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Monday, March 16, 2009
Douglass F. Taber
University of Delaware

Best Synthetic Methods: Carbon-Carbon Bond Construction

In the context of peptidyl ketone synthesis, Troels Skrydstrup of the University of Aarhus developed (J. Org. Chem. 2008, 73, 1088. ) the elegant SmI2-mediated conjugate addition of acyl oxazolidinones such as 1 to acceptors such as 2. Sadagopan Raghavan of the Indian Institute of Chemical Technology, Hyderabad reported (Tetrahedron Lett. 2008, 49, 1601. ) that the addition of a Pummerer intermediate, generated by exposure of 4 to TFAA, to the terminal alkene 5 and SnCl4 led to efficient C-C bond formation, to give the sulfide 6 as a single (unassigned) diastereomer.

Pd-catalyzed carbonylation of aryl halides and triflates is a well-established process. Stephen L. Buchwald of MIT has now (J. Am. Chem. Soc. 2008, 130, 2754. ) extended this transformation to much less expensive tosylates and mesylates such as 7. β-Amino acids have often been prepared from α-amino acids by Arndt-Eistert homologation. Geoffrey W. Coates of Cornell University has devised (Angew. Chem. Int. Ed. 2008, 47, 3979. ) a more practical alternative, the direct Co-catalyzed carbonylation of an oxazoline 9 to the 2-oxazine-6-one 10.

Eiji Shirakawa and Tamio Hayashi of Kyoto University also used (Chem. Lett. 2008, 37, 654. ) a Co catalyst to promote the coupling of aryl and alkenyl Grignard reagents with enol triflates such as 11. Alois Fürstner of the Max-Planck-Institut, Mülheim optimized (Chem. Commun. 2008, 2873. ) promoters for the Pd-catalyzed Stille-Migata coupling of iodo alkenes such as 14 with alkenyl stannanes such as 15 to give 16. It is particularly noteworthy that their system is fluoride free.

The stereocontrolled construction of trisubstituted alkenes continues to be challenging. We described (J. Org. Chem. 2008, 73, 1605. ) the facile preparation of the diioide 18 from the inexpensive 2-butyn-1,4-diol (17). Sequential coupling of 18 with an aryl Grignard followed by CH3Li delivered 19. Brian S. J. Blagg of the University of Kansas established (Tetrahedron Lett. 2008, 49, 141. ) that Still-Genari homologation of 20 with 21 gave (E)-22 with high geometric control. Biao Jiang of the Shangahi Institute of Organic Chemistry reported (Org. Lett. 2008, 10, 593. ) a convenient alternative protocol to give (Z)-α-bromo unsaturated esters.

Carsten Bolm of RWTH Aachen University found (Angew. Chem. Int. Ed. 2008, 47, 4862. ) that an inexpensive Fe catalyst could replace Pd in the Sonogashira coupling of 24 to 23. Hiriyakkanavar Ila of the Indian Institute of Technology, Kanpur devised (Org. Lett. 2008, 10, 965. ) a complementary route to aryl acetylenes, with concomitant ortho nitrile transfer. Matthias Brewer of the University of Vermont (29 → 31, J. Am. Chem. Soc. 2008, 130, 3766. ) and Gregory B. Dudley of Florida State University (not illustrated, J. Am. Chem. Soc. 2008, 130, 5050. ) devised fragmentation schemes for alkyne construction. Kazunori Koide of the University of Pittsburgh found (J. Org. Chem. 2008, 73, 1093. ) that exposure of an epoxide 32 to a silver acetylide 33 followed by stoichiometric Cp2ZrCl2 and catalytic AgOTf led to alcohol 34. This is as though the epoxide 32 had rearranged to the aldehyde 35, a long-sought goal of organic synthesis.

D. F. Taber, Org. Chem. Highlights 2009, March 16.