Synthesis of the 5-thiorotenoid system from thiochroman-3-one

Base-induced cyclisation of the diketones 10(X = H or F), prepared in a two-step sequence from thiochroman-3-one, affords 6H,12H-[1]benzothiopyrano[3,4-b][1]benzopyran-12-ones 11. Reduction with diisobutylaluminium hydride affords a separable mixture of the cis- and trans-5-thiorotenoids. Dieckmann cyclisation of ethyl 2-(ethoxycarbonylmethylthio)phenylethanoate affords a mixture of 4-ethoxycarbonyl- and 2-ethoxycarbonyl-thiochroman-3-ones. Attempts to prepare a thiorotenoid by condensation of these β-keto esters with 4-methoxyphenol resulted in formation of the novel bis[1]benzothiopyrano[3,2-b:4′,3′-e]pyran ring system