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Examination of Homo-[3 + 2]-Dipolar Cycloaddition:  Mechanistic Insight into Regio- and Diastereoselectivity

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posted on 21.12.2007, 00:00 by Avedis Karadeolian, Michael A. Kerr
The reaction of 2,3-disubstituted-1,1-cyclopropanediesters with nitrones under Lewis acid conditions produces tetrahydro-1,2-oxazines in which the cis/trans relationship of the cyclopropanes is not conserved. Reacting nitrones with 2,3-cis-disubstituted cyclopropanes lead to 5,6-trans-oxazines, and 2,3-trans-disubstituted cyclopropanes lead to 5,6-cis-oxazines. This observed stereochemical inversion provides evidence for a stepwise annulation mechanism in the preparation of tetrahydro-1,2-oxazines.