The structures of two derivatives of bicyclo[3.3.1]nonane-2, 4, 9-trione. A natural product: clusianone, C33H42O4, and trimethylated catechinic acid, C18H20O6 - McCandlish, L E; Hanson, J C; Stout, G H (Acta Crystallogr B, 1976)