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

The bromocyclization of several γ,δ-unsaturated oximes leads to respective bromomethylpyrroline N-oxides. Yields vary from 23 to 87 %, depending on the structural features. An unusual formal N-endo cyclization is found with a β,γ-enoxime. As a consequence, reversal of steps, i. e. addition of Br/OH to the C=C bond of the unsaturated aldehyde first, followed by oximation, opens access to 6-membered nitrones.

tate/methanol), whereby crystallization of the major isomer from dichloromethane furnished the title nitrone in the form of colorless crystals (m.p. 398-401 K). DiscussionThe pyrroline ring system has an envelope conformation, where C2 is situated out-of-plane. Both ring systems of the molecule have a nearly perfect perpendicular orientation of 88.9(2)° (figure, top). The double bond N1=C4 is clearly characterized by a distance of 1.287(3) Ä. The Ν-Ο distance of the neighboring TVoxide function is 1.293(5) Ä. In the view of the cell plot we observe alternate polar and non-polar channels along the c axis. The non-polar channels are built up by the phenyl moieties and the polar ones by the bromomethyl-nitrone fragments. There are also polar layers in the a,c plane along the b axis established by theNoxide functions (figure, bottom).

Source of materialThe title compound has been obtained by bromocyclization of 2-allylcyclohexanone oxime [1][2][3][4]. The two diastereoisomers (ratio 67:33) were separated by column chromatography using ethyl acetate/methanol (7:3), whereby crystallization of the minor isomer from dichloromethane furnished the title nitrone in the form of brownish crystals (m.p. 375-376 K). DiscussionThe six-membered ring system shows a chair conformation and the pyrrolidine ring has an envelope conformation, where C2 is out of plane. The double bond N1=C8 is clearly characterized by the distance of 1.298(7) Å. The NO distance of the neighboring N-oxide function is 1.309 (6) Å. In the packing diagram of the cell plot we observe an antiparallel orientation of the molecules along the c axis and a face-to-face stacking of the molecules along the a axis.Z. Kristallogr. NCS 221 (2006) 89-90

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