Flavanols, preparation

During fermentation in the preparation of black tea, oxidative polymerization of flavanols occurs with the formation of theaflavin, theaflavingallates, thearubigins, and epitheaflavic acid [44]. 

Fruit juices processing may seriously affect flavanol content. For example, the preparation of commercial apple juice decreased the flavanol content in a stepwise manner. In particular, crushing and pressing, storage of the concentrated juice at room temperature and decolorization by treatment with activated carbon destroy the flavanols almost entirely [46]. 

Flavanols are similarly accessed from 2-(2-(benzoyloxy)acetyl)phenyl benzoates 752 via a Baker-Venkataraman rearrangement to form 3-benzoyloxy flavones followed by deprotection of the hydroxyl group (Scheme 194) <2000JOC583>. 3-Aroyl flavones are prepared form or/ o-hydroxyacetophenones and aroyl chlorides in modest yield, using a Kostanecki-Robinson approach (Equation 305) <2005SC315>. 

Grape seeds are rich in polyphenols, especially flavanols (catechin, epicatechin) and gallic acid (Meyer et al., 1997). When grape extracts are prepared after crushing of the seeds, increased levels of phenols are obtained compared with extracts prepared with intact grape seeds. In particular, flavan-3-ol levels increased significantly. The antioxidant activity, as measured by the ability to inhibit LDL oxidation, also increased when the seeds were crushed prior to extraction. 

It must also be noted that the anthocyanin and the flavanol conq ete in the different steps of the reaction. Nevertheless, and due to the low proportion of the hydrated form of the anthocyanin at the pH value of the reaction medium (2.2) which considerably decreases its ability to act as a nucleophile, this competition is in favor of the flavanol which is predominantly attacked first. This was shown by detection by LC/ESI-MS of the flavanol intermediate at m/z 333 amu in the negative ion mode, while the corresponding anthocyanin intermediate was not detected. Isolation of the flavanol intermediate was achieved through HPLC at the semi-preparative scale and further reaction with malvidin 3-0-glucoside was shown to afford the two ethyl-bridged anthocyanin-flavanol adducts. 

A chiral ruthenium(II)—NHC complex catalyzes the asymmetric hydrogenation of chromones and flavones to prepare enantiomerically enriched chromanols and flavanols 49, chromanones, and flavanones 50 (Scheme 76) (13AGE8454). Chiral 2-alkylchromanones are prepared in good yields and... 

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