¹H Assignments of glycopeptide antibiotics by double quantum NMR and relayed correlation spectroscopy

Abstract

Two-dimensional double quantum NMR and relayed correlation spectroscopy were used to complete the ' H assignments of a series of glycopeptide antiiiotics. The double quantum N M R experiments were particularly useful in assigning the complicated aromatic region of the spectra by identifying the coupled protons with small differences in chemical shifts, and simplifying the contour maps by selectively enhancing the crosspeaks corresponding to either the orthe(343 Hz) or mefa-(&= 2 Hz) coupled protons. In addition, from the double quantum NMR experiments, protons belonging to the same spin system were unambiguously identified, even though not directly coupled. The comectivities between remote nuclei belonging to the same network were also established through relayed correlation spectroscopy.

Proton Double Quantum and Relayed Proton Double Quantum Coherence Two-Dimensional NMR Mapping of Proton-Proton Connectivity Networks in Natural Products: a Model Study of Strychnine

“…Thus, it is possible, to an extent, to determine the nature of the responses which will optimally appear in the data matrix. Fesik et al (4), in assigning the spectrum of ristocetin, have exploited the effects of variation of the t delay. The location and nature of responses in the proton double quantum experiment has been treated briefly in our previous work (3,5) and much more rigorously in the work of Braunschweiler et al (2) and will therefore not be discussed here.…”

“…The task of piecing together the structures of complex, unknown natural products has benefited significantly from the development of two-dimensional nmr techniques. Two-dimensional proton double quantum coherence (1H-2D-INADEQUATE) (1,2) is among the newest of such techniques and has recently been employed in the elucidation of spin coupling pathways involving small, long-range couplings in plumericin (3) and in the assignment of the 'H-nmr spectra of the antibiotic ristocetin (4) and the cembranoid diterpene jeunicin (5). Thus, we were interested in further evaluating the abilities of the proton double quantum experiment to elucidate complex proton connectivity networks.…”

2D Multiple-Quantum Spectroscopy

“…Moreover it is convenient to consider zero-quantum coherence as a rotating monopole, single-quantum coherence as a dipole, and double-quantum coherence as a quadrupole and so on, so that ZQC may be expected to be relatively insensitive to the phases of the applied pulses, thereby allowing a simpler 4-step cycle to be used in comparison to the 32-step time-consuming phase cycling procedure required in the HDQC experiment (ref. 25,26). The ZQC spectrum of strychnine is shown in Fig.…”

Proton Zero Quantum Two-Dimensional Nmr Spectroscopy: An Alternative to Proton Double Quantum Inadequate for Mapping Complex Proton Spin Systems of Natural Products

“…It was not until 1985, when the experiment was applied to plumericin, that the technique was actually introduced to the field of natural product structure elucidation (13). Subsequently, the HDQC experiment has been applied to ristocetin (14), the total assignment of the 1Hnmr spectrum of the cembranoid diterpene jeunicin (15) and the novel related C2l analog planaxool (16), and the elucidation of new long range coupling pathways in strychnine (17). Doubtless, HDQC applications will begin to increase in numbers as more natural product chemists begin to appreciate the capabilities of this rather powerful, albeit somewhat "exotic," experiment.…”

“…In addition, because zero quantum coherence (ZQC) can be considered as a rotating monopole, single quantum coherence (SQC) as a dipole, and double quantum coherence (DQC) as a quadrupole and so on, ZQC can thus be seen to be relatively insensitive to the phase of pulses applied to the spin system (20) thereby allowing a simplified four step phase cycle to be utilized (18) in combination with a homogeneity spoiling pulse that can been shown to suppress all higher order coherences (19). Thus, the four-step phase cycle employed in the HZQC nmr experiment shown in Figure 1 represents a considerable economy of time over the much more complex 32-step phase cycle required for the successful performance of the HDQC nmr experiment (13,14).…”