RAPID COMMUNICATIONS IN MASS SPECTROMETRY Rapid Commun. Mass Spectrom. 2008; 22: 261–264 Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rcm.3358
RCM Letter to the Editor
To the Editor-in-Chief Sir, Electron ionization mass spectrometric study of N-benzyl- and N-nitrobenzylsubstituted derivatives of cytisine. II (–)-Cytisine is a tricyclic skeleton quinolizidine alkaloid containing a 2-piperidone moiety (ring A), fused into a bispidine (rings B and C). This naturally occurring chiral compound is extracted from the seeds of Laburnum anagyroides and other Leguminosae plants.1,2 Cytisine is a very potent ligand for many neuronal nicotinic acetylcholine receptors (nAChRs),3–6 and it has been used as a radioligand in the study of these receptors.7,8 The nAChRs are widely distributed in the peripheral and central nervous system and are involved in several processes such as learning, memory, and movement.4 Cytisine has been marketed in central and eastern Europe for over 40 years for clinical use as an aid for smoking cessation (the most popular brand available is Bulgarian Tabex1).9 Moreover, some N-substituted cytisine derivatives have been found to have analgesic activity.3 In an earlier communication we described the mass spectra of N-amide and N-alkyl derivatives of (–)-cytisine.10 In this work we examine the decomposition pathways of the electron ionization (EI) molecular ions of N-benzylcytisine and newly synthesized N-nitrobenzylcytisine derivatives (2–4). For structures, see Fig. 1. The compounds analysed (1–4) were obtained from reactions of (–)-cytisine (1) with benzyl bromide derivatives.11,12 (–)-Cytisine was extracted from the seeds of Laburnum anagyroides according to the literature procedure.13 Low- and high-resolution EI mass spectra (1–4) were recorded using a
Figure 1. N-Benzylcytisine (1) and its nitro-substituted derivatives 2–4. model 402 double-sector mass spectrometer (AMD Intectra GmbH, Harpstedt, Germany) (ionizing voltage 70 eV, accelerating voltage 8 kV, mass resolution 1000 for low-resolution and 10,000 for high-resolution mass spectra). The compounds were introduced by a direct insertion probe at a source temperature of 2008C. The elemental compositions of all ions discussed were determined by accurate mass measurement using narrow-range high-voltage scans. All masses measured (1–4) were consistent with the compositions listed in Table 1 to within 2 ppm. The B/E
and B2/E linked-scan spectra were measured in the first field-free region using helium as the collision gas at a pressure of 1.73 105 mbar, ionization energy of 70 eV, and an accelerating voltage of 8 kV. All the mass spectral fragmentation routes of compounds 1–4 described in this paper have been determined on the basis of low- and high-resolution EI spectra, B2/E linked-scan mass spectra and linked scans at constant B/E. The elemental compositions and relative abundances of the ions in the EI mass spectra of compounds 1–4 are given in Table 1. The most significant fragmentation pathways of 1–4 have been interpreted according to Schemes 1 and 2. Some of the cyclic ions (e, g, h, i, j, n, p) shown in these schemes are similar to and in agreement with those reported previously.10,14,15 Depending on the substituents at the nitrogen atom N-12 (H, alkyl, benzoyl, acetyl, propionyl or benzyl group) we have observed significant differences in the abundances of relevant ions. Similar fragmentation routes of the quinolizidine skeleton have been observed for
Table 1. Elemental compositions and relative abundances of the ions in the spectra of 1–4 based on high-resolution data, and B/E and B2/E methods % Relative abundance Ion
m/z
Elemental composition
a
l m n o p q
280 325 308 278 190 189 179 134 160 148 147 146 136 91 132 130 118 117 109 93
r s t
92 90 78
C18H20N2O C18H19N3O3 C18H18N3O2 C18H18N2O C11H14N2O C11H13N2O C9H11N2O2 C9H12N C10H10NO C9H10NO C9H9NO C9H8NO C7H6NO2 C7H7 C8H6NO C9H8N C8H8N C8H7N C6H7NO C5H3NO (81%) C6H7N (19%) C7H8 C7H6 C5H4N
b c d e f g h I j k
1
2
3
4
46 — — — — 12 — 89 12 6 13 39 — 100 8 4 7 9 — —
— 40 22 — 60 26 62 — 30 21 69 60 100 — 22 13 20 25 17 18
— 43 100 8 — 7 76 — 12 5 10 31 58 — 7 5 9 11 4 6
— 21 37 7 7 6 100 — 14 8 19 41 44 — 8 6 13 17 6 7
10 — —
20 14 66
4 37 9
5 27 32
Copyright # 2007 John Wiley & Sons, Ltd.
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Letter to the Editor
Scheme 1. EI fragmentation pathways of the molecular ion of 1.
