A new route to diastereomerically pure cyclopropanes utilizing stabilized phosphorus ylides and y-hydroxy enones derived from 1,2-dioxines: mechanistic investigations and scope of reaction
Date
2000
Authors
Avery, T.
Taylor, D.
Tiekink, E.
Editors
Advisors
Journal Title
Journal ISSN
Volume Title
Type:
Journal article
Citation
Journal of Organic Chemistry, 2000; 65(18):5531-5546
Statement of Responsibility
Thomas D. Avery, Dennis K. Taylor, and Edward R. T. Tiekink
Conference Name
Abstract
A new chemical transformation for the construction of diversely functionalized cyclopropanes utilizing 1,2-dioxines and stabilized phosphorus ylides as the key precursors is presented. Through a series of mechanistic studies we have elucidated a clear understanding of the hitherto unknown complex relationship between 1,2-dioxines 1a-e, and their isomeric cis/trans -hydroxy enones (23 and 21a-e), cis/trans hemiacetals 24a-e, and -ketoepoxides (e.g., 26), and how these precursors can be utilized to construct diversely functionalized cyclopropanes. Furthermore, several new synthetically useful routes to these structural isomers are presented. Key features of the cyclopropanation include the ylide acting as a mild base inducing the ring opening of the 1,2-dioxines to their isomeric cis -hydroxy enones 23a-e, followed by Michael addition of the ylide to the cis -hydroxy enones 23a-e and attachment of the electrophilic phosphorus pole of the ylide to the hydroxyl moiety, affording the intermediate 1-25-oxaphospholanes 4 and setting up the observed cis stereochemistry between H1 and H3. Cyclization of the resultant enolate (30a or 30b), expulsion of triphenylphosphine oxide, and proton transfer from the reaction manifold affords the observed cyclopropanes in excellent diastereomeric excess. The utilization of Co(SALEN)2 in a catalytic manner also allows for a dramatic acceleration of reaction rates when entering the reaction manifold from the 1,2-dioxines. While cyclopropanation is favored by the use of ester-stabilized ylides, the use of keto- or aldo-stabilized ylides results in a preference for 1,4-dicarbonyl formation through a competing Kornblum-De La Mare rearrangement of the intermediate hemiacetals. This finding can be attributed to subtle differences in ylide basicity/nucleophilicity. In addition, the use of doubly substituted ester ylides allows for the incorporation of another stereogenic center within the side chain. Finally, our studies have revealed that the isomeric trans -hydroxy enones and the -keto epoxides are not involved in the cyclopropanation process; however, they do represent an alternative entry point into this reaction manifold.