HETEROCYCLES

Short Paper | Regular issue | Vol. 85, No. 9, 2012, pp. 2281-2290
Received, 18th June, 2012, Accepted, 17th July, 2012, Published online, 25th July, 2012.
DOI: 10.3987/COM-12-12528

■ The Structures of 8- and 10-Trifluoromethylquino[3,2-b]benzo[1,4]thiazines and Their Benzyl Derivatives

Małgorzata Jeleń, Kinga Suwińska, Céline Besnard, Krystian Pluta,* and Beata Morak-Młodawska

Department of Organic Chemistry, The Medical University of Silesia, Jagielloñska Str. 4, 41-200 Sosnowiec, Poland

Abstract

The modification of the phenothiazine structure via the substitution of the benzene ring with the quinoline ring may proceed through the Ullmann cyclization or the Smiles rearrangement of the appropriate sulfides followed by cyclization. Reactions of 2,2’-dichloro-3,3’-diquinolinyl disulfide or pentacyclic diquinodithiin with m-trifluoromethylaniline led to two isomeric tetracyclic trifluoromethylquinobenzothiazines. The product structures as the appropriate 6H-X-trifluoromethylquino[3,2-b]benzo[1,4]thiazines (X = 8, 10) were finally confirmed by X-ray analysis (one product was transformed into N-benzyl derivative). These results exclude the possibility of the reverse Smiles rearrangement and the existence of 5H-tautomers in these conditions. Although both compounds have the same quinobenzothiazine system, they differ in the spatial structures and geometric data. Molecule 6b is unexpectedly almost planar that is the result of larger than usual the C–S–C and C–N–C angles in the thiazine ring. Molecule 14 is folded along the S–N axis with the thiazine ring in boat conformation and the benzyl group in equatorial position.

Phenothiazines are known mainly as recognized antipsychotic, antihistaminic, antitussive and antiemetic drugs.1 Recent reports deal with very promising anticancer and antibacterial activities, reversal of multidrug resistance and potential treatment in Alzheimer’s and Creutzfeldt-Jakob diseases not only for classical but also for newly synthesized phenothiazines.2-6 The phenothiazine structures were modified by the introduction of new pharmacophoric substituents at the thiazine nitrogen atom and the substitution of the benzene ring with an azine ring. In continuation of our search for new anticancer and immunosuppressive azaphenothiazines containing azine rings (pyridine and quinoline)7-12 we synthesized tetracyclic quinobenzothiazines in original way from 2,2’-dichloro-3,3’-diquinolinyl disulfide 1 or pentacyclic diquinodithiin 2 in the reactions with various substituted anilines 3 (Scheme 1).13 The reaction ran through the formation of anilinium quinolinethiolate 4 which underwent the thiazine ring formation to quino[3,2-b]benzo[1,4]thiazines 5, the only product in the reaction with p-substituted anilines (Z1 = Z3 = X = H). In the case of the reaction with m-substituted anilines 3 (m-chloroaniline, m-bromoaniline, m-trifluoromethylaniline, Z1 = Cl, Br, CF3, Z2 = Z3 = X = H) additional quinobenzothiazines 6 were also observed as the result of two ways of the thiazine ring formation. Anilinium quinolinethiolate 4 can undergo the Smiles rearrangement to sulfide 11 which can give quinobenzothiazines 12 and 13. When o-chloroanilines 3 (X = Cl) were used, the reaction ran through the formation of sulfide 7 which could give quinobenzothiazines 8 or could undergo the Smiles rearrangement to amine 9 and further to quinobenzothiazines 10.

