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 C 1-5 hydrocarbyl and optionally substituted C 1-5 hydrocarbyloxy； R 4 is selected from hydrogen； a group -(O) n -R 7 where n is 0 or 1 and R 7 is an optionally substituted acyclic C 1-5 hydrocarbyl group or a monocyclic carbocyclic or heterocyclic group having 3 to 7 ring members； halogen； cyano； hydroxy； amino； and optionally substituted mono- or di-C 1-5 hydrocarbylamino； 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.
