Carbon-13 nuclear magnetic resonance spectroscopy of naturally occurring substances. XXV. Carbon-13 nuclear magnetic resonance spectral analysis of tobramycin and related antibiotics

Abstract

The natural abundance 13C nmr spectra of the antibiotics tobramycin, kanamycin B, and their carbamates were recorded at several pHs. The chemical shifts of the four antibiotics, of nebramine, neamine, deoxystreptamine, and of the methyl 2-amino-, 3-amino-, and 6-amino-a-D-glucopyranosides, were assigned and the conformation of all compounds in water solution was determined.A mixture of aminoglycoside antibiotics3 whose main components are nebramycin factors 2, 4, and 5' is produced by Streptomyces tenebrarius. These substances have been shown to be apramycin4 and carba-mates5 of kanamycin B (lc)6 and of tobramycin (la).7 coupled spectra were sufficient for the 13C chemical shift assignments indicated in Table I. The spectral analysis of the tricyclic compounds was aided by the spectra of the amino monosaccharides methyl 2-amino-2-deoxy--D-glucopyranoside (2a), methyl 3-amino-3-deoxy-a-A 13C nmr spectral analysis of the carbamates and their hydrolysis products now was undertaken in order to discern the position of the carbamyl moiety and to lay the groundwork for structure analysis of aminoglycoside antibiotics by this new, diagnostically powerful research tool.8Natural abundance 13C nmr spectra of 0.3-0.5 M aqueous solutions of componds 1 and others (vide infra): containing 5% dioxane as internal reference, were recorded on a Fourier transform spectrometer operating at 15.08 MHz.10 Proton resonance de-(1) Carbon-13 Nuclear Magnetic Resonance Spectroscopy of Naturally Occurring Substances. XXV. For the preceding paper, see N. J.

Nuclear magnetic resonance studies of microemulsions and related systems: 2-butoxyethanol in water

“…Thus by analogy with amines, the increased shielding at P carbons when dissolving the alcohol in water is the result of a decrease of effective negative charge on the oxygen in water. Using the amine results (19), one would expect C(2) of cyclohexanol to be shielded by about 5 ppm in the neutral species relative to the anion. The chemical shift data at 8°C (see Table 2) show a similar dependence on concentration to that obtained at 32°C.…”

“…The effect of changes in polarity and solvent effects are difficult to ascertain (1 8). One example where a polar effect is documented is the deprotonation of a protonated amine which deshields the p carbon by 5 ppm and the a, y, and 6 carbons by about 1 ppm (19).…”

Aminoglycoside antibiotics: Studies directed toward the selective modification of hydroxyl groups: Synthesis of 3′-epiparomamine and 3′-epineamine

“…Like in the monosaccharide series, the effect of an inversion of configuration at C-3 in 3'-epiparomamine and 3'-epinearnine, compared t o the naturally occurring pseudodisaccharides (26,27), is ~iianifested by a n upfield shift in the absorption of the I3C nuclei corresponding t o C-2',C-5' of the amino sugar moiety. Shielding of C-5', however, was much less pronounced c o~n~a r e d to 8, possibly owing to orientational (rotamer) effects (28,29) associated with the presence of a larger aglycone in the pseudodisaccharides. Upon acidification t o p H -.…”

“…Extension of the bromination reaction with the reagent pair Ph3P-NBS to 27 was also successful a n d led to the formation of tri-N-benzyloxycarbonyl-6'-bro1iio-6'-deoxy-3'-epiparoma1nine (29) in high yield. In this case, the selective bromination was done a t room temperature in HMPT as solvent, which illustrates the preparative utility of this reaction in this series (Scheme 6).…”

“…An interesting feature in the spectrum of 3'-epineamine is the apparent large upfield shift of the C-4' signal compared to that found in 3'-epiparomamine. This could be due to increasing 1,3-syn interactions with H-4-0-5, or more likely, to the presence of different rotamer populations (28,29) associated with the -CH,NH, group in 3'-epineamine. Such a y effect (30) would be reflected in the spectrum of 3'-epineamine, upon changing the pH, but not as much in the case of the 3'-epimeric pseudodisaccharide 28.…”