Molecular structure, determination from mass spectra

A new amorphous alkaloid has been recently isolated from the Chinese plant T. bufalina (Ervatamia hainanensis) collected on Hainan Island (53). Its mass spectrum showed a molecular ion at m/z 382, corresponding to C23H30N2O3. From the fragmentation pattern, this compound would appear to be a coronaridine derivative in which a C2H50 unit is attached to the aliphatic moiety of the molecule. The structure 111 with (S) configuration at C-3 was determined by a detailed analysis of its H-NMR spectrum (Table IV) in comparison with the data of other ibogan alkaloids. 

The general procedure is to use reconstructed ion chromatograms at appropriate m/z values in an attempt to locate compounds of interest and then look at the mass spectrum of the unknown to determine its molecular weight. MS-MS can then be employed to obtain spectra from this and related compounds to find ions that are common to both and which may therefore contain common structural features. Having the same m/z value does not necessarily mean the ions are identical and further MS-MS data or the elemental composition may be required. If these data do not allow unequivocal structure identification, then further MS" information may be required. 

Oxidation products from tetrahydropapaveroline with potassium ferricyanide were reinvestigated by Mak and Brossi 21), who confirmed structure 28 for the initial product. Air oxidation of 29 gave after acetylation a product with the same physical properties reported by Harley-Mason, but its mass spectrum showed a parent ion at m e 900 and molecular composition C48H40N2O16, suggesting it to be a dehydro dimer of 12. The H-NMR spectrum of the dimer showed that the double bonds at the 5,6 positions were intact. The point of dimerization was determined by comparing the multiplicities of carbons in the C-NMR spectra of the dimer and dibenzopyrrocoline models. For monomeric compounds the peak ascribed to C-12 in compound 11 was a triplet and a doublet in 12, but a singlet in the dimer. Thus, the products from oxidation of 28 had structures 31 and 32 rather than 29 and 30, respectively. 

Mass spectrometry (MS) is based on the generation of gaseous ions from an analyte molecule, the subsequent separation of those ions according to their mass-to-charge (m/z) ratio, and the detection of those ions [23]. The resulting mass spectrum is a plot of the (relative) abundance of the produced ions as a function of the m/z ratio. MS is important in determining the molecular mass, molecular fonnula and elemental composition and in elucidating the structure of the compound. Moreover, MS is the most sensitive spectrometric method for molecular analysis. Now, on-line... 

Q Given a structure, predict the major ions that will be observed in the mass spectrum from fragmentation of the molecular ion. Use these predictions to determine whether a proposed structure is consistent with the spectrum. 

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