Scheme 2. EI fragmentation pathways of the molecular ions of 2–4. Copyright # 2007 John Wiley & Sons, Ltd.
other compounds from this group of quinolizidine alkaloids.14 For compounds 1–4 a typical feature of the fragmentation of molecular ions is the cleavage of the external bonds N12–CH2 and C7–C13, and C9–C11 in ring C of cytisine. In the first step of fragmentation, N-benzylcytisine (1) (a, Scheme 1, Figs. 2 and 3) undergoes cleavage of the N12–CH2 bond starting at the site of the radical in the phenyl ring of the substituent, leading to formation of the benzyl cation k as the base peak at m/z 91. The breaking of this bond also leads to the appearance of the even-electron (EEþ) ion e (12%). The homolytic bond N12–CH2 cleavage preceded by hydrogen-atom rearrangement from ring C of cytisine skeleton to the ring of the benzyl group gives the low. abundance odd-electron (OEþ ) ion r (10%). The spectrum of N-benzylcytisine also shows the high abundance EEþ ion f (89%) appearing as a result of the characteristic cleavage of the C9– C11 and C7–C13 bonds accompanied by the rearrangement of a hydrogen atom. Further fragmentation of ion f takes place by inductive cleavage (i) and leads to formation of the EEþ benzyl cation k (100%). The cleavages of the bonds C13–C7 and C9–C11 without hydrogen-atom rearrangement lead to the odd-electron distonic ion i of 13% abundance. The next step in the fragmentation of ion i is connected with . . elimination of the radical H or CH3 and this process leads to even-electron ions j or l, respectively. The two-bond cleavages C11–N12 and C7–C13 of ring C of the molecular ion of 1 with neighbouring hydrogen-atom rearrangement leads to the even-electron fragment ion g [M–CH3 NCH2Ph]þ, whereas the two-bond cleavage C7–C13 and C9–C11 with a hydrogenatom rearrangement gives the lowabundance even-electron ion h (6%). Some of the fragmentation pathways of nitrobenzylic compounds (2–4) (e–o ions) are similar to those of unsubstituted N-benzylcytisine (1). The molecular ions of N-nitrobenzyl-substituted cytisines 2, 3 are of medium abundance (40% and 43%), while in 4 the abundance is somewhat lower (21%) (Table 1). In the spectrum of compound 2, as in N-benzylcytisine (1), the base peak is ion k (EEþ) at m/z 136 Rapid Commun. Mass Spectrom. 2008; 22: 261–264 DOI: 10.1002/rcm
Letter to the Editor
Figure 2. Low-resolution EI mass spectrum of N-benzylcytisine (1).
(Schemes 1 and 2 and Table 1). In the spectrum of N-(m-nitro)benzylcytisine (3) the base peak is the EEþ ion b (m/z 308), formed as a result of one hydrogen-atom rearrangement to the oxygen atom in the nitro group and a cleavage of the hydroxyl radical, whereas the base peak of N-( p-nitro) benzylcytisine (4) is the EEþ ion f at m/z 179. The low-resolution spectra of 2–4 do not show the typical ions for the loss of NO and NO2 molecules from the nitro aromatic compounds described in the literature.16,17
The characteristic feature of the fragmentation of 2–4 is the cleavage . of the OH radical (2–4, ion b), while in compounds 3 and 4 the loss of a HNO2 molecule from the N-nitrobenzylic group . leads to the low abundance OEþ ion 18 c. Rupture of the N12–CH2 bond in the molecular ion a of 2–4 accompanied by the loss of the NO2 group leads to the odd-electron ion s (m/z 90, Scheme 2, Table 1). This ion s has also been observed as a result of the loss of the radical from EEþ ion f or k (Scheme 2) forbidden by the even-
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electron rule, formulated by Biemann19 and Karni and Mandelbaum.20 Further fragmentation of ion a also gives the EEþ ion t at m/z 78 and distonic ion i which is a precursor of ions q and l (Table 1). The high-resolution spectra (HRD) of ion q have revealed that two elemental compositions C5H3NO and C6H7N correspond to the mass m/z 93. In the spectra of compounds 2–4 ion q occurs with relative abundances of 18%, 6% and 7%. The decomposition of compounds 2 and 4 leads to the . formation of the OEþ ion d (60% and 7%, respectively) appearing as a result of the a cleavage of the N12–CH2 bond, preceded by the rearrangement of a hydrogen atom to the nitrogen atom N12. In conclusion, the EI-induced fragmentation of the molecular ions of 1–4 proceeds mainly by the cleavage of the bonds C7–C13 and C9–C11 of ring C of the cytisine skeleton as well as of the N12–C(sp3) and the N12–CH2 substituted group. The same rupture of ring C accompanied by the rearrangement of a hydrogen atom leads to the base peaks in the mass spectra of N-alkylsubstituted cytisine derivatives.10 Fragmentation of the molecular ion leads to the base peak but the actual route depends on the position of the nitro group. The presence of a nitro substituent in the meta or para position of the benzyl group results in a significant decrease in the abundance
Figure 3. B/E spectrum of the molecular ion of N-benzylcytisine (1). Copyright # 2007 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2008; 22: 261–264 DOI: 10.1002/rcm
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of cation k relative to those in the spectra of the ortho-substituted or unsubstituted compounds (2, 1). The same substituent in the ortho or meta position of the benzyl group slightly decreases the abundance of ion f in comparison with the para-nitro- (4) or unsubstituted compound (1). There is evidence for deviation from the ‘even-electron rule’ of nitrosubstituted compounds. The typical loss of the groups NO and NO2 has not been observed,16,17 but the fragmentation of N-nitrobenzyl-substituted cytisine derivatives (2–4) is characterised . by the loss of the OH radical ion, while two of them, 3, 4, additionally lose an HNO2 group.18
Acknowledgements The authors wish to express their appreciation for help to Prof. Władysław Boczon´ and Mariena Mattson.
Anna K. Przybył* and Wiesław Prukała Faculty of Chemistry A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland
Copyright # 2007 John Wiley & Sons, Ltd.
*Correspondence to: A. K. Przybył, Faculty of Chemistry A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan´, Poland. E-mail:
[email protected]
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Received 9 October 2007 Revised 13 November 2007 Accepted 14 November 2007
Rapid Commun. Mass Spectrom. 2008; 22: 261–264 DOI: 10.1002/rcm
