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À propos de : A Cycloaddition Approach toward the Synthesis of SubstitutedIndolines and Tetrahydroquinolines        

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  • A Cycloaddition Approach toward the Synthesis of SubstitutedIndolines and Tetrahydroquinolines
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  • The intramolecular Diels−Alder reaction of 2-substituted aminofurans (IMDAF) results in theformation of various indolines and tetrahydroquinolines. The isolation of these ring systems fromthe IMDAF reaction can be rationalized in terms of an initial [4 + 2]-cycloaddition that first producesan oxa-bridged cycloadduct, which was not detected since it readily underwent nitrogen-assistedring opening. Proton exchange followed by an eventual dehydration provides the aromatic product.In certain cases, the intermediate cyclohexadienol can be isolated and independently converted tothe final product in high yield. The starting 2-aminofurans were readily prepared from furanylacyl azide by a Curtius rearrangement in the presence of an alcohol. Alkylation of the resultingN-alkyl carbamate with an alkenyl bromide allows for the synthesis of a wide variety of cycloadditionprecursors. The scope of the IMDAF reaction was evaluated by using mono- and disubstitutedalkenes, electron rich and electron deficient olefins, and acetylenic tethers. Cyclic 2-amidofuranswere also synthesized using a related intramolecular Diels−Alder reaction of 2-amido-substitutedoxazoles which contain a tethered alkyne. This transformation represents a new route to this rareheterocyclic ring system. The sequential cycloaddition method was used for a formal synthesis ofthe pyrrolophenanthridone alkaloid hippadine.
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