Submicro Inverse-Detection Gradient NMR:  A Powerful New Way of Conducting Structure Elucidation Studies with <0.05 μmol Samples

Quantities of material required for structural analysis were reduced substantially following the introduction of 3 mm microinverse and microdual NMR probes in 1992. We now report the first very low-level results obtainable with a new 1.7 mm submicro-inverse-detection gradient or SMIDG NMR probe. Using this technology at 600 MHz, it was possible to fully characterize an 8% impurity contained in a 0.55 μmol sample of cryptolepine (1) that had been standing in excess of 2 years since its initial isolation. The impurity was unequivocally identified as cryptolepinone (2) through the concerted interpretation of GHSQC, GHMBC, homonuclear TOCSY, and ROESY spectra in conjunction with APCI LC/MS and CID data acquired from a portion of the serial dilution solution used to prepare the NMR sample. Submicro-inverse-detection gradient probes offer the prospect of reducing still further the quantities of sample required for full characterization under favorable circumstances, making rare and potentially novel natural products amenable to structural determination. SMIDG NMR technology is equally applicable to a range of small samples requiring characterization such as isolated impurities from drug substances, isolates from drug degradation studies, and secondary metabolites.