of chalcones byan aldol condensation is a common high-yielding procedure, difficulty with thecondensation, due to the attachment of the glycosyl unit to the acetophenone wasanticipated and encountered. Synthesis of chalcone glycosides was accomplished, but noappropriate protection and deprotection protocols could be established.Flavonoids constitute an important part of polyphenols in apple juice, and their ability toinfluence a wide variety of biological functions has been asserted. Antioxidants haverecently come under much investigation and the obvious advantages to human health,makes the understanding of these compounds in apple juice and its products (e.g. cider)important. Vitamins E and C, flavonoids and other polyphenols act as primaryantioxidants, having the ability to quench superoxides, hydroxy and peroxy radicals.The results of this investigation clearly indicate the presence of compounds with potentialantioxidant properties in the cider.

[发布机构] University of the Free State