Total synthesis of (+/-)- and (-)-actinophyllic acid

J Am Chem Soc. 2010 Apr 7;132(13):4894-906. doi: 10.1021/ja100178u.

Abstract

Development of efficient sequences for the total syntheses of (+/-)-actinophyllic acid (rac-1) and (-)-actinophyllic acid (1) are described. The central step in these syntheses is the aza-Cope/Mannich reaction, which constructs the previously unknown hexacyclic ring system of actinophyllic acid in one step from much simpler tetracyclic precursors. The tetracyclic hexahydro-1,5-methano-1H-azocino[4,3-b]indole ketone rac-37 is assembled from o-nitrophenylacetic acid in four steps, with oxidative cyclization of a dienolate derivative of tricyclic precursor rac-35 being the central step. In the first-generation synthesis, this intermediate is transformed in two steps to homoallyl amine rac-43, whose formaldiminium derivative undergoes efficient aza-Cope/Mannich reaction to give pentacyclic ketone rac-44. In four additional steps, this intermediate is advanced to (+/-)-actinophyllic acid. The synthesis is streamlined by elaborating ketone rac-37 to beta-hydroxyester intermediate rac-53, which is directly transformed to (+/-)-actinophyllic acid upon exposure to HCl and paraformaldehyde. This concise second-generation total synthesis of (+/-)-actinophyllic acid is realized in 22% overall yield from commercially available di-tert-butyl malonate and o-nitrophenylacetic acid by a sequence that proceeds by way of only six isolated intermediates. The first enantioselective total synthesis of (-)-actinophyllic acid (1) is accomplished by this direct sequence from tricyclic keto malonate (S)-35. Catalytic enantioselective reduction of alpha,beta-unsaturated ketone 66 is the key step in the preparation of intermediate (S)-35 from the commercially available Boc-amino acid 65. Discussed also is the possibility that the aza-Cope/Mannich reaction might be involved in the biosynthesis of (-)-actinophyllic acid.

Publication types

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

MeSH terms

  • Crystallography, X-Ray
  • Indole Alkaloids / chemical synthesis*
  • Indole Alkaloids / chemistry
  • Models, Molecular
  • Molecular Structure
  • Stereoisomerism

Substances

  • Indole Alkaloids
  • actinophyllic acid