Teubrevin G and Teubrevin H: The first total syntheses of rearranged neo-clerodanes including solutions to the problems of chirality merger and furanring assembly

Total syntheses of teubrevins G (2) and H (3) are described. The reported s trategy relies on a highly regioselective cycloaddition-fragmentation appro ach to the construction of a 2,3,4-trisubstituted furan and features effici ent ring-closing metathesis chemistry made possible through the application of a 1.3-dimesityl-4,5-dihydroimidazol-2-ylideneruthenium precatalyst. The key building blocks 39 and 48 were constructed by asymmetric processes and coupled under conditions where good remote asymmetric induction was realiz ed. The diastereoselection observed in this alkylation reaction appears to be intimately associated with the conformational properties of the beta -ke to ester enolate. While the readily separated major diastereomer was transf ormed via a short route to 2, the minor component served as the precursor t o 3. The efficiency of the synthesis was thereby well served.