Total Syntheses of the Dihydrofuranonecarboxylate Natural Products Gregatin B and E: Gram-Scale Synthesis of (+)-Gregatin B and Unambiguous Assignment of the Stereostructure of (+)-Gregatin E
摘要:
We synthesized the dextrorotatory enantiomers of gregatin B (1.3 g scale) and E, establishing their diene side chains in the last step by Heck coupling with variable iodoolefins. Their counterpart was synthesized in 30% overall yield and with high enantiopurity (97% ee) from benzyl tiglate, beginning with an asymmetric dihydroxylation. The stereostructure of gregatin E followed from HPLC comparisons between the four synthetic stereoisomers of this compound and natural gregatin E, which we isolated from Cadophora gregata.
The first totalsynthesis of (+)-gregatin E and a new totalsynthesis of (+)-gregatin B are described. Key features of our synthetic approach involve a palladium-catalyzed cyclization–methoxycarbonylation of optically active propargylic acetate and a Suzuki–Miyaura coupling or CuTC-mediated coupling reaction. The absolute configuration of (+)-gregatin E (5R,5′S) is proposed.