A quinonoid naphthopyranone as a model for the synthesis of the pigment xylindein: Photochemical formation of the lactone ring

3-Propyl-3,4,5,10-tetrahydronaphtho[2,3-c]pyran-1,5,10(1 H)- trione (5) has been synthesized with a view to determining a satisfactory pathway to its 7,9-dihydroxy-analogue (4), which it is hoped will ultimately provide a route to the extended quinonoid pigment, xylindein. A key step in the reaction sequence is the photo-rearrangement of 1,4-dimethoxy-frans- 3-pent-1 -enylnaphthalene-2-carboxylic acid (11) into the related δ-lactone (12) having the required ring system. Subsequent silver(li) oxide oxidative demethylation affords the target quinone (5)