2,2'-spirobi[2H-indene]-1,1'(3'H,3H)-dione, 4,6'-dimethyl- 4,6'-dimethyl-2,2'-spirobi[indene]-1,1'(3H,3'H)-dione InChI=1/C19H16O2/c1-11-6-7-13-9-19(18(21)15(13)8-11)10-16-12(2)4-3-5-14(16)17(19)20/h3-8H,9-10H2,1-2H