A Versatile Stereoselective Synthesis of endo,exo-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 alpha-acetyl-gamma-butyrolactones 16 and 18-20, which reacted efficiently with in situ-generated TfN(3) to secure the key alpha-diazo-gamma-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.