A Versatile Stereoselective Synthesis of <i>e</i><i>ndo,</i><i>e</i><i>xo</i>-Furofuranones: Application to the Enantioselective Synthesis of Furofuran Lignans

A new stereoselective route to endo,exo-2,6-diarylfurofuranones has been developed using Mn(III)-mediated intramolecular cyclopropanation and C−H insertion reactions as key C−C bond-forming steps. Mn(III)-mediated oxidative cyclization of acetoacetate derivative 11 afforded 1-acetyl-4-aryl-3-oxabicyclo[3.1.0]hexan-2-one (12) with excellent diastereocontrol (d.r. 22:1). Subsequent Lewis acid-catalyzed opening of the activated cyclopropane ring present in 12 with benzylic alcohols then gave α-acetyl-γ-butyrolactones 16 and 18−20, which reacted efficiently with in situ-generated TfN3 to secure the key α-diazo-γ-butyrolactones 22−25. Highly stereoselective rhodium-catalyzed C−H insertion reactions of diazolactones 22−25 completed the synthesis of endo,exo-2,6-diarylfurofuranones 26−29 in overall yields ranging from 41 to 48% from 1-phenylallyl alcohol (±)-10. The approach developed for the furofuranones 26−29 was then applied to the asymmetric syntheses of four furofuran lignans, (+)-xanthoxylol (1), (+)-methylxanthoxylol (2), (+)-epipinoresinol (3), and (+)-epieudesmin (4), starting from enantiomerically enriched 1-arylallyl alcohol (S)-31.