Received 7 March 2016; accepted 17 April 2016; published 20 April 2016
Isolation of open-chain intermediates plays a key role in many synthetic organic reactions. Mass spectra data of the condensed indoles as pyrroloquinoline and the stability of the intermediates confirm the structure of the pyrroloquinolie product.
The indole ring is an important pharmocophore in modern drug discovery. Pyrroloquinoline quinone (PQQ) presenting the indole moeity, also known as methoxatin, was first identified in methylotrophic bacteria as a coenzyme for methanol dehydrogenase in 1979  . It is widely distributed in a variety of food and other sources   , and is considered to be a redox active nutrient that can produce or scavenge superoxide depending on different cellular context  . In recent years, PQQ has garnered much attention due to its versatile roles in biological processes, including antioxidant function, cognitive promotion, neuroprotection and cardioprotection  -  . Also, PQQ is reported to have anticancer activity by inducing apoptosis in human Jurkat cells and promonocytic leukemia U937 cells, and this event is associated with generation of reactive oxygen species (ROS)   .
This report is concerned with the mass spectra of indole derivatives in order to confirm the structure of the obtained pyrroloquinolines.
The electron-impact mass spectra were recorded on Varian Mat 311 spectrometer at 70 eV in the Centre de Mesures Physiques de l’Ouest (CRMPO) at Rennes 1 University. The electron ionization ion source was kept at 145˚C. The EI mass spectra were obtained over the range of m/z 10 - 700.
3. Results and Discussion
3.1. Synthesis of the Studied Compounds
The studied compounds were synthesized as shown in Scheme 1. Details of the synthetic methods are reported in our articles   . Also, all the compounds were previously characterized by mass, 1H, and 13C-NMR spectra.
Scheme 1 shows the details of the synthetic methods that are reported in our articles   .
3.2. The Open Chain Intermediates I and II
The composition of ions determined by exact mass measurements of these compounds are reported in Scheme 2 and Scheme 3.
In mass spectrometry, the presence of acetyl group is proved by ejection of one methyl radical from the molecular ion followed by the loss of one molecule of carbon monoxide. The loss of CH3CN then HCN fragments leads to the ions m/z = 116 and 89 for I. This fragment which eliminate one molecule of HCN leading to the ion m/z = 89 is characteristic in indole fragmentation  . The cation m/z = 171 for I was transformed to your tautomer then eliminate a methylene radical leading to the fragment m/z = 157 for I.
For the compound II, the presence of the function ester is deduced by splitting of a molecule of ethylene C2H4 following by ejection of one molecule of carbon dioxide giving the fragment m/z = 172. The next fragmentations are similar to those of compound I.
3.3. The Condensed Indoles: Pyrolloquinolines
In the spectra of the compounds III and IV, the observed principal fragmentation processes are similar to those observed in open chain intermediate derivatives I and II.
The ions pyrrolobenzazépinium m/z = 195 for III and m/z = 197 for IV take place from the ejection of one radical hydrogen by the molecular ion which leading to the extension of the pyridinic nucleus characteristic of the aromatic ring bearing a methyl group. This data shows that the indolic proton is not concerned by the cyclisation of the open chain intermediate (Scheme 4 and Scheme 5).
In this work, mass fragmentation pathways of open chain intermediates of indole and pyrroloquinoline derivatives were investigated by electron impact mass spectrometry (EI-MS). The principal fragmentation processes in indole series are reported. The ion pyrrolobenzazépinium fragments were obtained from the extension of the pyridinic nucleus characteristic of the aromatic ring bearing a methyl group. These data show that the indolic proton is not concerned by the cyclisation of the open chain intermediate.
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