Abstract: The invention provides compounds of formula (I): and salts thereof, wherein R1, R2, R3, B, X, Y, and Z have any of the values defined herein. The invention also provides pharmaceutical compositions comprising a compound of formula (I) or a pharmaceutically acceptable salt thereof, processes for preparing compounds of formula (I) and salts thereof, intermediates useful for preparing compounds of formula (I) and salts thereof, and therapeutic methods for treating cancer using a compound of formula (I) or a pharmaceutically acceptable salt thereof.