Bonding marine terminals

Ruthenium-catalyzed oxidative coupling of 2-aryl-substituted indoles and pyrroles with a variety of alkynes to generate structural analogs of bioactive marine alkaloids was reported (Eq. (7.26)) [34]. Ambient air was used as the terminal oxidant together with the addition of cocatalytic amounts of Cu(OAc)2 H2O in this transformation. The annulation reactions favored the electron-deficient alkyne and the more acidic C-H bond. Mechanistic experiments showed that the catalytic process took place via a concerted, acetate-assisted deprotonative metalation step. 

In 2008, Blechert et al. reported the total synthesis of e/jt-lepadin F (89) and G (90) by a tandem ene-yne-ene RCM [74]. Lepadins are members of marine alkaloids with decahydroquinoline framework. As a key step in their synthesis (Fig. 26) they planned to construct the decahydroquinoline core skeleton by a selective tandem ene-yne-ene RCM of the dienyne precursor (91). Craiceivably, two different reaction pathways could be expected (1) initiation of metathesis may occur at the terminal double bond followed by two consecutive RCMs to afford the desired 6/6 bicycle (92) or (2) initiation may occur on the disubstituted alkene followed by tandem RCMs to produce the undesired 5/7 bicycle (93). Considering the preference of initiation on monosubstituted double bond as well as the directing effect of free hydroxyl group, pathway (1) may be more favored. Gratilyingly, treatment of dienyne (91) with 10 mol% Gmbbs I catalyst smoothly provided the desired 6/6 bicycle (92) in 90% yield. 

---