We found the products with m-chloroaniline to be the same as those with 2,3- and 2,5-dichloroanilines, and therefore we assigned them as 8-chloro- and 10-chloroquino[3,2-b]benzo[1,4]thiazines 5b and 6b, respectively. The kind of fusion of the thiazine ring with the quinoline ring [3,2-b] in obtained quinobenzothiazines was confirmed by homonuclear NOE experiment for the 6-methyl derivative. A discrimination of 8-substituted and 10-substituted quinobenzothiazines was based on the 1H NMR analysis of the benzene ring proton signals (as 1,2,4- and 1,2,3-trisubstituted benzenes, respectively) and their coupling constants Jortho and Jmeta.13
Phenothiazines with biological activities are obtained most often from N-unsubstituted phenothiazines in N-alkylation reactions. Such alkylations mainly occurred at the thiazine nitrogen atom but there are some evidences that the azine nitrogen atom, sulfur atom or even carbon atom were also alkylated, even in basic conditions.14-16 A ring contraction was also observed during alkylation.17 As biological activities of phenothiazines depend on their structures, configurations and conformations, especially on the nature of the substituent at the thiazine nitrogen atom, the place of additional substituents, and on the nature of the phenothiazine and azaphenothiazine ring system, it is obvious the structures of phenothiazines should be determined without any doubts. The proper determination of the phenothiazine products is difficult as the Smiles rearrangement, which is widespread during synthesis via appropriate sulfide, is not always easy to observe.15-17 The aim of this paper is to confirm unequivocally the kind of fusion of the quinoline ring with thiazine ring and proper discrimination of the two isomeric trifluoromethylquinobenzothiazines, to verify the place of the hydrogen atom (Nthiazine–H or Nquinoline–H) and an alkyl substituent at the nitrogen atom, and finally to find out about planar or folded tetracyclic quinobenzothiazine ring system.

Synthesis and the product structures
The reaction of disulfide 1 or diquinodithiin 2 with m-trifluoromethylaniline 3a (or its hydrochloride) in boiling monomethyl ether of diethylene glycol (MEDG) at 194 oC or without solvent at 200–205 oC, respectively, led to two products with melting points of 225–226 oC and 219–220 oC assigned as trifluoromethylquinobenzothiazines 5b and 6b13 but other tautomeric structures 5c and 6c should be also taken into consideration. On the other hand intermediate 4a can undergo the reverse Smiles rearrangement to sulfide 10 and further to trifluoromethylquinobenzothiazines 5d and 6d or their tautomers 5e and 6e (Scheme 2).
The direct reaction products were changed into N-benzyl derivatives in 81% and 86% yield in the model alkylation reaction with benzyl chloride in DMF in the presence of sodium hydride. As we found that N-substituted quinobenzothiazines with pharmacophoric aminoalkyl groups exhibited very promising antiproliferative and anticancer activities,18 this model benzylation showed which nitrogen atom underwent alkylation.
For X-ray study, we obtained proper quality monocrystals of the direct product of higher melting point. The monocrystal quality of the second product (of lower melting point) was insufficient, therefore this compound was transformed into N-benzyl derivative. X-Ray study confirmed the correctness of the assumed structures of 6H-8-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 6b and 6-benzyl-10-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 14 and proved that the model N-benzyl reactions occurred at the thiazine nitrogen atom (giving compounds 14 and 15). These results exclude the possibility of the reverse Smiles rearrangement and the existence of 5H-tautomers in these conditions.

X-Ray analysis
The isomeric trifluoromethylquinobenzothiazine systems have different spatial structures. Contrary to all known structures of tricyclic phenothiazins, molecule 14 and other tetracyclic compounds i.e. dihydroquinobenzothiazine 1619 and naphthobenzothiazine 1720 (Scheme 3), molecule 6b is unexpectedly almost planar (Figure 1). The dihedral angle between the halves of the thiazine ring (N6/C5a/C11a/S11 vs. N6/C6a/C10a/S11) and between the benzene ring and quinoline ring (C6a/C7/C8/C9/C10/C10a vs. C1/C2/C3/C4/C4a/N5/C5a/C11a/C12/C12a) are 171.86(10)o and 174.02(8)o, respectively. We found only one case in literature when the phenothiazine structure was planar, despite some complexes with inorganic and organic compounds. Out from two reported structures of related linear fused azaphenothiazines possessing two quinoline rings instead of the benzene rings, pentacyclic 6-phenyldiquinothiazine 18 was folded but 6-(p-nitrophenyl)diquinothiazine 19 was unexpected planar due to the interaction of the thiazine nitrogen atom with electron-withdrawing p-nitrophenyl group.21,22
The similar dihedral angles in molecule 14 are 139.20(9)o and 145.97(6)o, respectively. The thiazine ring is in boat conformation and the benzyl group is located in equatorial position with the S11···N6–C17 angle of 163.8(2)o and torsion angles C10a–C6a–N6–C17 and C11a–C5a–N6–C17 of -164.4(2)o and 168.1(2)o. The benzyl group is turned around the N6–C17 bond (the torsion angles C5a–N6–C17–C18 and

