Photolysis of NSAIDs. Part 3: Structural educidation of photoproducts of tolmetin in methanol

Photolysis of NSAIDs. Part 3: Structural elucidation of

photoproducts of tolmetin in methanol

Ming-Thau Sheu,

Jender Wu,

Chih-Jui Chen,

Su-Hui Chao

and An-Bang Wu

a

Graduate Institute of Pharmaceutical Sciences, College of Pharmacy, Taipei Medical University, 250 Wu Hsing Street, Taipei 110, Taiwan

b

Department of Pharmacy, College of Pharmacy, Taipei Medical University, Taipei 110, Taiwan

Received 6 February 2004; revised 15 August 2004; accepted 23 August 2004 Available online 21 September 2004

Abstract—A sample of 10 mM tolmetin in methanol was photo-irradiated with a Hanovia 200 W high-pressure quartz Hg lamp for four days. In total, eight photoproducts were observed from the HPLC chromatogram. Three major photoproducts were separated, and their structures were elucidated by spectroscopic methods. The structures of all photoproducts were further determined by LC– ESI-MS. A reaction scheme of tolmetin was proposed.

2004 Elsevier Ltd. All rights reserved.

Tolmetin (TLM), 1-methyl-5-(p-toluoyl)-1H-pyrrole-2-acetic acid sodium salt dihydrate, is a non-steroidal anti-inflammatory drug (NSAID), which is widely used as an analgesic and antirheumatic agent.1 _However,

drug-induced photosensitivity or phototoxicity is always related to photoproducts.2 _{Photolysis showed that}

0.1 mM TLM in phosphate-buffered saline solution undergoes photodecarboxylation to 1,2-di-methyl-5-pyrrolyl ketone in nitrogen and to p-toluoyl-1-methyl-2-hydroxymethyl-5-pyrrolyl ketone and 5-(p-toluoyl)-1-methyl-2-pyrrole carbaldehyde in air.3

In the present study, an amount of 10 mM TLM in methanol was irradiated with a Hanovia 200 W high-pressure mercury lamp4 _{for four days. In total, eight}

photoproducts were observed from the HPLC5 _and

LC–MS6 _{chromatograms. Three major photoproducts}

3, 7, and 8, were separated according to the elution order by the preparative HPLC. The structures of photo-products 3, 7, and 8 were identified by spectroscopic methods to be p-methylbenzaldehyde, 5-(p-toluoyl)-1-methyl-2-pyrrole carbaldehyde, and p-toluoyl-1,2-dimethylpyrrolyl ketone, respectively.7 _{The molecular}

weights of the remaining five minor photoproducts were determined by LC–ESI-MS with a positive mode of polarity.

A novel a-cleavage of a ketone: Photoproduct 2 had a molecular weight of 109 g mol
1, and its structure is most likely to be N-methyl-2-formylpyrrole. The suc-cessful identification of 2 represents an interesting find-ing, which indicates that a typical radical reaction of a-cleavage of a ketone8 _{into two moieties occurred.}

TLM is a diaryl ketone with a phenyl ring on one side and a pyrrolyl group on the other. Consideration of the resonance energy of benzene versus pyrrole9_in

con-jugation with the carbonyl group, which equals 36 ver-sus 22 kcal mol
1, implies the double bond character of C80_–C10_{is stronger than C8}0_{–C5 due to the greater}

elec-tron distribution of the former bond. The counterpart of the cleavage is photoproduct 3, p-methylbenzaldehyde, as verified by NMR spectroscopy. Due to the apparently weaker bond strength of C80_{–C5 in the parent drug,}

TLM, the bond fission occurs mainly along C80_{–C5 to}

generate acyl (m/z 119) and pyrrolyl radicals (m/z 138). Next, each of the two radicals picks up a hydrogen atom from the solvent molecules and the former becomes pho-toproduct 3. The latter pyrrolyl radical forms firstly an acetic acid derivative, which decarboxylates and follows by oxidation with singlet oxygen producing photoprod-uct 2 (MW = 109 g mol
1 equivalent to m/z 138
30 + 1; when the functional group changed from –CH2(C@O)OH to –CHO, m/z would decrease by 30

units). Meanwhile, TLM is a weak acid, which releases protons as the catalyst. An equilibrium reaction of nucleophilic addition of methanol to the carbonyl group of 2 (m/z increases by 32 units) produces a hemiacetal 1 (MW = 141 g mol
1).

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2004Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2004.08.177

Keywords: Tolmetin; Photoproducts; LC–ESI-MS.

