The minor theaflavins of black tea

This thesis describes the isolation, characterisation and synthesis of three novel tea pigments. This isolation was achieved by the solvent extraction of a Malawian black tea (Ruo Mimosa) followed by the chromatography of the ethyl acetate-soluble portion on Sephadex LH-20 (ethanol elution), which produced three fractions containing orange pigments. Further separation of these fractions under various chromatographic conditions yielded ten orange pigments ( 1, 3, 4, 5, 6, 7, 8. 9, 10, 11). These compounds have been structurally characterised by a number of spectroscopic techniques including ID and 2D NMR. Theaflavate B (1) has been shown to be formed from (-)-epicatechin-3-0-gallate (ECG) and (-)-epicatechin (EC). This is the first compound to be isolated from black tea which contains a benzotropolone moiety formed from a galloyl ester group of a flavan-3-ol gallate. In addition, the theaflavin isomers, isotheaflavin-3'-0-monogallate (3) and neotheaflavin-3-(9-monogallate ( 4) have also been characterised. Compound 5 was identified as isotheaflavin, compound 6 as epitheaflagallin-3-O-monogallate, compound 10 as theaflavic acid and compound 11 as epitheaflavic acid. Compounds 7 - 9 were found to be theaflavin, theaflavin-3-O-monogalIate and theaflavin-3'-O-monogallate. This thesis also describes the investigation of the formation of theaflavins and related compounds under chemical oxidation conditions. The results clearly showed the importance of size (galloylation of flavan-3-ol at the 3 position on the C-ring) and configuration (2,3-cis vs 2,3-trans) of the flavan-3-ol precursors in determining the yields of these compounds. An attempt was made using molecular modelling techniques, to further our understanding of the relative yields of theaflavins obtained in chemical oxidation studies of flavan-3-ols.