people
|Faculty
|Theodore Cohen
 |
Theodore Cohen Department of Chemistry |
Our synthetic mode involves: (1) the design or discovery of new chemical reactions; (2) sequencing these so as to produce new synthetic methodology; (3) the application of the latter to particularly novel and efficient syntheses of natural products. When novel reactions are discovered, they are often probed mechanistically in order to discover new principles.
The discovery that aromatic radical-anions are capable of reductively cleaving carbon-sulfur bonds has led to some of the most versatile methods known for producing carbanions for synthetic purposes, including the simplest and most general procedures for producing a-lithioethers, a-lithiosilanes, cyclohexenyl- and cycloheptenyllithiums, homoallyllithiums, and allyllithiums. In the latter case, considerable regiochemical, as well as stereochemical, control can be exerted.
The use of a d-lithiooxyanion in the most efficient synthesis from commercially available materials of a bee pheromone 1 in its natural pure enantiomeric state is shown. The new finding that allylic oxyanionic groups both accelerate and control the stereochemistry of intramolecular carbometalations is demonstrated by highly efficient syntheses of terpenes 2 and 3 by Li-ene and Mg-ene cyclizations from a common precursor. Other natural product syntheses which are among the most efficient known are those of hirsutene and pheromones of the olive fruit fly, the Southern pine beetle, the Comstock mealybug, the house mouse, and the California red scale.
Awards
Ramsay Memorial Fellow (Great Britain), 1955-56; Chancellor's Distinguished Research Award, University of Pittsburgh, 1992.
Selected Publications
"Organolithiums by Reductive Lithiation: The Catalytic Aromatic Method vs the Use of Preformed Aromatic Radical-Anions. Naphthalene Can Behave as a Catalyst or an Inhibitor," Yang, A.; Butela, H.; Deng, K.; Doubleday, M. D.; Cohen, T., Tetrahedron, 2006, 62, 6526-6535
"Allyl Sulfones as Precursors to Allylzincs in the Palladium-Catalyzed Zinc-ene Cyclization: Highly Efficient Synthesis of Enantiopure(-)-Erythrodiene," Deng, K.; Chalker, J.; Yang, A.; Cohen, T., Org. Lett., 2005, 7, 3637-3640
"Cyclization by Intramolecular Carbolithiation of Alkyl- and Vinyllithiums Prepared by Reductive Lithiation. Surprising Stereochemistry in the Lithium Oxyanion Accelerated Cyclization," Deng, K.; Bensari, A.; Cohen, T., J. Am. Chem. Soc., 2002, 124, 12106-12107
"The Allylic Lithium Oxyanionic Directed and Facilitated Simmons-Smith Cyclopropanation: Stereoselective Synthesis of (±)-cis-Sabinene Hydrate and a Novel Ring Expansion," Cheng, D.; Kreethadumrongdat, T.; Cohen, T., Org. Lett., 2001, 3, 2121-2123
"The Magnesium-ene Cyclization Stereochemically Directed by an Allylic Oxyanionic Group and Its Application to a Highly Stereoselective Synthesis of (±)-Matatabiether. Allylmagnesium Compounds by Reductive Magnesiation of Allyl Phenyl Sulfides," Cheng, D.; Zhu, S.; Yu, Z.; Cohen, T., J. Am. Chem. Soc., 2001, 123, 30-34
"Use of Aromatic Radical-Anions in the Absence of THF. Tandem Formation and Cyclization of Benzyllithiums Derived from the Attack of Homo- and Bishomoallyllithiums on a-Methylstyrenes: Two-Pot Synthesis of Cuparene," Cohen, T.; Kreethadumrongdat, T.; Liu, X.; Kulkarni, V., J. Am. Chem. Soc., 2001, 123, 3478-3483
"Tandem Lithium-Ene Cyclization and Thiophenoxide Expulsion to Produce Fused Vinylcyclopropanes. Synthesis of dl-cis-Sabinene Hydrate Featuring Intramolecular Allylic Lithium Oxyanion-Induced Stereoselectivity and Reactivity," Cheng, D.; Knox, K. R.; Cohen, T., J. Am. Chem. Soc., 2000, 122, 412-413
"The Lithium-Ene Cyclization and the Purported Carbanion-Induced Ene Cyclization," Cheng, D.; Zhu, S.; Liu, X.; Norton, S. H.; Cohen, T., J. Am. Chem. Soc., 1999, 121, 10241-10242
"The Cyclopropylalkylidenecyclopropane Thermal Double Ring Expansion. A Novel Route to the Bicyclo[5.3.1]undecane Skeleton of the AB Ring System of Taxanes," Liu, H.; Shook, C. A.; Thiruvazhi, M., Jamison, J. A.; Cohen, T., J. Am. Chem. Soc., 1998, 120, 605-606
"Highly Efficient Terpenoid Pheromone Syntheses via Regio- and Stereocontrolled Processing of Allyllithiums Generated by Reductive Lithiation of Allyl Phenyl Thioethers," McCullough, D. W.; Bhupathy, M.; Piccolino, E.; Cohen, T., Tetrahedron, 1991, 47, 9727-9736