Xiaoxing Wu

Synthesis of (−)-Berkelic Acid

An extremophilic challenge: Stereospecific condensation of a fully functionalized ketal aldehyde and a 2,6-dihydroxybenzoic acid is the key step in the synthesis of (-)-berkelic acid confirming Fürstners reassignment of the stereochemistry at C18 and C19, establishing the absolute stereochemistry, and tentatively assigning the stereochemistry at C22.

Synthesis of the Spiroiminal Moiety of Marineosins A and B

A model for the spiroiminal moiety of marineosins A and B was prepared starting from methylvalerolactone. Addition of vinylmagnesium bromide, protection of the alcohol, and reaction of the vinyl ketone with a protected pyrrole-2-carbonitrile N-oxide gave an isoxazoline. Hydrogenolysis of the N-O bond with Raney nickel gave a keto imine that cyclized to a hemi-iminal. O-Methylation, acid-catalyzed cleavage of the TES group and spiroiminal formation, and deprotection completed a seven-step synthesis.

Introduction of the (-)-berkelic acid side chain and assignment of the C-22 stereochemistry.

A Kiyooka aldol condensation of an aldehyde with a trimethylsilyl ketene acetal and the oxazaborolidinone prepared from N-Ts-(S)-valine gives two of the four possible aldol adducts, which were oxidized and deprotected to complete the synthesis of (-)-berkelic acid and (-)-22-epi-berkelic acid. This synthesis establishes the absolute stereochemistry and assigns the stereochemistry at C-22. A biosynthetic pathway is proposed that is consistent with the known absolute stereochemistry at the quaternary carbon of spiciferone A, spicifernin, and berkelic acid and provides a simple explanation for the differing stereochemistry at C-18 and C-19 of spicifernin and berkelic acid.