The present invention provides compounds of the formula (I) ##STR00001##
wherein: X.sub.1, X.sub.2 and X.sub.3 are heteroatoms; R.sub.4, R.sub.5,
R.sub.6, R.sub.7, R.sub.8, R.sub.9 and R.sub.10 are independently
selected from H, halo, --OH, --NO.sub.2, --CN and optionally substituted
aliphatic, cycloalkyl, heterocycloalkyl, aryl or heteroaryl; and Z is
optionally substituted C.sub.1-8 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)NR.sup.1--,
--C(O)NR.sup.1NR.sup.2--, --C(O)O--, --OC(O)--, --NR.sup.1CO.sub.2--,
--O--, --NR.sup.1C(O)NR.sup.2--, --OC(O)NR.sup.1--, --NR.sup.1NR.sup.2--,
--NR.sup.1C(O)--, --S--, --SO--, --SO.sub.2--, --NR.sup.1--,
--SO.sub.2NR.sup.1--, --NR.sup.1R.sup.2, or --NR.sup.1SO.sub.2--, wherein
R.sup.1 and R.sup.2 are independently selected from H and optionally
substituted aliphatic, cycloalkyl, heterocycloalkyl, aryl or heteroaryl;
or where R* is NR.sup.1R.sup.2, R.sup.1 and R.sup.2 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--, --SO.sub.2-- and --PO--; or a pharmaceutically
acceptable salt, ester or prodrug thereof.
