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Free keywords:
DIELS-ALDER REACTIONS; AMINO-ACID-ESTERS; X=Y-ZH SYSTEMS; CYCLO-ADDITIONS; DIASTEREOFACIAL SELECTIVITY; POTENTIAL 1,3-DIPOLES; PYRROLIDINES; QUINOCARCIN; IMINES; ASYMMETRIC SYNTHESES; AZOMETHINE YLIDES; CHIRAL AUXILIARIES; CYCLOADDITIONS; PYRROLIDINES;
Abstract:
Upon treatment with triethylamine or DBU in the presence of LiBr, aromatic and aliphatic imines of amino acid esters are converted to N-metalated azomethine ylides, These 1,3-dipoles undergo highly stereoselective cycloadditions with N-acryloyl-(S)-proline esters in THF at - 78 to -40 degrees C to afford highly substituted pyrrolidines with complete regiocontrol and good to excellent diastereomeric ratios, The chiral auxiliary groups can readily be removed from the cycloadducts by simple acid hydrolysis, To rationalize the observed stereoselectivity a transition-state model is proposed in which the lithium cation is coordinated to both the 1,3-dipole and the dipolarophile.
