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Total synthesis of (-)-spinosyn A: examination of structural features that govern the stereoselectivity of the key transannular Diels-Alder reaction

Academic Article
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Overview

related to degree

  • Winbush, SusAnn, Ph.D. in Chemistry, Scripps Research 2005 - 2010

authors

  • Winbush, SusAnn
  • Mergott, D. J.
  • Roush, William

publication date

  • March 2008

journal

  • Journal of Organic Chemistry  Journal

abstract

  • A study of elements of stereochemical control in transannular Diels-Alder reactions leading to the decahydro-as-indacene core of (-)-spinosyn A is described. Initial studies focused on macrocyclic pentaene 9, which includes C(6)-Br and C(8)-OTBS substituents. Excellent selectivity (>95:5) was observed in the cycloaddition of 9 as a consequence of 1,3-allylic strain interactions involving the C(6) and C(8) substituents in the disfavored TS-2. The major cycloadduct 22 was used in a formal synthesis of (-)-spinosyn A. The TDA cyclizations of 12 (which lacks the C(8)-OTBS unit of 9), 13 (which lacks the C(6)-Br substituent of 12), and 14 (which lacks the C(6)-Br and C(21)-Et substituents of 12) were also studied. Macrocycles 12 and 13 served as precursors to (-)-spinosyn A and the (-)-spinosyn A aglycon (34), respectively. It is striking that substrates 12-14 give very similar distributions of transannular Diels-Alder cycloadducts, indicating that the C(6)-Br and C(21)-stereocenter do not play a significant role in the diastereoselectivity of the TDA cycloaddition of spinosyn A precursor 12. It is likely that some as yet unidentified conformational or structural features of macrocycles 12-14 contribute to the levels of diastereoselectivity achieved, since these TDA reactions are more selective for the C(7)-C(9) stereochemical relationship found in the natural product than are the IMDA reactions of trienes 4 and 7.

subject areas

  • Cyclization
  • Macrolides
  • Magnetic Resonance Spectroscopy
  • Molecular Structure
  • Spectrometry, Mass, Electrospray Ionization
  • Spectrophotometry, Infrared
  • Stereoisomerism
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Identity

International Standard Serial Number (ISSN)

  • 0022-3263

Digital Object Identifier (DOI)

  • 10.1021/jo7024515

PubMed ID

  • 18215065
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Additional Document Info

start page

  • 1818

end page

  • 1829

volume

  • 73

issue

  • 5

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