The invention provides compounds of the formula (I): or salts, tautomers, solvates and N-oxides thereof wherein R<1> is hydroxy or hydrogen R<2> is hydroxy methoxy or hydrogen provided that at least one of R<1> and R<2> is hydroxy R<3> is selected from hydrogen halogen cyano optionally substituted C1-5 hydrocarbyl and optionally substituted C1-5hydrocarbyloxy R4 is selected from hydrogen a group -(O)n-R<7> where n is 0 or 1 and R<7> is an optionally substituted acyclic C1-5hydrocarbyl group or a monocyclic carbocyclic or heterocyclic group having 3 to 7 ring members halogen cyano hydroxy amino and optionally substituted mono- or di-C1-5 hydrocarbyl- amino or R<3> and R<4> together form a monocyclic carbocyclic or heterocyclic ring of 5 to 7 ring members and NR<5>R<6> forms an optionally substituted bicyclic heterocyclic group having 8 to 12 ring members of which up to 5 ring members are heteroatoms selected from oxygen, nitrogen and sulphur. The compounds have activity as Hsp90 inhibitors.
