Optimisation of pyrazolo[1,5-a]pyrimidin-7(4H)-one derivatives as novel Hsp90 C-terminal domain inhibitors against Ewing sarcoma

•New structural class of Hsp90 C-terminal domain inhibitors was optimised.•Compounds showed low micromolar antiproliferative activity against Ewing sarcoma.•Compound 11c inhibited the protein–protein interaction between Hsp90 and PPID.•Compound 11c lowered the levels of Hsp90 client proteins.•Compound 11c did not induce the heat shock response.