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Publications (2)4.56 Total impact

  • Roy C. Flanagan, Shiping Xie, Alan Millar
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    ABSTRACT: The asymmetric synthesis of an N-acylpyrrolidine for HCV inhibition features a unique and highly selective reduction of an ester to an alcohol with NaBH4−MeOH catalyzed by NaB(OAc)3H. This reagent combination provides excellent chemoselectivity while avoiding formation of the thermodynamically favored but undesired epimer. Significant process safety issues including delayed onset of reaction initiation and latent, abrupt release of heat and hydrogen gas are encountered. The pyridine impurity responsible for the reaction inhibition is identified in the reaction calorimetry investigation. A series of reaction calorimetry and modeling studies have led to the safe design of a process which has been scaled up to 300 gallons for production of multikilogram quantities of the N-acylpyrrolidine target.
    Organic Process Research & Development - ORG PROCESS RES DEV. 10/2008; 12(6).
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    ABSTRACT: A practical asymmetric synthesis of a highly substituted N-acylpyrrolidine on multi-kilogram scale is described. The key step in the construction of the three stereocenters is a [3+2] cycloaddition of methyl acrylate and an imino ester prepared from l-leucine t-butyl ester hydrochloride and 2-thiazolecarboxaldehyde. The cycloaddition features novel asymmetric catalysis via a complex of silver acetate and a cinchona alkaloid, particularly hydroquinine, with complete diastereomeric control and up to 87% enantiomeric control. The alkaloid serves as a ligand as well as a base for the formation of the azomethine ylide or 1,3-dipole. Experiments have shown that the hydroxyl group of hydroquinine is a critical element for the enantioselectivities observed. The cycloaddition methodology is also applicable to methylvinyl ketone, providing access to either alpha- or beta-epimers of 4-acetylpyrrolidine depending on the reaction conditions utilized. The synthesis also highlights an efficient N-acylation, selective O- versus N-methylation, and a unique ester reduction with NaBH4-MeOH catalyzed by NaB(OAc)3H that not only achieves excellent chemoselectivity but also avoids formation of the undesired but thermodynamically favored epimer. The highly functionalized target is synthesized in seven linear steps from l-leucine t-butyl ester hydrochloride with all three isolated intermediates being highly crystalline.
    The Journal of Organic Chemistry 05/2008; 73(8):3094-102. · 4.56 Impact Factor