2302701

BIOMEDICAL AND ENVIRONMENTAL MASS SPECTROMETRY, VOL. 15, 525-528 (1988)
Negative Ion Mass Spectra of Dihydropyridine
Calcium-channel Blockers
J. D. Ehrhardtt
Spectrometrie de Masse, Institut de Pharmacologie (UA 589 CNRS), Facultt de Medecine 11, rue Humann, 67000 Strasbourg,
France
J. M. Ziegler
Service de Spectrometrie de Masse, Faculte de Pharmacie, 30 rue Lionnois, 54000 Nancy, France
The negative ion mass spectra of some dihydropyridineanalogues of nifedipine are studied; they show a fragmentation which is highly dependent on the position of the nitro group on the phenyl ring: 3’-nitro derivatives give
essentially the molecular anion, whereas 2’-nitro derivatives lose successively H,O, RO and 0. In addition, (2,1,3benzoxadiazol4yl) derivatives show essentially a [M - ROH]- peak. Possible pathways for these fragmentations
are given.
INTRODUCTION
Synthesis
Dihydropyridine (DHP) calcium-channel blockers
became a new class of drugs important in the treatment
of angina pectoris (nifedipine, Adalate ; nicardipine,
Loxen).
As these compounds are administered orally at relatively low doses (less than 10 mg), it was necessary to
work out sensitive and specific assays.
As they contain groups with high electron affinity
(-NO,, halogen atoms), the method of choice seemed
to be gas chromatography combined with negative ion
detection mass spectrometry.’ But when we studied
these negative ion spectra it appeared that, depending
on the position and the kind of substituent on the aromatic ring, the fragmentation of these compounds under
electron-capture conditions was very different.
We would like to discuss here these observations.
Table 1 gives the structure, code and names of the compounds studied. Nilvadipine (9) was described in Ref. 1;
1,2, 6,7 and 8 were from Bayer, 8,14,15 from Sandoz;
the other compounds were synthesized as described in
this article.
Oxidation of the DHPs:, the DHPs are oxidized to
pyridines with 0.1 M hydrochloric acid and 0.15 M
sodium nitrite at 45 “C during 1 h.
Synthesis of 10 and 13:3 10 mmol of 3- or
4-nitrobenzaldehyde, 22 mmol of ethyl acetoacetate, 10
ml of ethanol and 12 mmol of 28% aqueous ammonia
are successively charged in a 100 ml autoclave and
heated to 110°C for a night. After cooling and evaporating the volatiles, the residue is purified to give 10 and
13 with 80% yield.
Compounds 3 and 11 were obtained by reduction of
the methyl 2- or 3-nitrobenzoates to the corresponding
deuterated alcohols with LiAID, in ether: and oxidation to 2- or 3-nitro deuterated ben~aldehydes,~
which
were used as for the synthesis of 10.
Compounds 4, 12 and 16 were obtained by dissolving
respectively 1, 10 and 14 in 0-deuterated ethanol; after
48 h, the exchange was of about 50%, which was sufticient for studying fragmentation.
5
was
obtained
by
treating
Compound
2-nitrobenzaldehyde and ethyl acetoacetate with
methylamine in ethanol, as for 10. The yield was low
(20%) and silica gel chromatography was required for
purification.
EXPERIMENTAL
The mass spectra were obtained by direct introduction
of the compounds into the ion source of an LKB 2091
mass spectrometer modified for negative ion detection.
Temperature of the ion source was 200 “C and the moderating gas ammonia. Tandem mass spectra were
obtained on a NERMAG R3010 triple-quadrupole
instrument with collision energy at 20 eV.
t Author to whom correspondence should be addressed.
08874134/88/10052S04 $05.00
0 1988 by John Wiley & Sons Ltd
RESULTS AND DISCUSSION
According to their structure, these compounds may be
subdivided into four groups; as can be seen from Tables
2-4, this structural subdivision correlates well with different types of fragmentation depending on the position
and the nature of the substituent in position 4.
