C10H6O3

from The Collaborative International Dictionary of English v.0.48

Juglone \Ju"glone\, n. [L. juglans the walnut + -one.] (Chem.)
   A yellow crystalline substance ({C10H6O3}) resembling
   quinone, extracted from green shucks of the walnut ({Juglans
   regia}); -- called also {nucin}. Chemically, it is
   5-hydroxy-1,4-naphthalenedione.
   [1913 Webster]