(−)-EGCG, the most abundant catechin, was found to be chemopreventive and anticancer agent. However, (−)-EGCG has at least one limitation: it gives poor bioavailability. This invention provides compounds of generally formulae below, wherein R11, R12, R13, R21, R22, R2, R3, and R4 are each independently selected from the group consisting of —H, and C1 to C10 acyloxyl group; and R5 is selected from the group consisting of —H, C1-C10-alkyl, C2-C10-alkenyl, C2-C10-alkynyl, C3-C7-cycloalkyl, phenyl, benzyl and C3-C7-cycloalkenyl, whereas each of the last mentioned 7 groups can be substituted with any combination of one to six halogen atoms; at least one of R11, R12, R13, R21, R22, R2, R3 and R4 is —H, which were found to be more potent than their non-protected counterparts, which can be used as proteasome inhibitors to reduce tumor cell growth.