Diastereoselective reduction of E and Z α-alkoxyimino-β-ketoesters by sodium borohydride

The reduction of (Z)-alpha-alkoxyimino-beta-ketoesters, R1COC(=NOR3)CO2R2 (R-1=Me, Et, MeOCH2, Ph; R-2=Me, Et, PhCH2, R-3=Me, PhCH2) with NaBH4/THF-MeOH at 0 degrees C gave the corresponding (Z)-alpha-alkoxyimino-beta-hydroxyesters in 72-85% yield while the E isomers gave a mixture of corresponding (Z)- and (E)-2-alkoxyimino-1,3-diols in 71-85% yield. The (E)-alpha-alkoxyimino-beta-hydroxyesters were obtained in 71-85% yield when the reaction temperature was -72 degrees C. The C-13-NMR and IR spectra of (E)- and (Z)-alpha-oxyimino-beta-ketoesters and (E)- and (Z)-alpha-alkoxyimino-beta-ketoesters, were used for determination of E/Z configuration. The C-gamma signals in C-13-NMR of Z are shifted to higher field and the IR spectra of E isomers show a splitting of C=O ester band. (C) 1999 Elsevier Science Ltd. All rights reserved.