Stereodivergent Synthesis of Tetrahydrofuroindoles through Pd-Catalyzed Asymmetric Dearomative Formal [3+2] Cycloaddition.

A stereodivergent synthesis of tetrahydrofuroindoles through palladium-catalyzed asymmetric dearomative formal [3+2] cycloaddition of nitroindoles with epoxybutenes was developed. The polarity of the solvent was found to play a key role in the diastereoselectivity. In toluene, good to excellent yields (70-99 %), diastereoselectivity (87/13->95/5 d.r.), and enantioselectivity (85/15-94/6 e.r.) were obtained, regardless of the properties of the substituents on nitroindoles. In acetonitrile, tetrahydrofuroindoles of a different diastereoisomer were produced with good to excellent yields (75-98 %) and stereoselectivity (78/22-93/7 d.r., 93/7-99/1 e.r.). Mechanistic studies were conducted to illustrate the origin of the diastereodivergency. The kinetic experiments indicate that the rate-determining step of this reaction is different in different solvents. ESI-MS experiments also support the existence of key palladium complex intermediates and the catalytic cycle of the reaction.