A compound of formula I: Formula I or a pharmaceutically acceptable salt or ester thereof； wherein R1 is H, C1-C8 alkyl, C2-C8 alkenyl or C2-C8 alkynyl； R2 is OH or -OC (O) R3； each R3 is independently H, OR4, NH (R4), N (R4) 2, SR4, C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, C3-C8 carbocyclyl, C4-C8 carbocyclyl, C1-C8 arylalkyl , C1-C8 heterocyclylalkyl, aryl, heterocyclyl or heteroaryl； each Ra, R4 or R6 is independently C1-C8 alkyl, C2-C8 alkenyl, C2-C8 alkynyl, C3-C8 carbocyclyl, C3-C8 carbocyclyl, C1-C8 arylalkyl, C1-C8 heterocyclyl, C6-C20 heterocyclyl -C20 or heteroaryl； one of W1 or W2 is and the other of W1 or W2 is OR4 orUn compuesto de fórmula I: Fórmula I o una sal o éster farmacéuticamente aceptable del mismo； en la que R1 es H, alquilo C1-C8, alquenilo C2-C8 o alquinilo C2-C8； R2 es OH u -OC(O)R3； cada R3 es independientemente H, OR4, NH(R4), N(R4)2, SR4, alquilo C1-C8, alquenilo C2-C8, alquinilo C2-C8, carbociclilo C3-C8, carbociclilalquilo C4-C8, arilalquilo C1-C8, heterociclilalquilo C1-C8, arilo, heterociclilo o heteroarilo； cada Ra, R4 o R6 es independientemente alquilo C1-C8, alquenilo C2-C8, alquinilo C2-C8, carbociclilo C3-C8, carbociclilalquilo C3-C8, arilalquilo C1-C8, heterociclilalquilo C1-C8, arilo C6-C20, heterociclilo C2-C20 o heteroarilo； uno de W1 o W2 es y el otro de W1 o W2 es OR4 o
