Total syntheses of echinopines A and B, sesquiterpenes with a unique tetracyclic [3-5-5-7] skeleton

The Total Synthesis of Diquinane‐Containing Natural Products

“…Based on the diquinane diol precursor 146 , both racemic and asymmetric total syntheses of pepluanol A were achieved for the first time, in which the conformation of the eight‐membered ring system was exclusively controlled by a bromo‐epoxide structure (Scheme 28 a). Literature precedents depicted in Scheme 28 b, together with our work, demonstrated that bicyclic diol 146 is a versatile building block for the synthesis of diquinane‐containing natural products, [75, 76] such as kelsoene, sulcatine G, cylindramide, and echinopines A and B [77] …”

Ready Access to the Echinopines Skeleton via Gold(I)-Catalyzed Alkoxycyclizations of Enynes

“…This complex carbon framework holds five contiguous stereogenic centers, two of them being adjacent quaternary stereocenters. Despite the fact that no biological activity has been reported to date for 1 and 2 , the unique architecture of these sesquiterpenes has constituted an appealing challenge for the synthetic community and several syntheses of echinopines have been accomplished to date. − The key feature in all these syntheses is the establishment of the unique [3,5,5,7] skeleton, and to this aim conceptually very different ring-forming sequences have been successfully established . However, the assembly of the complex polycyclic framework of the echinopines skeleton is not easily addressed by conventional methods, as evidenced by the lengthy existing syntheses, and it is in most of the cases delayed to one of the last steps of the sequence.…”

“…2−7 The key feature in all these syntheses is the establishment of the unique [3,5,5,7] skeleton, and to this aim conceptually very different ring-forming sequences have been successfully established. 8 However, the assembly of the complex polycyclic framework of the echinopines skeleton is not easily addressed by conventional methods, as evidenced by the lengthy existing syntheses, and it is in most of the cases delayed to one of the last steps of the sequence.…”