3- indole, reaction with methyl maleate

The reaction with dienophiles of the sulfur-substituted 3-vinylpyrrole, generated in situ by the alkylation of the 3-thioacetylpyrrole, has also been utilized to obtain indoles. Reactions with dimethyl maleate, dimethyl fumarate, methyl acrylate, acrylonitrile, and acrolein gave the corresponding cycloadducts which were directly transformed to the corresponding indoles by treatment with DDQ (91CPB489). 

The stereochemistry of the cycloaddition has been studied through the reaction with methyl fumarate and maleate. The geometry is preserved and trans- and d.v-adducts are obtained. The di-adduct isomerizes rapidly into the frans-adduct on standing at room temperature. Indications of the regiochemistry were obtained from reaction with acrylonitrile which gives 2,3-dihydro-l-cyano-9-methyl-lH-pyrrolo[l,2a]indole exclusively.407,408... 

Another pioneer in the Diels-Alder reactions of vinylpyrroles was Noland, who also developed the reactions of vinylindoles to yield carbazoles. Some examples of the former are shown in Scheme 2 (equations 1 and 2) [4-7], Jones and his colleagues were equally active in this cycloaddition chemistry of vinylpyrroles (equations 3 and 4) [8-12], These workers measured the rates of the reaction between 1-methyl-2-vinylpyrrole and seven dienophiles, with maleic anhydride being 4800 to 50,000 times more reactive than the other dienophiles (DMAD, maleonitrile, fumaronitrile, dimethyl maleate, methyl acrylate, and acrylonitrile) [8], In a clever tactic to thwart the formation of dihydroindoles, Jones used an excess of methyl propio-late to convert the initial adduct to a second Diels-Alder cycloadduct that subsequently loses ethene by a retro-Diels-Alder reaction to afford the dimethyl 1-methyl (phenyl)-4,7-dicarboxylates (equation 4). The reactions are concerted and were consistent with FMO calculations (HOMO[vinylpyrrole]-LUMO[alkene]). The yields are 54% to 81%, but attempts to dehydrogenate the tetrahydroindole products to indoles were unsuccessful. 2-Vinylpyrrole itself undergoes Michael additions and polymerization with these dienophiles. Domingo, Jones, and coworkers subsequently... 

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