Bromocyclization of unsaturated oximes. Synthesis of five-membered cyclic nitrones (pyrroline N-oxides)

Abstract

The cyclization of a ribose-derived pentenose oxime with various halogen electrophiles showed bromine to be the most effective reagent, leading to 80% of L-lyxo/D-ribo-pyrroline N-oxides in an 84:16 diastereomeric ratio. In order to explore the scope of this facile process, several other γ,δ- unsaturated oximes were submitted to this reaction. Depending on the substitution pattern, 23-87%, yields of pyrroline N-oxides of were registered. With α-allyl-β-ketoester oximes the alkoxycarbonyl group proved a similar (ethoxy) or even better (t-butoxy) trapping nucleophile, leading preferentially to the corresponding bromolactone oxime. - With 2,2-dimethyl-3-butene aldoxime, the corresponding 3-bromopyrroline N-oxide was formed, due to a formal, unusual N-endo cyclization to the chain terminus. This was exploited for a new access to six-membered nitrones from a γ,δ-pentene aldoxime, with addition of Br/OH to the C=C of the 4-pentenal first, and oximation/cyclization following then. © 2006 Verlag der Zeitschrift für Naturforschung.