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 alpha-amino-alpha'-
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.