The present invention provides compounds of formula:whereinRa and Rb together with the carbon to which they are attached form a group of formula:where each X1 is independently selected from hydrogen or substituted or unsubstituted R, OR, -(C=O)R, substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted aryl, and substituted or unsubstituted aralkyl wherein R is selected from the group consisting of H, OH, NO2 NH2, SH, CN, halogen, C(=O)H, C(=O)CH3, CO2H, CO2CH3, C1-C6 alkyl, alkenyl, alkynyl, aryl, aralkyl and heterocyclicwhere Rd is a substituted or unsubstituted R, OR,-(C=O)R, substituted or unsubstituted alkyl, substituted or unsubstituted alkoxy, substituted or unsubstituted alkenyl, substituted or unsubstituted alkynyl, substituted or unsubstituted aryl, or substituted or unsubstituted aralkyl wherein R is selected from the group consisting of OH, NO2, NH2, SH, CN, halogen, C(=O)H, C(=O)CH3, CO2H, CO2CH3, C1-C6 alkyl, alkenyl, alkynyl, aryl, aralkyl and heterocyclicR7 is -OCH3 and R8 is -OH or R7 and R8 together form a group -O-CH2-O-R12a is hydrogen, acyl, alkenyl, or alkyl-O-CO-R21 is -H, OH or CNX is -O- or -NH-R5 and R18 are -OX1, wherein each X1 is independently selected from hydrogen, substituted or unsubstituted C1-C18 alkyl, substituted or unsubstituted C2-C18 alkenyl, substituted or unsubstituted C2C18 alkynyl, substituted or unsubstituted aryl, and -C(=O)R wherein R is selected from hydrogen, OH, NO2, NH2, SH, CN, halogen, C1-C6 alkyl, alkenyl, alkynyl, aryl, aralkyl, and heterocyclic.
