Reactions of Cyclopropanone Acetals with Alkyl Azides: Carbonyl Addition versus Ring-Opening Pathways

The Lewis acid-mediated reactions of substituted cyclopropanone acetals with alkyl azides were found
to strongly depend on the structure of the ketone component. When cyclopropanone acetal was treated
with alkyl azides, N-substituted 2-azetidinones and ethyl carbamate products were obtained, arising from
azide addition to the carbonyl, followed by ring expansion or rearrangement, respectively. When 2,2-dimethylcyclopropanone acetals were reacted with azides in the presence of BF3·OEt2, the products obtained
were α-amino-α‘-diazomethyl ketones, which arose from C2−C3 bond cleavage of the corresponding
cyclopropanone, giving oxyallyl cations that were captured by azides. Aryl-substituted cyclopropanone
acetals, when subjected to these conditions, afforded [1,2,3]oxaborazoles exclusively, which were also
the result of C2−C3 bond rupture, azide capture, and then loss of nitrogen. In the reactions of n-hexyl-substituted cyclopropanone acetals with alkyl azides, a mixture of 2-azetidinones and regioisomeric [1,2,3]oxaborazoles was obtained. The reasons for the different behavior of the various systems are discussed.