University of Illinois Urbana-Champaign

Total synthesis of 9,10-syn copalol, an intermediate in the biosynthesis of 9,10-syn diterpenes

Yee, Nathan Kwok Nang

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Description

Title
Total synthesis of 9,10-syn copalol, an intermediate in the biosynthesis of 9,10-syn diterpenes
Author(s)
Yee, Nathan Kwok Nang
Issue Date
1991
Doctoral Committee Chair(s)
Coates, Robert M.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Organic
Language
eng
Abstract
  • A total synthesis of 9,10-syn copalol (181) from silyl phosphonate 76 was accomplished in 11 steps with an overall yield of 6.6% (Scheme 54). The key transformation involved electrophilic cyclization of epoxy silyl triene 158 to construct the 9,10-syn bicyclic nucleus with the exocyclic methylene group in 181. The cyclization of 158 afforded a mixture of 9,10-syn- and 9,10-anti isomers 166 and 167 which were separated by preparative HPLC. A mechanism, involving an early transition state for the allyl silane ring closure step is proposed to rationalize type low stereo-selectivity observed in the cyclizations of the epoxy silanes. In the course of this synthesis, procedures were developed for direct C$\sb{10}$-C$\sb{10}$ Biellmann coupling of epoxy sulfone 155 and allylic bromide 103, and subsequent regioselective reduction with lithium in EtNH$\sb2$-Et$\sb2$O.
  • Tritium-labelled 9,10-syn CPP (222), ent-CPP (224), (E,Z,E)-GGPP (226), and (E,E,E)-GGPP (228) were prepared and tested as intermediates in the biosynthesis of 9,10-syn diterpene. Preliminary results, obtained in the laboratory of Professor West at UCLA, from the incubations of these substrates with cell-free enzyme extracts from elicited rice tissue (Oryza sativa vc Shesheniski) demonstrated that 9,10-syn CPP is efficiently incorporated into a hydrocarbon presumed to be 9$\beta$H-pimara-7,15-diene (17), the biosynthetic precursor for the momilactone phytoalexins. In addition, (E,E,E)-GGPP, and not (E,Z,E)-GGPP, proved to be the acyclic substrate for the diterpene cyclase that produced 9,10-syn CPP. These enzymatic experiments indicate that 9,10-syn pimara-7,15-diene is biosynthesized via a chair-boat cyclization of (E,E,E)-GGPP to give 9,10-syn CPP, followed by an S$\sb{\rm N\sp\prime}$ cyclization to form the C-ring.
Type of Resource
text
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http://hdl.handle.net/2142/19629
Copyright and License Information
Copyright 1991 Yee, Nathan Kwok Nang

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