Compounds of formula (I), pharmaceutical compositions comprising compounds of formula (I) and methods of inhibiting Hsp90 in a cell, treating or preventing a proliferation disorder in a mammal and treating cancer in a mammal comprising administering a compound of formula (I) to a patient or a cell. ##STR00001## Variable R.sub.5 is an optionally substituted heteroaryl; an optionally substituted 6 to 14-membered aryl; a bicyclic 9-member heterocycle optionally substituted at any substitutable nitrogen or carbon atoms; or a substituent R.sub.18, defined herein. Ring A is an aryl or a heteroaryl optionally further substituted with one or more substituents in addition to R.sub.3. Substituent R.sub.3 is defined herein.