Abstract: Compounds of formula (I) are HSP90 inhibitors, of utility in the treatment of, for example, cancers: wherein R1 is -(Alk1)p-(Z)r(Alk2)-Q wherein AIk1 and AIk2 are optionally substituted divalent C1C3 alkylene or C2-C3 alkenylene radicals, p, r and s are independently 0 or 1, Z is —O—, —S—, —(C?O)—, —(C?S)—, —SO2—, —C(?O)O—, —C(?O)NRA, —C(?S)NRA-, —SO2NRA—NRAC(?O)—, —NRASO2— or —NRA— wherein RA is hydrogen or C1-C6 alkyl, and Q is hydrogen or an optionally substituted carbocyclic or heterocyclic radical; R2 is optionally substituted aryl or heteroaryl; R3 is hydrogen, an optional substituent, or an optionally substituted (C1-C-6)alkyl, aryl or heteroaryl radical; and R4 is (i) a carboxylic ester, carboxamide or sulfonamide group, or (ii) a —CN group, or (iii) an optionally substituted C1-C6alkyl, aryl, heteroaryl, aryl(C1-C6alkyl)-, or heteroaryl(C1-C6alkyl)- group, or (iv) a group of formula —C(?O)R5 wherein R5 is hydroxyl, optionally substituted C1-C6alkyl, C1-C6alkyoxy, aryl, aryloxy, heteroaryl, heteroaryl