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

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.