This invention provides compounds of Formula (I), having the structure
##STR00001## where G.sup.1, G.sup.2, G.sup.3, G.sup.4, A.sup.1,
A.sup.2, Y.sup.1, Y.sup.2, L.sup.1, Z, e and f are defined herein, or a
pharmaceutically acceptable salt thereof, which are useful for treating
or preventing cancer.