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Monday, June 18, 2012
Douglass F. Taber
University of Delaware

C-C Bond Formation: The Petrov Synthesis of Combretastatin A-4

Janine Cossy of ESPCI Paris (Org. Lett. 2011, 13, 4084. ) and Yasushi Obora of Kansai University (Chem. Lett. 2011, 40, 1055. ) independently developed conditions for the "borrowed hydrogen" condensation of acetonitrile with an alcohol 1 to give the nitrile 2. Akio Baba of Osaka University showed (Angew. Chem. Int. Ed. 2011, 50, 8623. ) that a ketene silyl acetal 4 could be condensed with a carboxylic acid 3 to give the β-keto ester 5. Timothy W. Funk of Gettysburg College found (Tetrahedron Lett. 2010, 51, 6726. ) that the cyclopropanol 6, readily prepared by Kulinkovich condensation of an alkene with an ester, opened with high regioselectivity to give the branched ketone 7. In an elegant application of C-H functionalization, Yong Hae Kim of KAIST and Kieseung Lee of Woosuk University added (Tetrahedron Lett. 2011, 52, 4662. ) the acetal 9 in a conjugate sense to 8 to give 10.

Hitoshi Kuniyasu and Nobuaki Kambe of Osaka University devised (Tetrahedron Lett. 2010, 51, 6818. ) conditions for the Pd-catalyzed carbonylation of a silyl alkyne 11 to the ester 12, with high geometric control. Dennis G. Hall of the University of Alberta also observed (Chem. Sci. 2011, 2, 1305. ) good geometric control in the rearrangement of the vinyl carbinol 13 to the alcohol 14. Takashi Tomioka of the University of Mississippi condensed (J. Org. Chem. 2011, 76, 8053. ) the anion 16, prepared in situ from lithio acetonitrile and 1-iodobutane, with the aldehyde 15 to give a nitrile, that was carried on to the aldehyde 17, again with good control of geometry. Bruce H. Lipshutz of the University of California, Santa Barbara established (Org. Lett. 2011, 13, 3818. ) conditions for the Negishi coupling of an alkenyl halide 18 to give 20 with retention of alkene geometry.

Daesung Lee of the University of Illinois, Chicago found (J. Am. Chem. Soc. 2011, 133, 12964. ) that a Pt catalyst rearranged a silyl cyclopropene 21 to the allene 22. Jan Deska of the Universität zu Köln prepared (Angew. Chem. Int. Ed. 2011, 50, 9731. ) the enantiomerically-enriched allene 25 by lipase-mediated esterification of the prochiral 23.

Masaharu Nakamura of Kyoyo University described (Angew. Chem. Int. Ed. 2011, 50, 10973. ) an iron catalyst for the coupling of a halide 26 with 27 to give the alkyne 28. Xile Hu of the Ecole Polytechnique Fédérale de Lausanne reported (Angew. Chem. Int. Ed. 2011, 50, 11777. ) similar results with a nickel catalyst. Mustafa Eskici of Celal Bayar University showed (Tetrahedron Lett. 2011, 52, 6336. ) that a secondary cyclic sulfamidate 29 could be opened with an acetylide 30 to give 31.

Colvin showed (J. Chem. Soc., Perkin Trans. 1 1977, 869. ) that ketones could be converted to alkynes by exposure to lithio trimethylsilyldiazomethane 33. Ognyan I. Petrov of the University of Sofia used (Synthesis 2011, 3711. ) this transformation to good effect, converting 32 into 34 en route to Combretastatin A-4 (35).

D. F. Taber, Org. Chem. Highlights 2012, June 18.