1 H nuclear magnetic resonance spectroscopy of flavonoids and their glycosides in hexadeuterodimethylsulfoxide

Abstract

The early use of CCl4-soluble flavonoid trimethyl- silyl (TMSi) ether derivatives for lH nuclear magnetic resonance (NMR) spectroscopy has been well documented (Mabry et al. 1970; Markham and Mabry, 1975; Markham, 1982), but in recent years the use of these derivatives has been largely superseded. The advent of more sophisticated, higher-field spectrometers has brought with it the capability to run 13C as well as 1H NMR spectra, and, more recently, two-dimensional (2D) homonuclear and heteronuclear spectra. The consequent demand for a solvent suitable for both 1 H and 13C NMR spectroscopy and for dissolving a wide range of flavonoid aglycones and glycosides without the need for prior derivatization has resulted in the popularity of hexadeuterodimethylsulfoxide (DMSO-d6) as a solvent. While solvents such as pyridine-d5, methanol-d4 and acetone-d6 have found some applications in situations where the DMSO signals (centred at 2.5 ppm) or the often associated water signal (centred around 3.3 ppm) obscure key portions of the spectrum, DMSO-d6 remains the most effective general solvent for flavonoid glycosides. Troublesome water signals can now effectively be removed from spectra by the application of recently developed double-quantum filtering techniques (see discussion of DOF-COSY). While a number of comprehensive review articles and compilations of 13C NMR data for flavonoids in DMSO-d6 have appeared in the literature (e.g. Markham et al., 1978, 1982; Agrawal, 1989), no similar review is available of lH NMR studies. Early papers by Batterham and Highet (1964) and Hillis and Horn (1965) are now very much dated. The present chapter attempts to fill this gap by providing a pictorial display of a wide range of reference spectra measured on purified authentic samples under identical conditions (measured in DMSO-d6 solutions with a TMS internal standard at 30°C using a Bruker AC300 spectrometer with a 30° pulse angle and total recycle time of 3.8s), together with tabulated data from the literature, analyses of these data and discussion of some of the newer ID and 2D NMR techniques.