Furans, from alkylidenecyclopropane

Another strained alkylidenecyclopropane, 2-methylbicyclo[3.1.0]hex-l-ene (21), formed by cyclization of the carbenoid generated from dibromide 20 at 0 °C gave, in the presence of an excess of 1,3-diphenylisobenzofuran (10), a very small amount (5%) of a 2 1 mixture of diastereoisomeric Diels-Alder adducts 22 (Scheme 4) [11a]. The parent furan does not capture 21 even when used as solvent for the carbenoid cyclization. 

Trisubstituted furans can be realized from a novel palladium-catalyzed hydrofurylation of alkylidenecyclopropanes 73 <00JA2661>. 

The parent methylenecyclopropane (16, R = H) as well as alkylidenecyclopropanes react with carbon dioxide under palladium(O) catalysis to yield furan-2(5//)-ones 17 and 18. Although a complex mixture of cyclotrimers, cyclotetramers and higher oligomers is obtained from the parent MCP, under optimized conditions with regard to the palladium/phosphorus ratio, polarity of the solvent, carbon dioxide pressure and reaction temperature, the [3-f2] cycloadduct 17 (R = H) can be obtained in 80% yield. 

Tokuda and coworkers discovered easy conditions for the preparation of furans starting from fused-ring alkylidenecyclopropanes under solvent-free conditions... 

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