SUBMICRO INVERSE-DETECTION GRADIENT NMR - A POWERFUL NEW WAY OF CONDUCTING STRUCTURE ELUCIDATION STUDIES WITH LESS-THAN-0.05 MU-MOL SAMPLES

Quantities of material required for structural analysis were reduced s ubstantially following the introduction of 3 mm microinverse and micro dual NMR probes in 1992. We now report the first very low-level result s 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 mu mol sample of cryptolepine (1) that had been standing in excess of 2 years since it s initial isolation. The impurity was unequivocally identified as cryp tolepinone (2) through the concerted interpretation of GHSQC, GHMBC, h omonuclear 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, mak ing 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 impuritie s from drug substances, isolates from drug degradation studies, and se condary metabolites.