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| - A Facile and Efficient Synthesis of Thieno[2,3-c]furans andFuro[3,4-b]indoles via a Pummerer-Induced Cyclization Reaction
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| - The α-thiocarbocation generated from the Pummererreaction of an o-heteroaroyl-substitutedsulfoxide is intercepted by the adjacent keto group to produce anα-thio-substituted heteroaromaticisobenzofuran. In the presence of a suitable dienophile, thereactive o-xylylene undergoes a Diels−Alder cycloaddition followed by an acid-catalyzed ring-opening andaromatization to give heteroaromatic naphthalene derivatives. This one-pot procedure occurssmoothly with electron-deficientdienophiles. The tandem Pummerer cyclization−cycloadditionsequence also occurs intramolecularlyusing unactivated alkenyl tethers of variable length. Withacetylenic dienophiles, the primarycycloadducts undergo in situ ring-opening to producehydroxynaphthalene derivatives. In theabsence of a dienophile, it was possible to prepare4-(ethylthio)-6-phenylthieno[2,3-c]furanand1-ethyl-4-(phenylsulfonyl)-4H-furo[3,4-b]indole.Various synthetic approaches were used for thepreparation of the requisite thiophene- and indole-derived sulfoxideprecursors. The facility of thetandem Pummerer−Diels−Alder reaction was very dependent on theexperimental conditions usedto promote the reaction. The best results were achieved byemploying a mixture of acetic anhydrideand toluene which contained a catalytic quantity ofp-toluenesulfonic acid. The presence oftheacid effectively drives the reaction in the desired direction bypreventing formation of the acetoxysulfide, which corresponds to the normal Pummerer product.
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