Chromium tricarbonyl complexes of methyl O-methylpodocarpate: differentiation of diastereomers by high field NMR spectroscopy

The reaction of Cr(CO)6 with methyl O-methylpodocarpate yields diastereomeric complexes in which the Cr(CO)3 fragment binds to either the α- or the β-face of the aromatic C ring of the diterpenoid. These isomers are separable using reverse phase high pressure liquid chromatography and their 1H and 13C NMR spectra have been recorded at 500 and 125 MHz, respectively. By use of the 1H1H COSY and 1H13C shift-correlated two-dimensional spectra all the protons and carbons in the free ligand and in both complexes have been unambiguously assigned. The α- and β-Cr(CO)3 complexes can be clearly differentiated on the basis of their 500 MHz 1H NMR spectra. Protons proximal to the metal are markedly deshielded if they lie in the negative region of the cone of anisotropy of the carbonyl ligands. The changes in conformation of the diterpene skeleton are detectable by NMR and correlate very well with the X-ray crystallographic data. Analogies are drawn between the structures of the podocarpate complexes and those of related estrogenic steroids.