Stereochemical studies of tetracycline antibiotics and their common impurities by 400 MHz ¹H NMR spectroscopy

Abstract

The 400 MHz 'H NMR spectra of hydrochlorides of tetracycline, chlortetracycline, doxycycline and two epimeric derivatives are reported and interpreted in terms of solute conformation in DMSO-d, and configuration. Coupling constant magnitudes within the system H-5, H-4a, H-4 and H-12a and dimethylamino chemical shifts are applied to the configurational assignment of isomeric a-and p-apooxytetracyclines.

A carbon-13 nuclear magnetic resonance study of solid tetracyclines

“…The breadth and low intensity of this signal serve to distinguish it from the nearby C12 peak. In solution, the amide shift is always reported to be upfield of the C12 signal (14), but in the solid this order is occasionally reversed. Typically, the amide resonance appeared as an asymmetric doublet (41,(44)(45)(46) with an apparent splitting of approximately 100 Hz (Fig.…”

“…1 and chemical shifts are listed in Table 1. Assignment of the spectra was primarily based upon non-quaternary suppression experiments, comparison to solution state results (14), and the solid state I3C shifts of other tetracycline derivatives.…”

“…Exceptions include: (i) C7 and C9 signals that are very close and may be interchanged (except in CTC, demeclocycline, and minocycline where C7 is shifted to higher frequency by substituents); (ii) C l l a and C2, which are assigned by similarity to solution shifts (14), but are too close in epi-TC and non-ionized OTCO for the assignment to be reliable; and (iii) the group of low field carbonyls C11, C1, and C3. These last three have been tentatively assigned by consideration of the available X-ray data (6, 7 , 15, 23), and by comparison to pH induced shifts obtained in solution (12).…”

“…The results are discussed in terms of both corresponding solution values (14) and X-ray crystal data ( 6 , 7 , 10, 15-23), and should serve as the basis for further 13C nmr studies of tetracycline derivatives and their metal complexes (26,27). the method of Doerschuk et al (35).…”

“…They propose that biological activity of the antibiotic involves both conformations, and that interconversion occurs in vivo in aqueous or lipid-rich phases. Solvent induced changes in conformation have not been discussed in 13C nmr solution studies (1 1, 13, 14), although differences in the I3C chemical shifts of tetracycline derivatives in DMSO-d6 and in H 2 0 are noted (14). These differences, though subtle for most of the resonance peaks, are nonetheless critical for the understanding of chemical shifts due to structural changes (e.g., 4 vs. 5).…”