: [0082] R2 is a member selected from the group consisting of formulae (a)-(f); [0083] R is hydrogen, C1-4 alkyl, C1-4 alkylaryl, or CO2R9; [0084] W is OH or OCOR9; [0085] R3 is hydrogen, or methyl; [0086] R4 is hydrogen or methyl; [0087] R5, R6, R7 and R8 are each, independently, H or C1-4 alkyl. [0088] A most preferred compound of the present invention is compound 12 and 13 of FIG. 1. [0089] The