[4 + 3] Cycloaddition of cyclopropanone hemiacetals

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 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.

Eta3-pyranyl and eta3-pyridinyl molybdenum pi-complexes as chiral scaffolds for the enantioselective construction of substituted oxa- and aza[3.3.1]bicyclics: first enantio- and regiocontrolled [5+3] cycloaddition reactions

The first Mo-mediated [5+3] cycloadditions are reported. 3-Substituted pyranyl and pyridinyl molybdenum pi-complexes participate in enantiocontrolled [5+3] cycloadditions and provide a new and efficient synthetic approach to oxa- and aza[3.3.1]bicyclics of high enantiomeric purity. The reaction proceeds in good to excellent yields and with complete regio- and endo-selectivities; it diverts to a [2+3] cycloaddition pathway when 2-substituted heterocycle pi-complexes are used. This methodology, coupled with a variety of general demetalation protocols, holds much promise in synthetic applications.