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The diastereofacial selection in addition reactions to biased rigid systems can be modulated by the action of Lewis acids. As an example, the stereoselectivity of the nucleophilic addition of vinyl magnesium bromide (VMB) to cyclic nitrones in the presence of diethylaluminum chloride (DEAC) shows a strong dependence on the temperature and the carbon substituent adjacent at the reaction center; it is remarkable that whereas a high selectivity is obtained at higher temperatures, in the presence of DEAC, a trend to invert the stereochemical course of the reaction is observed at lower temperatures, provided the substituent at C3 of the pyrrolidine ring allows delivery of the vinyl moiety. This behavior and difference in selectivity is to be attributed to the high conformational barriers of the intermediate nitrone-DEAC-VMB complex. A clear inversion of the selectivity is observed at -78 degrees C for the reaction of the nitrone protected as an O-methyl derivative. The present study provides a rationalization for the stereocontrolled addition of nucleophiles to rigid systems (cyclic nitrones).

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[reaction: see text]. Iteration of organometallic addition to chiral hydroxylated pyrroline N-oxides through an addition-oxidation-addition synthetic sequence allowed highly stereoselective double alkylation of pyrrolidine at C-2 or at C-2 and C-5 depending on the regioselectivity of the oxidation step. Application of this methodology has been exemplified by the synthesis of the all-substituted pyrrolidine alkaloid (-)-codonopsinine and of proline-type amino acid precursors possessing a quaternary stereogenic center, whose configuration can be controlled.