A mainly NMR-based structure elucidation of a surprising vindoline trimer with the aid of non-uniform sampled 1H-13C HSQC and HMBC spectra

Abstract

Two unexpected and unusual vindoline trimers, a ketone and a methyl ether cation, were isolated from a reaction aimed at producing new, synthetically modified vinca alkaloids. The presence of several very similar molecular subunits in these products resulted in highly congested 1H and 13C NMR spectra, whereby their structure elucidation and proper spectroscopic characterization turned out to be a rather challenging task. To overcome this problem, non-uniform sampling was applied as a resolution enhancement tool on an 800 MHz NMR spectrometer. Our present description of how we accomplished both the structure determination and full NMR assignment of these molecules via the use of state-of-the-art NMR and MS techniques is not only paramount in proving their structure, but also exemplifies how—contrary to a common notion—difficult it can be to solve the structure of small molecules even with the aid of the latest NMR and MS methods.