Functional groups mass spectral data

The combination of IR and mass spectral data provides key information on the structure of an unknown compound. The mass spectrum reveals the molecular weight of the unknown (and the molecular formula if an exact mass is available), and the IR spectrum helps to identify the important functional groups. 

Once we know a compound s molecular formula from its mass spectral data and the identity of its functional group from its IR spectrum, we can then use its H NMR spectrum to determine its structure. A suggested procedure is illustrated for compound X, whose molecular formula (C4Hjj02) and functional group (C=0) were determined in Section 13.7. 

The chemical structure of baogongteng A (1), a new myotic agent from Erycibe obtusifolia, has been determined. High-resolution mass-spectral data indicated the presence of a 3,6-disubstituted tropane skeleton. Infrared and and n.m.r. data indicated the presence of secondary amine, secondary hydroxyl, and acetoxy functions, and the location of the latter group at C-6 was based on comparison of n.m.r. chemical shifts with those of 6B-acetoxy-3a-tropanol. The hydroxyl group was assigned to the 23-position from the n.m.r. spectrum of the N-methylated alkaloid. 

MeONa loses formaldehyde by reverse aldol reaction. Methylation of (119) with diazomethane yields a methyl ester, with concomitant appearance of an indolenine chromophore. N.m.r. and mass spectral data reveal the presence of the epoxide function, and determine the configuration at C-16. The remainder of the stereochemical details, and the position of the aryl methoxy-group, which rests at present solely on u.v. evidence, have yet to be established. 

Acetylation of delectine yielded an jV,0-diacetyldelectine (160). Basic hydrolysis of delectine gave a parent amino alcohol (161) and anthranilic acid. Treatment of alkamine (161) with methyl iodide and sodium hydride afforded derivative 162, which was identical with 0,0-dimethyllycoc-tonine. Soviet chemists also reported the isolation of 0,O-dimethylly-coctonine (162) from the same plant. The complete assignments of all functional groups and their stereochemistry were made on the basis of comparisons of H NMR and mass spectral data with those of other lycoc-tonine-type alkaloids. 

The chloroform extracts of the roots of D. dictyocarpum (138) yielded 1.83% total alkaloids consisting of the known alkaloids methyllycaconitine (155) and lycoctonine (58) and two new bases. A partial structure (18 ) has been suggested for the amorphous alkaloid with molecular weight 453 on the basis of chemical and extensive spectral data. The presence of two secondary hydroxyl groups was confirmed by acetylation with acetic anhydride in pyridine. The IR, H NMR, and mass spectral data for the second amorphous alkaloid with molecular weight 541 indicated the presence of an N-ethyl, three methoxyl, and a benzoyl ester function. 

Mass spectrum interpretation is essential to solve one or more of the following problems establishment of molecular weight and of empirical formula detection of functional groups and other substituents determination of molecular skeleton (atom connectivity) elucidation of precise structure and, even in favorable cases, certain stereochemical features. As discussed in the previous chapters, electrospray (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most effective and successful interfaces for the liquid chromatography—mass spectrometry (LC—MS) that have been developed. Thus, we will focus on how to interpret the mass spectral data generated by either ESI or APCI in this section. 

Until just recently, the structure of the profoundly active (experimentally) oncolytic alkaloid leurosidine has eluded elucidation Spectral data (UV, IR and UMR), functional group determination, elemental analysis and mass spectral data for the parent alkaloid and its derivatives agree with a formulation of isomeric with VLB ... 

Segall and coworkers described the in vitro mouse hepatic microsomal metabolism of the alkaloid senecionine (159) (Scheme 34). Several pyrrolizidine alkaloid metabolites were isolated from mouse liver microsomal incubation mixtures and identified (222, 223). Preparative-scale incubations with mouse liver microsomes enabled the isolation of metabolites for mass spectral and H-NMR analysis. Senecic acid (161) was identified by GC-MS comparison with authentic 161. A new metabolite, 19-hydroxysenecionine (160), gave a molecular ion consistent with the addition of one oxygen atom to the senecionine structure. The position to which the new oxygen atom had been added was made evident by the H-NMR spectrum. The three-proton doublet for the methyl group at position 19 of senecionine was absent in the NMR spectrum of the metabolite and was replaced by two signals (one proton each) at 3.99 and 3.61 ppm for a new carbinol methylene functional group. All other H-NMR spectral data were consistent for the structure of 160 as the new metabolite (222). 

The correct analysis of the homologous ion series has certain limitations. Low abundances of peaks in some series require the attention and experience of a researcher. Usually alkane series are dominated in the mass spectra of the most various compounds. Fragmentation initiated by one functional group may completely suppress or notably camouflage other reactions of polyfunctional substances. In the latter case it is useful to consider IR-spectroscopy data in mass spectral interpretation. 

Volume 15 of this series features four important reviews of research on alkaloids. Chapter 1 by B. S. Joshi, S. W. Pelletier and S. K. Srivastava is the first comprehensive review of the carbon-13 and proton NMR shift assignments and physical constants of diterpene alkaloids and their derivatives. In addition to the catalogue of spectral and physical data, the chapter includes a table of the occurrences of these alkaloids in various plant species, tables containing molecular formulas versus calculated high-resolution mass values, and calculated high-resolution mass values versus the molecular formulas of diterpenoid alkaloids, as well as seven tables summarizing the carbon-13 chemical shifts of various functional groups in diterpenoid alkaloids. 

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