Synthesis of the cis-3a,8b-dihydrofuro[3,2-b]benzofuran-2(3H)-one ring system via a furofuran annulation to activated benzoquinones

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Abstract
The uncatalysed addition of 2-(trimethylsiloxy)furan (4) to the 1,4-benzoquinones (5a–e) containing electron-withdrawing groups at C-2 gave the cis-3a,8b-dihydrofuro[3,2-b]benzofuran-2(3H)-ones (6a–e) in 51–76% yields. The 1,4-benzoquinones (5f, g, h) without electron-withdrawing groups at C-2 failed to undergo the furofuran annulation, with none of the desired adducts (6f, g, h) being isolated. The carboxylic acid adduct (6i; R = CO2H) was prepared indirectly by reductive hydrolysis of either the phenacyl adduct (6e) or the trichloroethyl ester adduct (6d) using zinc in acetic acid. Treatment of the methyl ketone adduct (6b) with acid effected a ring opening to the corresponding (benzofuran-2-yl)acetic acid (9).