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Monday, April 13, 2009
Douglass F. Taber
University of Delaware

Stereoselective C-O Ring Construction: The Oguri-Oikawa Synthesis of Lasalocid A

O-Centered radicals have been little used for C-O ring formation. Glenn M. Sammis of the University of British Columbia showed (Org. Lett. 2008, 10, 5083 ) that O-centered radicals could be generated efficiently, and that they cyclized with high diasterecontrol. Liming Zhang of the University of Nevada, Reno, continuing his studies of Au-activation of alkynes, uncovered (J. Am. Chem. Soc. 2008, 130, 12598 ) the bimolecular condensation of polarized alkynes such as 3 with aldehydes and ketones, including 4, to give the dihydrofuran with high diastereocontrol.

Margarita Brovetto of the Universidad de la República, Montevideo, Uruguay, prepared (J. Org. Chem. 2008, 73, 5776 ) the precursor to the enantiomercially triol 6 by fermentation of bromobenzene with Pseudomonas putida 39/D. Cyclization of 6 gave 7 with high diastereocontrol. Petri M. Pihko of the University of Jyväskylä, Finland, found (Org. Lett. 2008, 10, 4179 ) that cyclization of 8, prepared by Sharpless asymmetric epoxidation followed by Sharpless asymmetric dihydroxylation, also proceeded with high diastereocontrol.

Vincent Aucagne of the Université d’Orléans observed (Tetrahedron Lett. 2008, 49, 4750 ) that brief exposure of the sulfone 10 to t-BuOK at low temperature gave clean conversion to the kinetic diastereomer 11. At room temperature, similar conditions delivered the other, more stable diastereomer. Angeles Martín and Ernesto Suárez of the C. S. I. C., La Laguna, took advantage (Tetrahedron Lett. 2008, 49, 5179 ) of the facile generation of O-centered radicals in converting 12 to 14, having a stereocontrolled quaternary center. The transformation is thought to be proceeding by H-atom abstraction, then diastereocontrolled trapping of the C-radical so formed with the allyl stannane 13.

Much of the effort toward alkylated cyclic ether construction has been focused on alkyl group attachment adjacent to the ring oxygen. Torsten Linker of the University of Potsdam developed (J. Am. Chem. Soc. 2008, 130, 16003 ) a complementary approach, stereocontrolled oxidative radical addition of malonate 16 to glycals such as 15 to give the 3-alkyl substituted 17.

Richard C. Hartley of the University of Glasgow established (Tetrahedron Lett. 2008, 49, 4771 ) what promises to be a powerful strategy for the convergent assembly of spiro ketals, based on the condensation of the Ti alkylidene derived from a thioacetal such as 19 with a lactone such as 18. James A. Marshall of the University of Virgina nicely reduced to practice (J. Org. Chem. 2008, 73, 6753 ) the preparation and reductive cyclization of polyepoxides such as 22. Hiroki Oguri and Hideaki Oikawa of Hokkaido University demonstrated (J. Am. Chem. Soc. 2008, 130, 12230 ) that overexpressed enzyme Lsd19 converted 24 to lasolacid A (26). With acid, 24 cyclized to 25.

D. F. Taber, Org. Chem. Highlights 2009, April 13.