Hsp90 inhibitors havin are provided havin the formula: (I) with a 2′,4′,5′-substitution pattern on the right-side aryl moiety. X1 represents two substituents, which may be the same or different, disposed in the 4′ and 5′ positions on the aryl group, wherein X1 is selected from halogen, alkyl, alkoxy, halogenated alkoxy, hydroxyalkyl, pyrollyl, optionally substituted aryloxy, alkylamino, dialkylamino, carbamyl, amido, alkylamido dialkylamido, acylamino, alkylsulfonylamido, trihalomethoxy, trihalocarbon, thioalkyl, SO2alkyl, COO-alkyl, KH2, OH, CN, SO2X5, NO2, NO, C═SR2 NSO2X5, C═OR2, where X5 is F, NH2, alkyl or H, and R2 is alkyl, NH2, NH-alkyl or O-alkyl, C1 to C6 alkyl or alkoxy; or wherein X1 has the formula —O—(CH2)n—O—, wherein n is an integer from 0 to 2, preferably 1 or 2, and one of the oxygens is bonded at the 5′-position and the other at the 4′-position of the aryl ring. The compounds are useful in cancer therapy and as radioimaging ligands.