C6a–N6–C17–C18 of -133.4(2)o and 68.2(3)o) and less turned around the C17–C18 bond (the torsion angles N6–C17–C18–C19 and N6–C17–C18–C23 of -164.4(2)o and 26.1(3)o). The phenyl group plane (C18/C19/C20/C21/–C22/C23) is rather perpendicular to the C5a/C6a/C10a/C11a plane with the angle of 112.89(9)o.
Whereas the thiazine ring is symmetrical with similar S–C (1.753(2) and 1.754(2) Å) and N–C bond lengths (1.388(2) and 1.377(2) Å), the pyridine ring is distorted with different C–C (1.355(3) vs. 1.432(2) Å) and

N–C bond lengths (1.303(2) Å vs. 1.373(2) Å). The C–N–C and C–S–C angles in the thiazine ring are significantly large (128.22(15)o and 103.31(9)o) in molecule 6b in comparison with appropriate angles in molecule 14 (120.9(2)o and 98.78(12)o, what enables the thiazine ring to be almost planar. It is worth noting that the same angles in planar similar structure 19 are also large (125.7(4)o and 102.6(3)o) but in folded structure 18 are smaller (123.8(2)o and 100.1(2)o, respectively).21,22 Similarly to compound 6b, the thiazine ring in compound 14 is quite symmetrical (with the S–C bond lengths of 1.759(3) and 1.768(3) Å, and with the N–C bond lengths of 1.408(3) and 1.411(3) Å), but the pyridine ring is distorted with different the C–C (1.360(4) vs 1.429(4) Å) and N–C bond lengths (1.306(4) Å vs. 1.377(3) Å).
The electron-withdrawing trifluoromethyl group in compounds 6b and 14, similarly to the p-nitrophenyl group in compound 19, reduces the electron density at the thiazine nitrogen atom what makes possible to take a planar conformation of the thiazine ring and thereby the whole molecule 6b. In case of molecule 14, the electron-withdrawing action of the trifluoromethyl group is compensated by the electron-donating action of the benzyl group. The sum of three C–N6–C angles is 356.21° indicating nearly pyramidal configuration of the bonds around the central nitrogen atom.
Two molecules 14 related by crystallographic center of symmetry are arranged in dimers via two C–H···F hydrogen bonds [dH···F = 2.56 Å, dC···F = 3.394 Å, <C–H···F = 146.8 ° (Figure 2a). Each molecule interacts with neighboring three molecules with C–H···π interactions (Figure 2b). Additionally, in the crystal structure, π−π interactions between the quinoline moieties of the adjacent molecules are observed. The π−π stacking propagates along c crystallographic axis.
Figure 3 shows crystal packing along the b axis for both crystal structures.
The trifluoromethyl group rotates very fast in molecule 6b and is coplanar with the benzene ring as the torsion angles C13/C8/C9/C10 and C6a/C7/C8/C13 are -179.5(2)o and 179.76(18)o, respectively.
The CF3 group is disordered in molecule and was modelled using two components, whose geometry was restrained to be the same. Restrains were also applied on anisotropic displacement parameters.

