Further purification of individual catechins to high purity (>98 %) in large quantity has not been easy because of the ready water solubility and the structural similarities of the catechins. A US patent described a process involving three column chromatographic separations using expensive reverse phase column fillings to purify EGCG (Bailey, 2001). A more recent patent application described a process of separating catechins using chromatography on a macroporous polar resin with a polar elution solvent under pressure (Burdick, 2003). The lack of quantities of pure catechins of high purity at reasonable cost may well hamper the clinical development of using green tea polyphenols for possible therapeutic applications. We have recently devised an alternative method of purifying catechins to high purity by treating green tea leaves directly with acetic anhydride in pyridine. This acetylation reaction converted the mixture of catechins into fully acetylated catechins (Scheme 1) and rendered them less hydrophilic and separable by simple column chromatography over silica gel with ethyl acetate/hexane as eluent. In this way, EGCG octaacetate (6), EGC hexaacetate (7), ECG heptaacetate (8) and EC pentaacetate (9) (Fig. 1) were obtained as solids with >98 % purity (Huo, 2008). The amounts of the four acetates depended on the source of green tea. Selective removal of the acetate moiety by hydrolysis using ammonium acetate in aqueous methanol gives the original catechin back. In this way, for example, EGCG (1) can be obtained from EGCG octaacetate (6) (Chan, 2005) (Scheme 1).

Chemical structures of green tea polyphenols and synthetic analogs.