Mass spectrometry ring structures

Compound 165 was reduced with SDMA and the product was hydrolyzed with acid to effect ring contraction, affording 4-deoxy-4-C-[(jR,S)-ethylphosphinyl]-o ,/ -D-ribo- and -L-lyxo-furanoses (166), which were characterized by conversion into the peracetates. After separation by chromatography on silica gel, structures 167-170 were established for these peracetates by 400-MHz, -n.m.r. spectroscopy and high-resolution mass spectrometry the structures of these products, their probable conformations, and the yields from 165 are summarized in Scheme 7. 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

Losses of 28 and 29 Daltons from the molecular ion suggests that the hydroxyl group is attached to the benzene ring, thus eliminating structure I. By preparing a TMS derivative, the presence of the hydroxyl group can be established. However, the position of the aromatic substitution cannot be determined using only mass spectrometry. 

The structures of compounds 55a,c and 56a,c were established by means of NMR spectroscopy and mass spectrometry. Due to the different polarity of the C=N and C=P triple bonds, the silicon ring atom in 55a,c is bound to the nitrogen atom, and in 56a,c to the carbon atom of the C=P moiety. The molecular structure of 55a was further determined by single-crystal X-ray diffraction analysis (Fig. 16).14 The four-membered SiNAsC framework is slightly puckered (folding angle N—Si—C/Si—C—As 7°), and... 

The structure of the 3-oxo derivative 65 was determined by high resolution mass spectrometry, which demonstrated that a single oxygen atom had been incorporated into the alkaloid skeleton. Peaks in the mass spectrum at mte 174 and 188 provided evidence that the additional oxygen atom was not in the dihydroindole portion of the molecule, while a peak at mte 138 supported the presence of an oxygenated piperdine ring. This metabolite was also chemically compared with authentic oxodihydrovindoline derivatives previously prepared and provided for comparison by J. P. Kutney. 

An intramolecular cyclization of a tetrazole-containing nucleoside 64 has been observed by Chu and co-workers <1997JOC7267> (cf. other properties of this compound in Table 3). These authors reported that the 3-OH group on the arabinofuranose ring participates in an addition reaction at position 7 of the tetrazolo[l,5-c]pyrimidine ring to form 65 which is a dihydro derivative of the parent heterocycle. The final structure elucidation has been carried out with the help of the tricyclic tandem mass spectrometry (MS/MS) fragment 66. 

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