Dimethyl fumarate reaction with carbenes

Thermal decomposition of the diazo compounds 116a,b in methanol or ethanol, in the presence of Cu(acac)2 as catalyst, resulted in an intramolecular carbene reaction the benzo[c]furans 117a,b were not isolated, but 117b could be trapped with A-phenylmaleimide and dimethyl acetylenedicarboxylate, to give compounds 118 (endo exo =1 1, 90%) and 121 (92%). Dimethyl fumarate yielded adduct 119 (15%) and supposedly 120 (83%) concerning this structure the same objections can be raised as in the case of... 

Transition-metal mediated carbene transfer from 205 to benzaldehyde generates carbonyl ylides 211 which are transformed into oxiranes 216 by 1,3-cyclization, into tetrahydrofurans 212, 213 or dihydrofurans 214 by [3 + 2] cycloaddition with electron-deficient alkenes or alkynes, and 1,3-dioxolanes 215 by [3 + 2] cycloaddition with excess carbonyl compound120 (equation 67). Related carbonyl ylide reactions have been performed with crotonaldehyde, acetone and cyclohexanone (equation 68). However, the ylide generated from cyclohexanone could not be trapped with dimethyl fumarate. Rather, the enol ether 217, probably formed by 1,4-proton shift in the ylide intermediate, was isolated in low yield120. In this respect, the carbene transfer reaction with 205 is not different from that with ethyl diazoacetate121, whereas a close analogy to diazomalonates is observed for the other carbonyl ylide reactions. 

By contrast, 1,2,4-triazole carbene 354 displays electrophilic character. Thus, it reacts with alcohols and amines producing triazoline derivatives 355 in quantitative yields. Oxygen or sulfur gives triazolinone and triazolinethione derivatives 356 (similar reaction with tellurium is known for imidazol-2-ylidenes). Reactions of 354 with dimethyl maleate or dimethyl fumarate lead to compounds 357, probably via ring opening of a cyclopropane intermediate with subsequent 1,2-hydrogen shift. 

Diphenylcyclopropenylidene (391) reacts with electrophilic olefins such as dimethyl fumarate and maleate only. The fact that the same spiropentene 393 (stereochemistry unknown) is isolated from reaction of 391 with both alkenes (equation 111) supports the intervention of zwitterion 392 and augments the concept of cyclopropenylidenes being very nucleophilic carbenes. 

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