The present invention provides compounds of the formula (I) wherein: X1, X2 and X3 are heteroatoms; R4, R5, R6, R7, R8, R9 and R10 are independently selected from H, halo, —OH, —NO2, —CN and optionally substituted aliphatic, cycloalkyl, heterocycloalkyl, aryl or heteroaryl; and Z is optionally substituted C1-8 straight-chained or branched aliphatic, optionally containing 1 or more double or triple bonds, wherein one or more carbons are optionally replaced by R* wherein R* is optionally substituted cycloalkyl, heterocycloalkyl, aryl or heteroaryl; an amino acid residue, H, —CN, —C(O)—, —C(O)C(O)—, —C(O)NR1—, —C(O)NR1NR2—, —C(O)O—, —OC(O)—, —NR1CO2—, —O—, —NR1C(O)NR2—, —OC(O)NR1—, —NR1NR2—, —NR1C(O)—, —S—, —SO—, —SO2—, —NR1—, —SO2NR1—, —NR1R2, or —NR1SO2—, wherein R1 and R2 are independently selected from H and optionally substituted aliphatic, cycloalkyl, heterocycloalkyl, aryl or heteroaryl; or where R* is NR1R2, R1 and R2 optionally together with the nitrogen atom form an optionally substituted 5-12 membered ring, said ring optionally comprising 1 or more heteroatoms or a group selected from —CO—, —SO—, —SO2— and —PO—; or a pharmaceutically acceptable salt, ester or prodrug thereof.