Sc(OTf) ₃ -catalyzed Diastereoselective Addition of Enol Silanes and Silyl Ketene Acetals to Cr(CO) ₃ -complexed Aryl Aldimines

Abstract

Enolsilanes and silyl ketene acetals add to aryl aldimines complexed with tricarbonylchromium, often at ambient temperature, with very high diastereoselectivity in the presence of 5 mol% Sc(OTf)₃ to afford β-amino ketones and β-amino esters in excellent yield. The β-amino esters were cyclized to β-lactams by treatment with MeMgI.

Key words

arene-chromium - scandium triflate - imino aldol reaction - diastereoselective

References

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• 13b

  On decomplexation in air and sunlight, complex 5a afforded the known β-lactam (ref. [3] ) as a colorless solid (mp 158 °C, lit.: 155 °C)in quantitative yield and as a pure diastereomer.

9

All new compounds were characterized by satisfactory spectral (IR, NMR) and elemental analysis (C, H and N).

12

In all examples only a single diastereomer of each product was isolated, and only one enantiomer is represented in the diagrams for brevity. The expression ‘single diastereomer’ for these complexes refer to absence of peak due to the other diastereomer in 200 MHz proton NMR spectra, and hence reflect at least > 98% stereoisomeric purity.

14

Crystal data for 4a: C₂₃H₂₅NO₆Cr, Mw = 463.44, monoclinic, space group P2₁, (# 4), a = 8.310(1), b = 17.938(3), c = 15.736(2) Å, V = 2330.9(5) ų, Z = 2, D_c = 1.321g cm^-3, R = 0.0607, R_w = 0.149, GOF = 1.057 for 3103 reflections with I > 2σ (I) out of 4808 reflections collected.