Novel purine compounds of Formula I. ##STR00001## and tautomers, pharmaceutically acceptable salts, and prodrugs thereof, wherein X is S, S(O), or S(O).sub.2; and O is selected from alkyl, cycloalkyl, arylalkyl, aryl, heteroaryl, and heterocyclic, all optionally substituted, are described, as are pharmaceutical compositions comprising the same, complexes comprising the same, e.g., HSP90 complexes, and methods of using the same.