X=Y-ZH SYSTEMS AS POTENTIAL 1,3-DIPOLES .34. GENERATION OF NITRONES FROM OXIMES - TANDEM MICHAEL ADDITION-1,3-DIPOLAR CYCLOADDITION REACTIONS - CLASS-2 PROCESSES UTILIZING BIFUNCTIONAL MICHAEL ACCEPTOR-DIPOLAROPHILE COMPONENTS

Aldoximes and ketoximes react with a range of bifunctional Michael acceptor-dipolarophile substrates comprising functionalised 1,3-, 1,4- and 1,5-dienes via a tandem process involving an N-alkenyl nitrone intermediate. The 1,3-dienes react regio- and stereo-specifically to give 1-aza-7-oxabicyclo[2.2.1]heptanes whilst sterically unencumbered aryl aldoximes and 1,4-dienes give 1-aza-2-oxabicyclo[3.2.1]octane derivatives. Ketoximes and 1,4-dienes give mixtures of 1-aza-2-oxa- and 1-aza-8-oxa-bicyclo[3.2.1]octanes. 1,5-Dienes and ketoximes react regio- and stereo-specifically to give 1-aza-8-oxabicyclo[3.2.1]octane derivatives whilst benzaldoxime gives a 1:1 mixture of epimeric 1-aza-8-oxabicyclo[3.2.1]octanes together with traces of two epimeric 1-aza-2-oxabicyclo[3.2.1]octanes. The regio- and stereo-chemical outcome of the tandem process is controlled by the length and nature of the linking chain in the bifunctional substrate and the steric interactions between substituents on the oxime and dipolarophile in the transition state. A crystal structure of one of the 1-aza-7-oxabicyclo[2.2.1]heptanes is reported.