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.

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.