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The thermal decompositions of several phenyl(trihalomethyl)mercury compounds in the presence of substituted benzaldehydes 1 and dimethyl acetylenedicarboxylate (7) in dry benzene at 80 °C resulted in 2-halo-5-arylfuran-3,4-dicarboxylates 9 in isolated yields (not optimized) that ranged from 13% to 64%. These products appear to be the result of selective attack by dihalocarbene on the aldehyde followed by preferential capture of the resulting carbonyl ylide by the acetylenic dipolarophile and loss of hydrogen halide. The substitution of diethyl fumarate (12) for acetylene 7 produced a mixture of the E and Z isomers of dihydrofuran 14 in a ratio of 70:30. Tetrasubstituted dipolarophiles failed to give comparable products. The intermediate dihalocarbonyl ylides were shown to undergo halogen exchange.

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