Compounds of Formula I are described: preferably subject to the proviso that either (a) R
2
and R
3
together form —O—CH(R
10
)—O—, or (b) R
5
and R
6
together form —O—CH(R
10
)—O—, wherein R
10
is H, halo, or loweralkyl. Pharmaceutical salts, formulations, and methods of using the same in the treatment of cancer are also described.