Synthesis of a muscarinic receptor antagonist via a diastereoselective Michael reaction, selective deoxyfluorination and aromatic metal-halogen exchange reaction

J Org Chem. 2001 Oct 5;66(20):6775-86. doi: 10.1021/jo0157425.

Abstract

An efficient synthesis of a structurally unique, novel M(3) antagonist 1 is described. Compound 1 is conveniently disconnected retrosynthetically at the amide bond to reveal the acid portion 2 and the amine fragment 3. The synthesis of key intermediate 2 is highlighted by a ZnCl(2)-MAEP complex 19 catalyzed diastereoselective Michael reaction of dioxolane 7 with 2-cyclopenten-1-one (5) to establish the contiguous quaternary-tertiary chiral centers and a subsequent geminal difluorination of ketone 17 using Deoxofluor in the presence of catalytic BF(3).OEt(2). The synthesis of the amine moiety 3 is highlighted by the discovery of a novel n-Bu(3)MgLi magnesium-halogen exchange reaction for selective functionalization of 2,6-dibromopyridine. This new and practical metalation protocol obviated cryogenic conditions and upon quenching with DMF gave 6-bromo-2-formylpyridine (26) in excellent yield. Further transformations afforded the amine fragment 3 via reductive amination with 35, Pd-catalyzed aromatic amination, and deprotection. Finally, the highly convergent synthesis of 1 was accomplished by coupling of the two fragments. This synthesis has been used to prepare multi-kilogram quantities of the bulk drug.

MeSH terms

  • Amides / chemical synthesis
  • Animals
  • Humans
  • Hydrocarbons, Fluorinated / chemical synthesis
  • Muscarinic Antagonists / chemical synthesis*
  • Receptor, Muscarinic M3
  • Receptors, Muscarinic / drug effects
  • Stereoisomerism

Substances

  • Amides
  • Hydrocarbons, Fluorinated
  • Muscarinic Antagonists
  • Receptor, Muscarinic M3
  • Receptors, Muscarinic