X-Ray study confirmed the right structures of 6H-8-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 6b and 6H-10-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 5b (concluded from the 1H NMR study) as the direct reaction products which exist as the 6H-tautomers. The structure of 6-benzyl-10-trifluoromethylquino- [3,2-b]benzothiazine 14 proved that the model N-benzyl reaction occurred at the thiazine nitrogen atom. These results exclude the possibility of the reverse Smiles rearrangement and the existence of the 5H-tautomers in these conditions. Although both compounds have the same quinobenzothiazine system, they differ in geometric data and molecule 6b is unexpectedly almost planar. The thiazine ring in molecule 14 is in boat conformation with the benzyl group in equatorial position. The planarity of the thiazine ring in molecule 6b is the result of larger than usual the C–S–C and C–N–C angles.

EXPERIMENTAL
Melting points were determined in open capillary tubes on a Boetius melting point apparatus and were uncorrected. The 1H NMR spectra were recorded on a Bruker AV III spectrometers at 500 MHz in deuteriochloroform with tetramethylsilane as the internal standard. Electron impact (EI MS) mass spectra were run on a Finnigan MAT 95 spectrometer at 70 eV. The Ir spectra were recorded on a Shimadzu IR-Affinity-1 FT-IR spectrophotometer.
General synthesis of 6H-8-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 6b and 6H-10-trifluoromethylquino[3,2-b]benzo[1,4]thiazine 5b
The reaction of disulfide 2 (0.20g, 0.5 mmol) with m-trifluoromethylaniline 3a (2 mmol) in boiling monomethyl ether of diethylene glycol (5 mL) was carried out as described previously.13 The column chromatography separation (silica gel, CHCl3) gave two products:
a. 6H-8-Trifluoromethylquinobenzothiazine 6b (0.11 g, 34%), mp 225–226 oC, Rf = 0.65 (Al2O3, CH2Cl2),
b. 6H-10-trifluoromethylquinobenzothiazine 5b (0.09 g, 28%), mp 219–220 oC, Rf = 0.58 (Al2O3, CH2Cl2).
The same products were obtained in 28% and 22% yield in the reaction of diquinodithiin 1 with hydrochloride of m-trifluoromethylaniline 3a in an oil bath at 200–205 oC.13
Synthesis of 6-benzyl-10-trifluoromethylquino[3,2-b]benzothiazine 14 and 6-benzyl-8-trifluoromethylquino[3,2-b]benzothiazine 15
To a solution of 10-trifluoromethylquinobenzothiazine 5b or 8-trifluoromethylquinobenzothiazine 6b (0.32 g, 1 mmol) in dry DMF (10 mL) sodium hydride (0.24 g, 10 mmol, washed out from mineral oil with hexane) was added. The reaction mixture was stirred for 1 h in room temperature. Then benzyl chloride (0.35 mL, 3 mmol) was added and the stirring was continued for 24 h. The reaction mixture was poured into water (40 mL) and extracted with CH2Cl2. The extract was washed with water, dried with anhydrous sodium sulfate. The drying agent was filtered off and the solution was evaporated in vaccuo. The residue was purified by column chromatography (Al2O3, CH2Cl2) to give 6-benzyl-10-trifluoromethylquino[3,2-b]benzothiazine 14 (0.18 g, 86%), mp 127–128 oC (EtOH) or 6-benzyl-8-trifluoromethylquino[3,2-b]benzothiazine 15 (0.17 g, 81%), mp 115–116 oC (EtOH).
