Transition Metal-mediated Ring Construction: The Yu Synthesis of 1-Desoxyhypnophilin
Both 1 and 3 are inexpensive prochiral starting materials. Tae-Jong Kim of Kyungpook National University devised (Organomet. 2008, 27, 1026. ) a chiral Cu catalyst that efficiently converted 1 (other ring sizes worked as well) to the enantiomerically pure ester 2. Alexandre Alexakis of the University of Geneva found (Adv. Synth. Catal. 2008, 350, 1090. ) a chiral Cu catalyst that mediated the enantioselective coupling of 3 with Grignard reagents such as 4. The π-allyl Pd complex derived from 6 is also prochiral. Barry M. Trost of Stanford University showed (Angew. Chem. Int. Ed. 2008, 47, 3759. ) that with appropriate ligand substitution, coupling with the phthalimide 7 proceeded to give 8, readily convertible to (-)-oseltamivir (Tamiflu) (9), in high ee.
Jonathan W. Burton of the University of Oxford found (Chem Commun. 2008, 2559. ) that Mn(OAc)3-mediated cyclization of 10 delivered the lactone 12 with high diastereocontrol. John Montgomery of the University of Michigan observed (Org. Lett. 2008, 10, 811. ) that the Ni-catalyzed cyclization of 12 also proceeded with high diastereocontrol. Ken Tanaka of the Tokyo University of Agriculture and Technology combined (Angew. Chem. Int. Ed. 2008, 47, 1312. ) Rh-catalyzed ene-yne cyclization of 14 with catalytic ortho C-H functionalization, leading to 16 in high ee.
Eric N. Jacobsen of Harvard University designed (Angew. Chem. Int. Ed. 2008, 47, 1469. ) a chiral Cr catalyst for the intramolecular carbonyl ene reaction, that converted 17 to 18 in high ee. Using a stoichiometric prochiral Cr carbene complex 20 and the enantiomerically-pure secondary propargylic ether 19, Willam D. Wulff of Michigan State University prepared (J. Am. Chem. Soc. 2008, 130, 2898. ) a facially-selective Cr-complexed o-quinone methide intermediate, that cyclized to 21 with high ee.
A variety of methods have been put forward for the transition metal-mediated construction of polycarbocyclic systems. One of the more powerful is the enantioselective Rh-catalyzed stitching of the simple substrate 22 into the tricycle 23 devised (J. Am. Chem. Soc. 2008, 130, 3451. ) by Takanori Shibata of Waseda University. Inter alia, ozonolysis of 23 delivered the cyclopentane 24 containing two all-carbon quaternary centers. Zhi-Xiang Yu of Peking University has devised both carbonylative (J. Am. Chem. Soc. 2008, 130, 4421. ), illustrated, and non-carbonylative (J. Am. Chem. Soc. 2008, 130, 7178. ) Rh-catalyzed cylization of alkenyl cyclopropanes such as 26. Intramolecular aldol condensation followed by further functionalization converted 27 into 1-desoxyhypnophilin (28).