Diels-Alder reactions of fumarates

Comparing cis and trans isomers of propenyl propyl ether in their reactivity towards diphenylketene, a ratio kcislknans of about 180 has been found (in PhCN, 40°C) , confirmed by the value of 172 20 from experiments without solvent with dimethylketene the ratio is 60 10 . The opposite phenomenon has been observed in Diels-Alder reactions of fumarate/maleate and other 1,2-disubstituted ethylenes (Section 4.1.3). The fact that cis isomers are thermodynamically less stable than trans isomers seems to prevail in determining relative reactivities of these 1,2-cycloadditions. 

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