6-benzyl-10-trifluoromethylquinobenzothiazine 14 1H NMR (CDCl3) δ: 5.60 (s, 2H, CH2), 6.97 (d, 1H, H7), 7.07 (t, 1H, H8), 7.23 (m, 1H, H2), 7.24 (d, 1H, H9), 7.31 (m, 3H, C6H3), 7.38 (m, 2H, C6H2), 7.51 (m, 2H, H3), 7.58 (m, 1H, H1), 7.69 (m, 1H, H4), 7.83 (s, 1H, H12). EI MS m/z: 408 (M, 63), 317 (M-CH2C6H5, 100). Ir (KBr): 1130.33, 1309.72, 1399.42 cm-1. Anal. Calcd for C23H15F3N2S: C 67.64, H 3.70, N 6.86. Found: C 67.44, H 3.83, N 6.76.
6-benzyl-8-trifluoromethylquinobenzothiazine 15 1H NMR (CDCl3) δ: 5.60 (s, 2H, CH2), 6.98 (d, 1H, H7), 7.09 (dd, 1H, H9), 7.15 (d, 1H, H10), 7.23 (m, 1H, H2), 7.31 (m, 3H, C6H3), 7.39 (m, 2H, C6H2),7.50 (m, 2H, H3), 7.54 (m, 1H, H1), 7.70 (s, 1H, H12), 7.72 (m, 1H, H4). EI MS m/z: 408 (M, 55), 317 (M-CH2C6H5, 100). Ir (KBr): 1119.73, 1324.19, 1406.70 cm-1. Anal. Calcd for C23H15F3N2S: C 67.64, H 3.70, N 6.86. Found: C 67.41, H 3.85, N 6.69.
X-Ray analysis of 6H-8-trifluoromethylquino[3,2-b]benzothiazine 6b and 6-benzyl-8-trifluoromethylquino[3,2-b]benzothiazine 14
A. A yellow crystal of compound 6b (dimensions: 0.40 × 0.30 × 0.04 mm) was grown from ethanol-chloroform solution at room temperature. Crystallographic data were collected at 260 K on a Stoe IPDS diffractometer with graphite monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXS-86)23 and refined by full-matrix least-squares minimization based on all unique F2 (SHELXL).24 Crystal data: C16H9F3N2S, Mr = 318.32, monoclinic, a = 7.6423 (9), b = 6.0471 (4), c = 28.940 (3) Å, β = 97.60 (1)°, space group P21/c, V = 1325.7 (2) Å3, Z = 4, µ = 0.28 mm−1. The crystal system is monoclinic but the system could be indexed using an orthorombic unit-cell with parameters a = 7.665, b = 57.331, c = 6.054 Å. This cell is the result of a perfect superposition of two monoclinic twins with twin law [1 0 0, 0 -1 0 ,-1 0 -1]. The twin law was included in the refinement and the twin fraction refined to 0.53 /0.47. 7878 reflections were collected of which 2593 were independent and 2337 reflections with I > 2σ(I) (Rint = 0.039). The structure was refined to R[F2 > 2σ(F2)] = 0.031 and wR(F2) = 0.077. H-atoms were included in geometric positions and refined as 'riding' atoms with isotropic thermal parameters based upon the corresponding bonding carbon atom [Uiso = 1.2Ueq].
B. A yellow crystal of compound 14 (dimensions: 0.15 × 0.20 × 0.30 mm) was grown from ethanol-chloroform solution at room temperature. Crystallographic data were collected at 100 K on a Kappa ApexII diffractometer with graphite monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SHELXS-97)25 and refined by full-matrix least-squares minimization based on all unique F2 (SHELXL-97).26 Crystal data: C23H15F3N2S, Mr = 3408.44, monoclinic, a = 16.7752 (6), b = 16.2145 (7), c = 7.0434(2) Å, β = 99.971(2)°, space group P21/c, V = 1884.3 (2) Å3, Z = 4, µ = 0.212 mm−1. 24702 reflections were collected of which 3208 were independent and 2503 reflections with I > 2σ(I) (Rint = 0.042). The structure was refined to R[F2 > 2σ(F2)] = 0.049 and wR(F2) = 0.100. H-atoms were included in geometric positions and refined as 'riding' atoms with isotropic thermal parameters based upon the corresponding bonding carbon atom [Uiso = 1.2Ueq].

ACKNOWLEDGEMENTS
The work is supported by the National Centre of Science (grant N N405 101739).

