Stereocontrolled C-O Ring Construction: The Morimoto Synthesis of (+)-Omaezakianol
Tobin J. Marks of Northwestern University observed (J. Am. Chem. Soc. 2009, 131, 263. ) high geometric control in the cyclization of 1 to 2. Tristan H. Lambert of Columbia University found (Org. Lett. 2009, 11, 1381. ) that Bi could catalyze both the addition of the ketene silyl acetal 4 to 3, and the subsequent cyclization of the secondary alcohol so formed, to give the product ether 5 with high diastereocontrol. Glenn M. Sammis of the University of British Columbia devised (Org. Lett. 2009, 11, 2019. ) a radical relay cyclization of 6 to 7, again with high diastereocontrol. Eric Fillion of the University of Waterloo established (Org. Lett. 2009, 11, 1919. ) that conjugate addition to the Meldrum’s acid derivative 8 proceeded with high stereoselectivity, delivering the useful chiron 10.
Gregory C. Fu of MIT found (Angew. Chem. Int. Ed. 2009, 48, 2225. ) that both five- and six-membered ring ethers could be formed with high enantiocontrol from alkyne alcohols such as 11. The catalyst was a chiral phosphine. Christian M. Rojas of Barnard College established (Org. Lett. 2009, 11, 1527. ) a route to 2-amino sugars such as 15, by Rh-mediated intramolecular nitrene addition in the presence of the trapping agent 14. J. S. Yadav of the Indian Institute of Chemical Technology, Hyderabad devised (Tetrahedron Lett. 2009, 50, 81. ) a route to C-glycosides such as 18, by condensation of a glycal 16 with an isonitrile 17. Michel R. Gagné of the University of North Carolina developed (Org. Lett. 2009, 11, 879. ) a complementary route to C-glycosides such as 21, with control of side chain relative configuration. Note that the addition to the methacrylate 20 is likely proceeding by initial one-electron reduction, since reductive β-elimination is not observed.
It is also possible to construct larger rings. Frank E. McDonald of Emory University devised a flexible route to protected tetraols such as 22, and showed (Org. Lett. 2009, 11, 851. ) that it could be cyclized selectively to the septanoside 23. Kenshu Fujiwara of Hokkaido University found (Tetrahedron Lett. 2009, 50, 1236. ) that ring-closing metathesis of 24 delivered the eight-membered ring product 25 in near quantitative yield.
For the synthesis of the ladder ethers, six-membered ring formation, as illustrated by the cyclization of 26 to 27, is required. Timothy F. Jamison of MIT found (J. Am. Chem. Soc. 2009, 131, 6678, ; Angew. Chem. Int. Ed. 2009, 48, 4430, ) that six-membered ring formation can best be accomplished if the cyclization is carried out in water, without catalyst. The preference for six-membered ring formation is still dominant even in cases where methyl substitution would usually direct five-membered ring formation.
Acid-catalyzed cyclization of polyepoxides such as 28 strongly favors five-membered ring formation. In that cyclization, the central reaction in the synthesis of (+)-omaezakianol (29) reported (Angew. Chem. Int. Ed. 2009, 48, 2538. ) by Yoshiki Morimoto of Osaka City University, three of the four tetrahydrofuran rings of 29 are formed in a single step, each with high stereocontrol.