Rhodium(I)-Catalyzed Nucleophilic Ring Opening Reactions of Oxabicyclo Adducts Derived from the [4+2]-Cycloaddition of 2-Imido Substituted Furans

• Authors: Albert Padwa, Emory University; Qiu Wang, Emory University
• Journal Title: Journal of Organic Chemistry Research
• Volume: Volume 71, Number 8
• Publisher: American Chemical Society | 2006-04-14, Pages 3210-3220
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1021/jo060238r
• Subjects: Chemistry, Organic

Abstract

A series of 2-imido substituted furans containing tethered unsaturation were prepared by the addition of the lithium carbamate of furan-2-yl carbamic 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-1 H-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.