Diastereoface Selectivity During Phthalimidonitrene Additions to (E)- and (Z)-Configurated α,β-Unsaturated Esters, Induced by a Chiral Center in the γ-Position

Article abstract of DOI:10.1002/hlca.19890720528

In-situ-generated phthalimidonitrene was added to five α,β-unsaturated esters containing a chiral secondary O-function at C(γ).The additions were fully suprafacial, inasmuch as the (E)-isomers 1 afforded only the trans-aziridines 2 and 3 (J(β,γ)=4.8-5.1 Hz) and the (Z)-isomers 4 only the cis-aziridines 5 and 6 (8.2-8.5 Hz).The products 2, 3, 5, and 6 where shown to possess the arabino-, xylo-, ribo-, and lyxo-configuration, respectively, by X-ray structure analysis of 2b, 2d, and 6a.The diastereoface selectivity of the nitrene addition, induced by the chiral substructure around C(γ), resulted in more 2 than 3 from 1, but more 6 than 5 from 4, which means that the preference of attack at the double bond switches from one side to the other depending on the C=C configuration.The preferences were higher at lower temperature.The aziridines 2a, 2d, and 3d exhibit 1H-NMR-visible isomerism at the ring N-atom; the major (78-95percent) invertomer A is always the one with the phthalimido group in trans-position to the (larger) substructure around C(γ).The other aziridines only show 1H-NMR signals of one invertomer, which - by steric reasoning - ought to be A; this is confirmed by a 1H-NMR argument for 3a, 5a, 6a, 5c, and 6c.