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 π-bond
followed by a nitrogen-assisted cleavage of the carbon−oxygen bond occurs to furnish a π-allyl rhodium(III) species. Addition of the nucleophile then occurs from the least hindered terminus of the resulting
π-allyl rhodium(III) complex. Proton exchange followed by rhodium(I) decomplexation ultimately leads
to the cis-diastereomer.