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.
Padwa
Albert
A
Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
Wang
Qiu
Q
D001372:Aza Compounds / Q000737:chemistry; D002219:Carbamates / Q000737:chemistry; D002384:Catalysis; D005663:Furans / Q000737:chemistry*; D006574:Heterocyclic Compounds, 2-Ring / Q000138:chemical synthesis / Q000737:chemistry*; D007094:Imides / Q000737:chemistry*; D015394:Molecular Structure; D012238:Rhodium / Q000737:chemistry*
D001372:Aza Compounds; D002219:Carbamates; D005663:Furans; D006574:Heterocyclic Compounds, 2-Ring; D007094:Imides; D012238:Rhodium