Total synthesis of phorboxazole A via de novo oxazole formation: convergent total synthesis

J Am Chem Soc. 2011 Feb 9;133(5):1506-16. doi: 10.1021/ja1089099. Epub 2010 Dec 29.

Abstract

The phorboxazoles are mixed non-ribosomal peptide synthase/polyketide synthase biosynthetic products that embody polyketide domains joined via two serine-derived oxazole moieties. Total syntheses of phorboxazole A and analogues have been developed that rely upon the convergent coupling of three fragments via biomimetically inspired de novo oxazole formation. First, the macrolide-containing domain of phorboxazole A was assembled from C3-C17 and C18-C30 building blocks via formation of the C16-C18 oxazole, followed by macrolide ring closure involving an intramolecular Still-Genarri olefination at C2-C3. Alternatively, a ring-closing metathesis process was optimized to deliver the natural product's (2Z)-acrylate with remarkable geometrical selectivity. The C31-C46 side-chain domain was then appended to the macrolide by a second serine amide-derived oxazole assembly. Minimal deprotection then afforded phorboxazole A. This generally effective strategy was then dramatically abbreviated by employing a total synthesis approach wherein both of the natural product's oxazole moieties were installed simultaneously. A key bis-amide precursor to the bis-oxazole was formed in a chemoselective one-pot, bis-amidation sequence without the use of amino or carboxyl protecting groups. Thereafter, both oxazoles were formed from the key C18 and C31 bis-N-(1-hydroxyalkan-2-yl)amide in a simultaneous fashion, involving oxidation-cyclodehydrations. This synthetic strategy provides a total synthesis of phorboxazole A in 18% yield over nine steps from C3-C17 and C18-C30 synthetic fragments. It illustrates the utility of a synthetic design to form a mixed non-ribosomal peptide synthase/polyketide synthase biosynthetic product based upon biomimetic oxazole formation initiated by amide bond formation to join synthetic building blocks.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acrylates / chemistry
  • Biological Products / chemical synthesis
  • Biological Products / chemistry
  • Heterocyclic Compounds, 4 or More Rings / chemical synthesis*
  • Heterocyclic Compounds, 4 or More Rings / chemistry*
  • Macrolides / chemistry
  • Models, Molecular
  • Molecular Conformation
  • Oxazoles / chemical synthesis*
  • Oxazoles / chemistry*

Substances

  • Acrylates
  • Biological Products
  • Heterocyclic Compounds, 4 or More Rings
  • Macrolides
  • Oxazoles
  • phorboxazole A
  • acrylic acid