Indoles reaction with acetylenic esters

N-Methyl- and N-phenyl-2-vinylpyrroles 20a,b react with DMAD at reflux temperature in chloroform to give, in moderate yields, the dihydroindoles 22 via a 1,3-H shift from the Diels-Alder intermediate 21 (55-75%) (80JOC4515). These adducts were readily converted into the corresponding indoles 23 with Dichlorodicyanoquinone (DDQ). 2-Vinyl-pyrrole failed to give [4 + 2]-cycloadducts with acetylenic esters (80JOC4515). Spectroscopic analysis of the product mixtures indicated the presence of polymeric compounds resulting from Michael addition reactions. 

The cycloaddition reactions of 3-vinylindoles with acetylenic esters have been extensively studied. The reaction products generally obtained are [Z ]-annelated indoles, but Michael adducts can be isolated, depending on the substituents on the vinylindole (87HCA1419) (Scheme 1). 

Reactions between nitrogen-containing heterocycles and acetylenic esters often lead to complex mixtures, which may require chromatography and great experimental skill for successful resolution. The products sometimes depend on the precise conditions employed, and on the purity of the reactants or solvents. The choice of solvent can be important. Thus ether and tetrahydrofuran give different results for the combination of DM AD with 1-methylbenzotriazole. The products obtainable from indoles and quinazolines are particularly dependent on the reaction conditions and purity of the solvent. 

Acetylenecarboxylic esters, reactions with nitrogen-containing heterocycles, 23, 263 Acetylenic derivatives of pyrazoles, synthesis and properties of, 82, 1 Acetylenic esters, synthesis of heterocycles through nucleophilic additions to, 19,297 Acid-catalyzed polymerization of pyrroles and indoles, 2, 287... 

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