Total Syntheses of (+)-Polygalolide A and (+)-Polygalolide B: Elucidation of the Absolute Stereochemistry and Biogenetic Implications

Stereoselective 1,4-Addition of Primary Alcohols to γ-Alkoxy-α,β-unsaturated Esters

The scope and limitations of the diastereoselective 1,4-addition reaction of primary alcohols to γ-alkoxy-α,β-unsaturated esters were investigated. We found that a variety of sodium alkoxides, generated from the corresponding primary alcohols with NaH, underwent 1,4-addition reactions with (E)-enoates in CH2Cl2 at –23 °C to give β-alkoxy esters in modest yields with good to excellent syn-selectivity, whereas stereoselectivity was not observed with the use of glycerol derivatives as nucleophiles. Cyclic acetal protection was found to play a pivotal role for the reaction to proceed.

Stereocontrolled total synthesis of polygalolide A

The total synthesis of polygalolide A, a secondary metabolite that was isolated from a Chinese medicinal plant, is reported. A key issue in this synthesis was construction of an oxabicyclo[3.2.1] skeleton, which was solved by the development of an intramolecular Ferrier-type C-glycosylation of a glucal with siloxyfuran as an internal nucleophile. The substrate was prepared from D-glucal by the introduction of trimethylsilylacetylene and siloxyfuran groups. Although C-glycosylation did not occur under the conditions found from model experiments, further examination revealed that the combination of trimethylsilyl trifluoromethanesulfonate (TMSOTf) and 2,4,6-collidine successfully afforded the desired product as a single diastereomer. The siloxy group at the C3 position played a crucial role in the stereocontrol of this reaction. The product was further transformed into a tetracyclic compound as follows: The vinyl ether and acetylenic moieties were reduced and the siloxy group was removed with a Barton-McCombie reaction. The construction of the six-membered ether and the γ-lactone provided the tetracyclic compound. Finally, a phenolic moiety was introduced by using a Mukaiyama aldol reaction to furnish polygalolide A.