Diastereoselectivity of the metallo-enamine condensation; addition and cyclization reactions of vinyl isocyanates
Wolk, Diane Ridout
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https://hdl.handle.net/2142/21192
Description
Title
Diastereoselectivity of the metallo-enamine condensation; addition and cyclization reactions of vinyl isocyanates
Author(s)
Wolk, Diane Ridout
Issue Date
1990
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
The diastereoselectivity of the metallo-enamine condensation was examined using a variety of imines and aldehydes. The highest selectivity (5:95, anti:syn) was observed in the reaction of the lithium enamine of propylidene-t-butylamine with pivaldehyde. Therefore, high diastereoselectivities can be achieved through the use of aldehyde derivatives, although with lithium, sterically hindered substituents on both the imine nitrogen and on the aldehyde are required. A non-chelated transition state is proposed to account for the formation of the syn product from the ($E$)-metallo-enamine. Reduction of the intermediate alkoxide was developed as a way to prepare 1,3-amino alcohols and to avoid the risk of epimerization of the hydroxy imines. The use of other metals was examined. Titanium provided high levels of syn selectivity in all reactions ($<$10:90 in most cases), regardless of the size of the substituents although the yields were moderate (20-66%). An octahedral titanium complex is invoked to explain the observed results. Metallo-enamine condensations with zirconium, zinc, boron, and tin were nonselective. Hauser's work on the condensations of amide enolate anions with aldehydes was repeated and was found to be in error owing to the solubility of the minor isomer in the aqueous phase. The reactions were not exclusively anti selective, as reported, but instead provided hydroxy amides in ratios of 63:37 and 78:22. Imidate anions were also nonselective.
The reactions of carbonyl compounds with cyclohexenyl, cyclopentenyl, and styryl isocyanates were investigated. Although no reaction occurred with dried ethyl glyoxylate, ethyl glyoxylate hydrates provided a $\beta$-hydroxy ketone, presumably through the N-vinyl carbamic acid. The formation of a cyclic carbamate was found to be limited to diethyl oxomalonate in the presence of titanium tetrachloride. Addition of alkyl Grignard and alkyllithium reagents to cyclohexenyl isocyanate afforded a series of N-cyclohexenyl amides. Thermal and photochemical cyclizations of these enamides was also carried out. Cyclic amides were formed by photochemical electrocyclization along with $\beta$-amino ketones arising through photo-Fries rearrangement.
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