1-5
Group 1 : 4-(2’-nitropheny1)-DHPs
2: 4-(3’-nitropheny1)-DHPs
6-12
3 : 4-(4‘-nitropheny1)-DHP
13
4: 4-(2’,1’,3’benzoxadiazol-4’yl)-DHPs14-16
Received 31 July 1986
Revised 2 February 1987
J. D. EHRHARDT AND J. M. ZIEGLER
526
Table 1. Structure, name and code of the compounds studied
14-
1-13
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
X
R,
2-NO,
2-NO,
2-NO,
2-NO,
2-NO,
3-NO,
3-NO,
3-NO,
3-N02
3-NO,
3-NO,
3--NO,
4-NO,
-CH,
-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,
-CH -(CH 3)
-CH,
-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,
-CH,-CH,
R2
-CH,
-CH,-CH(CH,),
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-( CH ,),-O-CH,
-CH,-N(CH,)-CH,-Q
-CH -(CH ,)
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH,-CH,
-CH-(CH,),
-CH,-CH,
,
R3
R4
H
H
H
H
H
D
H
H
H
H
H
H
H
D
H
H
H
H
H
,
16
Nifedipine
Nisoldipine
Bay-a-1040
Bay-k-5552
Nitrendipine
Nimodipine
Nicardipine
Nilvadipine
Bay-e-5009
Bay-e-9736
YC-93
(*2-cyano)
D
-CH,
H
H
H
H
H
H
D
H
H
H
PY-108-068
PN-200-110
D
Table 2. Negative ion fragmentation of group 1 compounds
1
346 (18%)
345 (8%)
329 (1%)
328 (2%)
2
388 (18%)
387 (5%)
371 (1%)
370 (<1%)
3
375 (10%)
374 (1%)
4
5
347 (5%)
388 (100%)
345 (5%)
;;{
&l;
‘329 (1%)
371 (1%)
‘
297 (13%)
339 (6%)
297 (7%)
281 (100%)
323 (33%)
281 (100%)
270 (2%)
{312 (1%)
270 (1%)
269 (3%)
{311 (2%)
269 (2%)
356 (<1%)
311 (11%)
295 (100%)
285 (4%)
283 (6%)
328 (1%)
297 (12%)
281 (100%)
270 (1%)
269 (3%)
Table 3. Negative ion fragmentation of group 2 and 3 compounds
6
7
8
9
10
360
418
479
385
374
(100%)
(100%)
(100%)
(100%)
(100%)
359 (3%)
417 (3%)
478 (2%)
11
375 (100%)
374 (1 Yo)
12
13
375 (100%)
374 (100%)
374 (7)
373 (3%)
343
410
462
368
357
358
357
357
(20%)
(20%)
(11%)
(50%)
(16%)
(23%)
(2%)
(14%)
342 (3%)
400 (3%)
461 (8%)
356 (4%)
356 (2%)
356 (2%)
Table 4. Negative ion fragmentation of group 4 compounds
14
371 (3%)
370 (<1%)
15
371 (4%)
370 ( ~ 1 % )
16
372 (1%)
370 (<1%)
325
339
311
326
(100%)
(43%)
(100%)
(100%)
388 (4%) M-benzyl
DIHYDROPYRIDINE CALCIUM-CHANNEL BLOCKERS
Briefly, we observe that:
(i) Group 1 compounds give a rather complicated
fragmentation.
(ii) Group 2 compounds show the molecular anion as
base peak with an important M - 17.
(iii) Compound 13 shows only the molecular anion.
(iv) Group 4 compounds give spectra where the most
important peak(s) correspond to the loss from the
molecular anion of the alcohol(s) of the ester
groups.
When the ester groups in compounds of group 1 or 4
are different, two sets of peaks are shown corresponding
to the loss of either one or the other alcohol group, the
heavier being lost preferentially.
To understand these fragmentations, it seemed interesting to find out what hydrogen atom(s) is (are) lost
during the fragmentation; so, we first synthesized the
pyridine derivatives of compounds 1, 6 and 14: all
derivatives give only the molecular anion ; this means
that, at least in group 2 compounds, the hydrogen lost
in the M - OH peak does not originate from the aromatic ring. So most probably, it comes from position 1
or 4 of the dihydropyridine ring, the most labile hydrogen of these compounds. For this reason, we synthesized the deuterated derivatives 3, 4, 11, 12 and 16 as
well as the N-methyl derivative 5.
