Chemoselective cycloadditions of 3,4-dialkoxyfurans and alkyl coumalates. Novel loss of aromaticity of two nonbenzenoid aromatic rings in a mild thermal process

Alkyl coumalates 1 can serve as either dienes or dienophiles in cycloadditions depending on their reaction partners. They have often been used as dienophiles with simple substituted butadienes 2, giving good yields of the Diels-Alder adducts 3 involving the 5,6-double bond of the pyrone as the dienophile, a useful process for trichothecane synthesis3 However, with highly electron-rich olefins, e.g., the ketene acetal 4, they react as dienes in [4 + 21 cycloadditions affording good yields of the bicyclic lactones 5.4