The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.
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Lewis Acid-Promoted [2 + 2] Cycloadditions of Allenes and Ketenes: Versatile Methods for Natural Product Synthesis.
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Department of Chemistry, Indiana University, 800 E. Kirkwood Ave, Bloomington, Indiana 47405, United States.
ConspectusCycloaddition reactions are an effective method to quickly build molecular complexity. As predicted by the Woodward-Hoffmann rules, concerted cycloadditions with alkenes allow for the constructions of all possible stereoisomers of product by use of either the or geometry. While this feature of cycloadditions is widely used in, for example, [4 + 2] cycloadditions, translation to [2 + 2] cycloadditions is challenging because of the often stepwise and therefore stereoconvergent nature of these processes.
View Article and Find Full Text PDFTotal synthesis of gracilioether F: development and application of lewis Acid promoted ketene-alkene [2+2] cycloadditions and late-stage C-H oxidation.
Angew Chem Int Ed Engl
December 2014
Department of Chemistry, Indiana University, 800 E. Kirkwood Ave, Bloomington, IN 47401 (USA).
The first synthesis of gracilioether F, a polyketide natural product with an unusual tricyclic core and five contiguous stereocenters, is described. Key steps of the synthesis include a Lewis acid promoted ketene-alkene [2+2] cycloaddition and a late-stage carboxylic acid directed C(sp(3) )H oxidation. The synthesis requires only eight steps from norbornadiene.
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J Phys Chem A
June 2014
The College of Chemistry and Molecular Engineering, Center of Computational Chemistry, Zhengzhou University, Zhengzhou, Henan Province 450001, P.R. China.
The detailed mechanisms and diastereoselectivities of Lewis acid-promoted ketene-alkene [2 + 2] cycloaddition reactions have been studied by density functional theory (DFT). Four possible reaction channels, including two noncatalyzed diastereomeric reaction channels (channels A and B) and two Lewis acid (LA) ethylaluminum dichloride (EtAlCl2) catalyzed diastereomeric reaction channels (channels C and D), have been investigated in this work. The calculated results indicate that channel A (associated with product R-configurational cycloputanone) is more energy favorable than channel B (associated with the other product S-configurational cyclobutanone) under noncatalyzed condition, but channel D leading to S-configurational cyclobutanone is more energy-favorable than channel C, leading to R-configurational cycloputanone under a LA-promoted condition, which is consistent with the experimental results.
View Article and Find Full Text PDFLewis acid-promoted ketene-alkene [2 + 2] cycloadditions.
J Am Chem Soc
February 2013
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47401, United States.
Described are the first examples of ketene-alkene [2 + 2] cycloadditions promoted by Lewis acids. Notable features of this method include (1) substantial rate acceleration relative to traditional thermal reactions, (2) good diastereoselectivities and yields for the formation of the cyclobutanone products, and (3) inverse diastereoselectivity compared with related thermal cycloadditions for many examples. These studies not only provide access to synthetically versatile cyclobutanones that cannot be prepared by traditional thermal cycloadditions but also address important mechanistic questions regarding ketene-alkene [2 + 2] cycloaddition reactions.
View Article and Find Full Text PDFChallenge to detect 1,4-zwitterions spectroscopically in a ketene-alkene reaction.
J Am Chem Soc
January 2006
Department of Chemistry, Faculty of Science, Saitama University, Sakura-ku, Saitama 338-8570, Japan.
The first example of direct spectroscopic detection of transient species, 1,4-zwitterions, generated in a ketene-alkene reaction is reported. Also, a striking result of the intervention of an unprecedented "1,4-zwitterion neutral dimer" is presented in a new mechanistic pathway; the ketene-alkene reaction gives the product cyclobutanone from the initial cycloadduct alpha-methyleneoxetane.
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