Chiral dipoles, diastereoselectivity, asymmetric

Gawley and coworkers developed a methodology for the asymmetric syntheses of primary amines via acyclic dipole-stabilized organolithium intermediates [75]. In the case of the aminooxazoline 90,4 1 diastereoselectivity was observed, but the selectivity was improved significantly with oxazolidinone 92, as shown in Scheme 26. The chiral oxazolidinone was subsequently removed to provide the highly enantio enriched primary amines 94. 

Chiral ketene equivalent 60 was prepared from pulegone (63), a common monoterpene. Both enantiomers of 63 are known and thus, both enantiomers of 60 are available. A model that rationalizes the observed diastereoselectivity follows. The model emphasizes three points (1) The ester reacts from a conformation that minimizes dipole-dipole repulsion in terms of conformation around the 0-acyl bond. This is normally the lowest energy conformation for any ester. (2) Steric effects are minimized in the presumed reactive complex between 60 and the Lewis acid. The metal complexes opposite the large ester alkyl group, and the vinyl dienophile reacts from an -trans conformation to minimize metal-vinyl group interactions. (3) r-Stacking contributes to shielding of one face of the olefin from the diene. It is notable that the non-catalyzed process shows little asymmetric induction. 

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