References

1. R. R. Gupta and M. Kumar, ‘Synthesis, properties and reactions of phenothiazines’, in R. R. Gupta (Eds.) ‘Phenothiazines and 1,4-Benzothiazines – Chemical and Biological Aspects’. Elsevier, 1988, p. 1.
2. N. Motohashi, M. Kawase, S. Saito, and H. Sakagami, Curr. Drug Targets, 2000, 1, 237. CrossRef
3. N. Motohashi, M. Kawase, K. Satoh, and H. Sakagami, Curr. Drug Targets, 2006, 7, 1055. CrossRef
4. K. Pluta, B. Morak-Młodawska, and M. Jeleń, Eur. J. Med. Chem., 2011, 46, 3179. CrossRef
5. J. J. Aaron, M. D. Gaye Seye, S. Trajkovska, and N. Motohashi, ‘Bioactive Phenothiazines and Benzo[a]phenothiazines: spectroscopic studies and biological and biomedical properties and applications’, Top Heterocycl. Chem., Springer-Verlag, Berlin, 2009, 16, 153.
6. A. Dasgupta, S. G. Dastridara, Y. Shirataki, and N. Motohashi, ‘Antibacterial activity of artificial phenothiazines and isoflavones from plants’, Top Heterocycl. Chem., Springer-Verlag, Berlin, 2008, 15, 67.
7. B. Morak and K. Pluta, Heterocycles, 2007, 71, 1347. CrossRef
8. M. Jeleń and K. Pluta, Heterocycles, 2008, 75, 859. CrossRef
9. B. Morak-Młodawska and K. Pluta, Heterocycles, 2009, 78, 1289. CrossRef
10. M. Zimecki, J. Artym, M. Kocięba, K. Pluta, B. Morak-Młodawska, and M. Jeleń, Cell. Mol. Biol. Lett., 2009, 14, 622. CrossRef
11. K. Pluta, M. Jeleń, B. Morak-Młodawska, M. Zimecki, J. Artym, and M. Kocięba, Pharmacol. Reports, 2010, 62, 319.
12. B. Morak-Młodawska, K. Suwińska, K. Pluta, and M. Jeleń, J. Mol. Struct., 2012, 1015, 94. CrossRef
13. M. Jeleń and K. Pluta, Heterocycles, 2009, 78, 2325. CrossRef
14. T. Itoh, Y. Tomii, T. Noitoh, M. Yamamura, I. Ishikawa, N. Kawahara, Y. Mizuno, and H. Ogura, Chem. Pharm. Bull., 1989, 37, 2197. CrossRef
15. K. Pluta, B. Morak-Młodawska, and M. Jeleń, J. Heterocycl. Chem., 2009, 46, 355. CrossRef
16. W. Schindler and J. Peterli, U. S. Patent 3,010,961, 1961.
17. G. Riedel and W. Deuschel, British Patent 971,048, 1964.
18. K. Pluta, M. Jeleń, M. Zimecki, B. Morak-Młodawska, J. Artym, and M. Kocięba, Polish Patent Appl. P-398835, 2012.
19. R. L. Luck, K. Li, and D. K. Bates, Acta Cryst., 2003, E59, o302.
20. S. Yoshida, K. Kozawa, N. Sato, and T. Uchida, Bull. Chem. Soc. Jpn., 1994, 67, 2017. CrossRef
21. K. Pluta, M. Nowak, and K. Suwińska, J. Chem. Cryst., 2000, 30, 479. CrossRef
22. M. Nowak, K. Pluta, K. Suwińska, and L. Straver, J. Heterocycl. Chem., 2007, 44, 543. CrossRef
23. G. M. Sheldrick, SHELS-86, Program for crystal solution, University of Göttingen, Germany.
24. G. M. Sheldrick, SHELSXL, Acta Cryst., 2008, A64, 112.
25. G. M. Sheldrick, SHELS-97, Acta Cryst., 1990, A46, 467.
26. G. M. Sheldrick, SHELX97. Programs for Crystal Structure Analysis (Release 97-2). University of Göttingen, 1997, Germany.