Rhodium(I)-catalyzed nucleophilic ring-opening reactions of oxabicyclo adducts derived from the [4 + 2]-cycloaddition of 2-imido-substituted furans.
A series of 2-imido-substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-ylcarbamic acid tert-butyl ester to a solution of the mixed anhydride of an appropriately substituted 3-butenoic acid. The initially formed imido furans undergo a rapid intramolecular [4 + 2]-cycloaddition at room temperature to deliver the Diels-Alder cycloadducts in good to excellent yield. Isolation of the highly labile oxabicyclic adduct is believed to be a consequence of the lower reaction temperatures employed as well as the presence of the extra carbonyl group, which diminishes the basicity of the nitrogen atom, thereby retarding the ring cleavage/rearrangement reaction generally encountered with related systems. By using a Rh(I)-catalyzed ring opening of the oxabicyclic adduct with various nucleophilic reagents, it was possible to prepare highly functionalized hexahydro-1H-indol-2(3H)-one derivatives in good yield. The major stereoisomer obtained possesses a cis-relationship between the nucleophile and hydroxyl group in the ring-opened product. The stereochemistry was unequivocally established by X-ray crystallographic analysis. Coordination of Rh(I) to the alkenyl pi-bond followed by a nitrogen-assisted cleavage of the carbon-oxygen bond occurs to furnish a pi-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting pi-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads to the cis-diastereomer.
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