Item – Theses Canada

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OCLC number
46495329
Author
Liu, Yadong,1956-
Title
Asymmetric synthesis by sequential group selective reactions.
Degree
Ph. D. -- University of Saskatchewan, 1995
Publisher
Ottawa : National Library of Canada = Bibliothèque nationale du Canada, 1995.
Description
3 microfiches.
Notes
Includes bibliographical references.
Abstract
The synthesis of enantiomerically pure compounds is one of the most important research areas in organic chemistry today. In the first chapter of this thesis a classification of stereoselective reactions according to the topicity relationships of the groups or faces of the substrates and the requirements for a stereoselective process are described. A description of various synthetic strategies for stereoselectivity and a brief review of the evolution of stereoselective synthesis is presented.(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\vbox{\vskip90pt}$$(TABLE/EQUATION ENDS) A relatively new stereoselective synthesis strategy using sequential enantiotopic group selective reactions of bifunctional substrates has gained popularity in synthetic organic chemistry. An analysis of underlying theory of this process is discussed and indicates that a kinetic enhancement of stereoisomeric purity of products can be achieved even with reactions with modest group selectivity. To demonstrate this conclusion, the stereoselective reduction of stereochemically pure 2,3,4,5-tetramethoxy-1,6-hexanedials (prepared from D-galactose and from D-glucose) with the chiral reducing agent Alpine-Borane$\sp\circler$ (group selectivity ca.3-4:1) was studied. In the case of the galactose derived dialdehyde, the enantiomeric purity of the chiral hydroxyaldehyde product (40-50% yield) was 7-15:1. The observed results were well approximated by a calculation using a simple mathematical model. In the case of the D-glucose derived dialdehyde, hydroxyaldehyde product was obtained with good selectivity (20-44% yield, 83-90% de) from reaction with (S)-Alpine-Borane$\sp\circler$ (matched selectivity). Poor diastereoselectivity was observed for reduction with (R)-Alpine-Borane$\sp\circler$ (mismatched selectivity). The sequential group selective reactions of a stereoisomeric mixture of dl and meso substrates is analyzed in the third chapter of the thesis. This analysis suggests a strategy for obtaining desymmetrized derivatives of meso substrates with excellent diastereoselectivity and enantiomeric excess from these processes. High diastereoselectivity is achieved by processing the starting material with both R- and S-group selective reactions (after recycling). The Sharpless epoxidation of a bis-allylic alcohol derived from furfural (6,6-ethylenedioxyundeca-1,10-diene-3,9-diol) was studied to test the strategy. The corresponding monoepoxide $\lbrack(2R,\ 3S\ 9R)$-1,2-epoxy-6,6-ethylenedioxy-10-undecene-3,9-diol) was produced with $>$98% diastereoselectivity and $>$98% enantiomeric excess thereby demonstrating the feasibility of the strategy.
ISBN
0612002500
9780612002500