Fragmentation of 4-(3’-nitrophenyl) derivatives
The comparison of the spectra of compounds 1&12
shows clearly that the hydrogen lost during the fragmentation is that located in position 1 (on the nitrogen
of the DHP ring), as 10 and 11 give an M - 17 peak
and 12 an M - 18 peak.
What is not clear with these compounds is if the
hydrogen and the oxygen atoms are lost as an OH
species or separately. The fact that the 4-nitrophenyl
DHP (13), where the distance between the NO, group
and the hydrogen atom in position 1 is greater than in
10, does not give the M - 17 peak supports the hypothesis of the transfer of the hydrogen atom as depicted in
Scheme 1.
Fragmentation of 4-(2’,1’,3’-benzoxadiazol4’yl)
derivatives
The comparison of the spectra of 14 and 16 which both
give an M - 46 (ethanol) peak shows that, in this case,
it is not the hydrogen atom in position 1 which is lost,
but most probably that in position 4, but we could not
527
0
H
Scheme 2. Fragmentation of 4-(2’,1‘,3-benzoxadiazol)-DHPs.
get the starting material to synthesize the corresponding
4-deuterated analogue.
Scheme 2 shows the possible fragmentation of this
class of compounds.
Fragmentation of 4-(2’-nitrophenyl) derivatives
The elucidation of the fragmentation of nifedipine (1)
and nisoldipine (2) is much more difficult. The spectrum
of nifedipine contains ions corresponding formally to
M, M - H, M - OH, M - H,O, M - (H,O + CH,O),
M-(H,O C H 3 0 0) (base peak). The hydrogen
atoms at position 1 and 4 are both lost in the
M - (H,O CH,O) and M - (H,O + CH,O 0)
peaks as compounds 3 and 4 give the same ions as
nifedipine (the difference of 14 mass units is due to the
fact that 3 and 4 are ethyl ester instead of methyl and
that one ester group is lost).
As we could not observe metastable ions which could
explain the fragmentation pattern, we turned to tandem
mass spectrometric experiments with nifedipine 1. These
showed that: peaks at m/z 281 and 297 are daughters of
m/z 328 and 346; peak m/z 281 is not a daughter of
peak mfz 297 which could happen through loss of an
oxygen atom.
So it is probable that, following the loss of water to
give m/z 328 from the m/z 346 peak, the peaks at m/z
281 and 297 are obtained through two ways, as depicted
in Scheme 3.
Capture of an electron gives an anion at m/z 346 and
then transfer of either a proton or a hydrogen radical
gives the species l a and lb, which both lose water. The
localization of the free electron may then direct the fragmentation: loss of either a methoxy radical alone (to
give m/z 297) or a methoxy radical and an oxygen atom
(to give m/z 281).
In conclusion, the negative ion mass spectra give
information allowing location of the nitro substituent
+
+
+
Scheme 1. Fragmentation of 4-(3’-nitropheny1)-DHPs.
+
J. D. EHRHARDT AND J. M. ZIEGLER
528
\OH
8
la
,
- H,O
N
- 0,
-
OCH,
H,CO,C
8
7
\
m/z
328
m/z
281
CO,CH,
" 3H3C
c 0 2 c ~ c H 3
H
m/z 3 4 6
-
Q-.PQ
\OH
- H,O
H3C0,C
-'OCH,
H,C 0,C
lb
H ,CO,C
m/z
328
m/z 297
Scheme 3. Fragmentation of 4-(2'-nitrophenyl)-DHPs
on the phenyl cycle. On the other hand, owing to high
yield of negative ions, electron-capture mass spectrometry with negative ion detection will be a very sensitive method of detection of these compounds.'
Acknowledgement
We thank the Sociiti Nermag, 92500 Rueil-Malmaison, France, for
realization of the tandem mass spectrometric experiments.
REFERENCES
1. Y. Tokuma, T. Fujiwara and H. Noguchi, J. Chromatogr. 345,
51 (1985).
2. S. Higuchi and Y. Shiobara, Biomed. Mass Spectrom. 5, 220
(1 978).
3. Y. Watanabe, K. Shiota, T. Hoshiko and S. Ozaki, Synthesis 761
(1 983).
4. P. Newman. P. Rutkin and K. Mislow, J. Am. Chem. SOC.80,
465 (1958).
5. K. E. Pfitzner and J. G. Moffat, J . ,4m. Chem. SOC.87, 5661
(1965).