* Corresponding author. Tel.: +886 2 27361661x6121/27366518; fax: +886 2 27366518; e-mail:[email protected]

Tetrahedron Letters 45 (2004) 8107–8109

Tetrahedron

Letters

Decarboxylation and oxidation: By decarboxylation of TLM and N-methyl-2-pyrrolyl acetic acid (m/z decreases by 44 units) initially forms methylene radical intermedi-ates. The following reactions split into two ways. First, the former intermediate intercepts a hydrogen atom

from methanol to form compound 8 (MW =

213 g mol
1). Second, the two intermediates proceed via similar reaction pathways by oxidation with the singlet oxygen to generate the oxidized forms of an

alco-hol 4 (MW = 229 g mol
1), two aldehydes 3 (MW = 120 g mol
1), and 7 (MW = 227 g mol
1), respectively.3

Photoproduct 5 is simply a hydrated form of 7 (MW = 245 g mol
1; m/z increases by 18 units indicating that 1 mol of H2O is added to 7), just like its parent drug

(TLM) with a dihydrate structure. Photoproduct 6 forms neither a negative nor positive charged species, thus its molecular weight could not be determined. In all, seven of the eight photoproducts with their chemical

Table 1. Structure elucidation based on LC–MS of TLM and photoproducts

Compound Retention time (min) Quasimolecular ion and fragment (m/z) Difference in m/za Chemical structureb

1 2.41 [MH]+: 14 2
116 (
118
30 + 32) Fragmentation: 110 2 2.51 [MH]+_{: 110}
148 (
118
30) Fragmentation: 82, 99 3 6.04[MH]+_{: 119}
139 (
109
30) Fragmentation: N.D.c 4 9.99 [MH]+_{: 230}
28 Fragmentation: 212, 119 TLM 10.77 [MH]+_{: 258 —} Fragmentation: 119 5 12.29 [MH]+_{: 24 6}
12 (
30 + 18) Fragmentation: 228, 214, 119 6 15.49 — — — 7 24.20 [MH]+: 228
30 Fragmentation: 214, 119 8 41.21 [MH]+: 214
44 Fragmentation: 119 a

[MH]+in m/z units. The difference in MW by subtracting the molecular ion of TLM from compound 1–8.

b

IUPAC names: TLM, 1-methyl-5-(4-methylbenzoyl)-1H-pyrrole-2-acetic acid; 1, a hemiacetal of 2; 2, N-methyl-2-formylpyrrole; 3, methyl-benzaldehyde; 4, toluoyl-1-methyl-2-hydroxymethyl-5-pyrrolyl ketone; 5, hydrate of 7; 7, 5-(toluoyl)-1-methyl-2-pyrrole carbaldehyde; 8, p-toluoyl-1,2-dimethyl-5-pyrrolyl ketone. The structures of photoproduct 3, 7, and 8 confirmed by spectroscopic methods.

c

N.D., no data.

Figure 1. A proposed photodegradation reaction scheme of TLM: (a) a-cleavage of a ketone; (b) decarboxylation; (c) oxidation with singlet oxygen. 8108 M.-T. Sheu et al. / Tetrahedron Letters 45 (2004) 8107–8109

structures were further checked by LC–ESI-MS10 _and

listed inTable 1. A proposed reaction scheme of photo-lysis of TLM in methanol is shown inFigure 1.

References and notes

1. Squires, J. E.; Mintz, P. D.; Clark, S. Transfusion 1985, 25, 410–413.

2. Moore, D. E. Drug Safety 2002, 25, 345–372.

3. Giuffrida, S.; De Guidi, G.; Sortino, S.; Chillemi, R.; Costanzo, L. L.; Condorelli, G. J. Photochem. Photobiol. B-Biol. 1995, 29, 125–133.

4. Irradiation conditions: A Hanovia 200 W high-pressure quartz mercury lamp (Union, NJ) was used as a light source. The light intensity of the monochromatic radiation was measured at 310 nm to be 0.65 mW/cm2_.

5. HPLC apparatus and separation conditions: An Alcott 760 HPLC pump system (Norcross, GA) equipped with a Jasco 875-UV detector (Tokyo, Japan) set at 254nm, and

a preparative YMC-Pack Pro C18 of 250· 20 mm id

column (Tokyo, Japan) was used with a mobile phase of

CH3CN–CH3OH–1% HOAc (4:63:33, v/v/v). The flow

rate was 10 mL/min, and the injection volume was 200 lL. 6. LC–MS instrument and conditions: An HP series 1100LC/ MSD (Palo Alto, CA) instrument consisted of an Inertsil 5 ODS-80A column (150· 2.1 mm id) and a mobile phase of

CH3OH–0.1% HOAc (55:45, v/v). The UV detector was

set at 254nm, the flow rate at 0.3 mL/min, and the injection volume at 10 lL. The MS conditions were optimized as follows: API electron spray interface, posi-tive mode polarity, a drying gas flow of 10 L/min, a nebulizer gas pressure of 60 psi, a drying gas temperature of 350C, a fragmentor voltage of 100 V, a capillary voltage of 3500 V, and a scan range of m/z 0–600, at 1.15 s/ scan.

