Components from South African apple cider: structure and synthesis
Apple cider, fermented from assorted apple cultivars, contains a variety of polyphenols including 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 their products. The cider under investigation is artificially sweetened after fermentation, using sugar cane by-products. In our investigation both commercially available sweetened, and partially processed unsweetened products were investigated. Extraction with ethyl acetate followed by chromatographic separation (column Sephadex and preparative thin-layer) afforded flavonols, dihydrochalcones, flavan-3-ols, dihydroflavonols and one C6C3-type phenol. 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 are known 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 the diglycoside, 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 of dihydrochalcones with more complex glycosides, was attempted. A glycoside was attached to an appropriate acetophenone and used in a base-catalyzed aldol condensation with a benzaldehyde to yield the precursor chalcone to dihydrochalcones. In a second procedure, the chalcone was synthesized first, usmg the same aldol-type reaction, followed by attachment of the glycoside. Although the synthesis of chalcones by an aldol condensation is a common high-yielding procedure, difficulty with the condensation, due to the attachment of the glycosyl unit to the acetophenone was anticipated and encountered. Synthesis of chalcone glycosides was accomplished, but no appropriate protection and deprotection protocols could be established. Flavonoids constitute an important part of polyphenols in apple juice, and their ability to influence a wide variety of biological functions has been asserted. Antioxidants have recently 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 primary antioxidants, having the ability to quench superoxides, hydroxy and peroxy radicals. The results of this investigation clearly indicate the presence of compounds with potential antioxidant properties in the cider.