DFT-GIAO 1H and 13C-NMR Chemical Shifts Calculation of Uncaria longiflora Alkaloids
Recently, the structure elucidation of three new chiral alkaloids, namely isoformosaninol (1), formosaninol (2), and longiflorine (3), isolated from the leaves of U. longiflora var. pteropoda (Miq.) Ridsdale has been reported. Their molecular structures were initially determined by using experimental NMR data, following a systematic method for establishing the absolute configuration of pentacyclic oxindole alkaloids (POAs), and the chemical correlation method based on the known chirality of their precursor, secologanin. Indeed, the integration of the information from experimental and theoretical data can be of fundamental importance for the successful elucidation of the configurational and stereostructural assignments of organic compounds, including natural products. Thus, the present work was conducted to further support the success of this integration by focusing on the NMR data, which is the most critical spectroscopic analysis in the structural elucidation of natural products. The 1H and 13C-NMR chemical shift values for the three alkaloids were calculated using density functional theory-gauge including atomic orbitals (DFT-GIAO) approximation at the B3LYP/6-311+G(d,p) level of theory in integral equation formalism polarizable continuum model (IEF-PCM) concerning to tetramethylsilane (TMS). Statistical error analysis between the experimental and calculated supported an excellent correlation with linearity of higher than 93% and 99% for 1H and 13C-NMR chemical shifts, respectively, for the three alkaloids. The correct correspondences between experimental and calculated data sets were further supported by the mean absolute error (MAE) parameter. The present findings provide insights on the usefulness of integrating experimental and calculated NMR data to ascertain structural elucidation in easing ambiguity.