7. Characterization the photoproducts: NMR: The 1_{H and} 13_{C NMR spectra were taken on a Bru¨ker, ACE-500}

FT-NMR (500 MHz) (Ettlingen, Germany). 2D FT-NMR of heteronuclear multiple quantum coherence (HMQC) for determining 1J (C, H) correlation, and heteronuclear multiple bond connectivity (HMBC) for showing2J (C, H) and3J (C, H) long-range coupling relations were used.

p-Methylbenzaldehyde (3): 1_{H NMR (in CD} 3OD): d in ppm relative to TMS, 9.89 (s, 1H, C80_{O@C–H), 7.88–7.90} (m, 2H, C20 _{and C6}0_{), 7.25–7.27 (m, 2H, C3}0 _{and C5}0_), 2.39 (s, 3H, p-CH3). 13 C NMR (in CD3OD): 191.7 (C80), 144.9 (C40_{), 130.8 (C2}0 _{and C6}0_{), 130.1 (C3}0 _{and C5}0_),

129.4(C10_{), 21.6 (C7}0_{). EI-MS (70 eV): m/z (rel int. %) 136}

(47), 119 (45), 91 (100). IR: 1681 (strong, C@O), 1612.3

(medium, C@C). UV, kmax in nm (absorbance): 203

(1.543), 236 (1.335). 5-(p-Toluoyl)-1-methyl-2-pyrrole carbaldehyde (7): 1H NMR (in CD3OD): d 9.79 (s, 1H, C7, O@C–H), 7.73– 7.74(m, 2H, C20 and C60), 7.33–7.34(m, 2H, C30 and C50), 7.03 (d, 1H, J = 4.2 Hz, C2@C3–H), 6.67 (d, 1H, J = 4.3 Hz, C5@C4–H), 2.41 (s, 3H, C70_{, p-CH} 3). 13C NMR (in CD3OD): 189.0 (C80), 183.6 (C7), 145.2 (C40), 138.1(C2), 137.3 (C10_{), 130.9 (C2}0 _{and C6}0_{), 130.4(C5),} 130.2 (C30_{and C5}0_{), 122.4(C3), 120.6 (C4), 35.0 (C6), 21.6}

(C70_{). EI-MS (70 eV): m/z (rel int. %): 227 (66), 212}

(83), 119 (37), 91 (100). IR (KBr) in cm
1: 1682.1 (strong,

H–C@O) and 1635.4(strong, RR0_{C@O), 1607.4(weak,}

C@C). UV, kmax in nm (absorbance): 202 (0.398), 317

(0.608).

p-Toluoyl-1,2-dimethylpyrrolyl ketone (8): 1H NMR (in CD3OD): d 7.59–7.61 (m, 2H, C20and C60), 7.26–7.28 (m, 2H, C30_{and C5}0_{), 6.61 (d, 1H, J = 4.0 Hz, C4, C@C–H),} 5.98 (d, 1H, J = 3.9 Hz, C3, C@C–H), 2.41 (s, 3H, C70_, p-CH3), 2.31 (s, 3H, C7, C@C–CH3). 13C NMR (in CD3OD): 187.4(C80), 143.2 (C40), 142.0 (C2), 138.9 (C10), 131.6 (C5), 130.3 (C30 and C50), 129.7 (C20 and C60), 125.1 (C4), 109.7 (C3), 33.2 (C6), 21.5 (C70), 12.5 (C7). EI-MS (70 eV): m/z (rel int. %): 212 (100), 198 (40), 122 (33), 91 (24). IR (KBr) in cm
1: 3103, 2947, 2919, and

2857 (weak, C–H stretching), 1614.0 (strong, C@O),

1569.1 (weak, C@C). UV, kmax in nm (absorbance): 205

(1.267), 254(0.850), 316 (1.751).

8. Turro, N. J. Modern Molecular Photochemistry, 1st ed.; University Science Book: Mill Valley, CA, 1991; pp 224– 228.

9. Carey, F. A. In E-Book t/a Organic Chemistry, CD-ROM; 5th ed.; McGraw-Hill Science: New York, 2003; Chapter 11.

10. Sheu, M. T.; Ho, H. O.; Wang, P. Y.; Liou, Y. P.; Wu, A. B. J. Chromatogr. Sci. 2003, 41, 200–204.