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Title:Total synthesis of 9,10-syn copalol, an intermediate in the biosynthesis of 9,10-syn diterpenes
Author(s):Yee, Nathan Kwok Nang
Doctoral Committee Chair(s):Coates, Robert M.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Organic
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.
Issue Date:1991
Rights Information:Copyright 1991 Yee, Nathan Kwok Nang
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9136774
OCLC Identifier:(UMI)AAI9136774

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