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

Total syntheses of teubrevins G (2) and H (3) are described. The reported strategy relies on a highly regioselective cycloaddition-fragmentation approach to the construction of a 2,3,4-trisubstituted furan and features efficient 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 realized. The diastereoselection observed in this alkylation reaction appears to be intimately associated with the conformational properties of the beta -keto ester enolate. While the readily separated major diastereomer was transformed via a short route to 2, the minor component served as the precursor to 3. The efficiency of the synthesis was thereby well served.