Abstract
Abstract The 1H NMR spectrum of a solution of neomycin B free base (Structure 1) in D2O has been assigned completely by two-dimensional, homonuclear J-resolved NMR spectroscopy and spin decoupling at 400 MHz. Proton chemical shifts and proton-proton couplings are reported for all glycoside residues in neomycin B along with their computer simulated spectra. The 4 C 1 chair conformation has been assigned to the 2,6-diamino-2,6-dideovy-β-l-idopyranosyl (ring D) portion of the antibiotic (1b) by analysis of the proton coupling constants and chemical shifts. The β-furanose form of the ribosyl portion (ring C) has been assigned. Vicinal proton couplings for the 2-deoxystreptaminyl group (ring B) are consistent with a chair conformation in which all ring substituents are equatorial, and proton chemical shift assignments are based on protonation studies. A computer simulated composite of the individual calculated spectra is presented for comparison with the experimental spectrum of neomycin B.
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