Carbon-13 nuclear magnetic resonance spectra have been obtained for the coumarin-related compounds warfarin and phenprocoumon. The spectral assignments indicate that warfarin exists as a mixture of cyclic hemiketal diastereomers in dimethyl sulfoxide solution. The sodium salt of warfarin exists as the ringopen form in water solution.Warfarin has been used as a rodenticide and anticoagulant since the late 1940's, while the sodium salt of warfarin (Coumadin®) has become the most widely used anticoagulant in many hematological diseases and surgical procedures. The rationale for structures 1 and 2 assigned to warfarin and its sodium salt, respectively, has come from chemical synthesis (1) and spectroscopic methods (2). These structures have been assumed to be correct for many years, although no rigorous structural proof has as yet been presented in the literature. Recently, it has been suggested that crystalline warfarin exists as the hemiketal, 3. (Diffraction data indicates a cyclic hemiketal structure in the solid state; E. Valente, W. Clearly, structural information for the dissolved substance is of great interest and importance to those concerned with the mode of action of warfarin. Solution studies of warfarin utilizing infrared and proton nuclear magnetic resonance (NMR) spectra, are difficult to interpret and are ambiguous due to the complexity of the spectra, although the proton NMR spectrum of warfarin is similar to that of the cyclocoumarol, 4 (K. K. Chan, unpublished data). This fact suggests that the keto structure 1 for warfarin in solution may also be incorrect. Much more definitive information seemed possible through use of carbon-13 NMR to differentiate between structures 1 and 3 because the hemiketal carbon (3, position 13) would have a grossly difAbbreviations: NMR, nuclear magnetic resonance; TMS, tetramethylsilane; Me2SO, dimethyl sulfoxdde. Chemical-shift measurements were made using a Brukarian, pulsed FT spectrometer (3) operating at 15.09 MHz for carbon-13. The temperature was maintained at 37.0 i 1°with a Bruker B-ST temperature control unit. Off-resonance and a gated-decoupling (4) technique which turns off the proton noise decoupler during acquisition of data were used in facilitating spectral assignments. A typical spectrum was run with an acquisition time of 0.8 sec, pulse delay of 1.0 sec, sweep width of 5000 Hz and 8K data points.
RESULTS AND DISCUSSIONThe assignment of spectral peaks is based on several wellestablished methods which are now common in 18C spectroscopy (for discussion see refs. 5 and 6). For example, the alkyl carbons, Cli, C12, and C14, are known to have characteristic chemical shifts (4) based upon model studies with hydrocarbons. The peaks can be further differentiated by the multiplicity patterns obtained from off-resonance experiments. However, ambiguities were encountered for warfarin and, for example, the C2 and C4 peaks were not assignable by analogy with the resonances of coumarin or 4-hydroxycoumarin. Also, off-resonance did not provide sufficient inf...
