A Beta-lactone-based route to 1,2-disubstituted cyclopentanes ; mechanistic studies of the tandem Mukaiyama aldol-lactonization (TMAL) reaction ; studies of the zwitterionic intermediate in the general Mukaiyama aldol reaction /
The ability of []-lactones to undergo intramolecular ring opening by allylsilanes leading to O-alkyl cleavage products was demonstrated. A "S[]2-like'' reaction mechanism was proposed. In addition, a novel intramolecular Friedel-Crafts alkylation reaction of []-benzyloxy-[]-lactones w...
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| Format: | Thesis Book |
| Language: | English |
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[Place of publication not identified] :
[publisher not identified] ;
1999.
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| Online Access: | http://proxy.library.tamu.edu/login?url=http://proquest.umi.com/pqdweb?did=730298611&sid=1&Fmt=2&clientId=2945&RQT=309&VName=PQD |
| Summary: | The ability of []-lactones to undergo intramolecular ring opening by allylsilanes leading to O-alkyl cleavage products was demonstrated. A "S[]2-like'' reaction mechanism was proposed. In addition, a novel intramolecular Friedel-Crafts alkylation reaction of []-benzyloxy-[]-lactones was discovered. As a result of extensive mechanistic studies of the ZnCl[] mediated tandem Mukaiyama aldol-lactonization (TMAL) reaction of thiopyridyl ketene acetate, a unique mechanism with a highly ordered, boat-like trigonal bipyramidal transition state has been proposed. The efficiency of the TMAL reaction was improved by employing less bulky sibyl groups in the ketene acetals. More importantly, our understanding of this TMAL reaction led to several important discoveries. First, a catalytic TMAL reaction of thiophenyl ketene acetate has been proposed and discovered. Secondly, some important mechanistic details of the general Mukaiyama aldol reaction have been uncovered regarding the zwitterionic intermediate. Thirdly, improved reaction conditions for the TMAL reactions of two substrates, 4-t-butyldimethylsiloxybutanal and 7-trimethylsilyl- 5-hexenal, have been proposed and demonstrated. Finally, a stereocomplementary TMAL reaction leading to cis-[]-lactones has been discovered. Mechanistic studies of the ZnCl[] mediated TMAL reaction indicate that this is a catalytic TMAL reaction and three reaction pathways leading to three different products are involved in this reaction. A silicon-catalyst formed in situ has been discovered that leads to two by-products. This study provides further support for the cyclic transition state proposal in the TMAL reaction of thiopyridyl ketene acetate and suggests methods whereby the efficiency of this TMAL reaction might be improved. Four reaction pathways emanating from this zwitterionic intermediate in the general Mukaiyama aldol reaction have been proposed and supported. Importantly, we found that the nucleophilicity of the Lewis acid ligand serving as a desalinating agent was unexpectedly poor in the Mukaiyama aldol reaction. Based on this finding, we proposed that an equilibration between the zwitterionic intermediate and starting materials might be involved and this equilibration may effect the stereochemical outcome of some Mukaiyama aldol reactions. Based on this proposal, the Lewis acid and substrate-dependent stereochemical outcomes, which are considered as a "puzzling'' behavior in many Mukaiyama aldol reactions, can be reasonably understood. |
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| Item Description: | In title, symbol for Greek letter Beta is used. Vita. "Major Subject: Chemistry". |
| Physical Description: | xiv, 258 leaves : illustrations ; 28 cm. Issued also on microfiche from University Microfilm Inc. |
| Bibliography: | Includes bibliographical references (leaves 154-167). |