Synthesis of new derivatives of 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines

Introduction

As part of our studies in the field of fused benzothiazines as inhibitors of 15-lipooxygenase (15-LO) [1–3], it was of interest to make a series of 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines that are structurally homologous to pyrimido[4,5-b][1,4]benzothiazine, which is a potent 15-LO inhibitor. Recently, 15-LO has emerged as an attractive target for therapeutic intervention. 15-LO has been implicated in the progression of certain cancers and chronic obstructive pulmonary disease. Evidence for the inhibition of 15-LO in the treatment of vascular disease is, however, most compelling [4]. A perusal of the literature revealed that this family of heterocyclic compounds has not been investigated since the first report by Yoneda et al. in 1966 [5], when they described the synthesis of 3-chloro-10H-benzo[b]pyridazino[3,4-e][1,4]thiazine from 3,5,6-trichloropyridazine and 2-aminothiophenol and explained the mechanism of the reaction using a molecular orbital method. In the present article, we wish to report on the synthesis of some new derivatives of 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines and give a brief account of their inhibitory activity against the soybean 15-LO enzyme.

Results and discussion

The synthesis of 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines 6a–g started from 3-bromomaleic anhydride (1 in Scheme 1). Compound 1 was allowed to react with hydrazine hydrochloride to give 4-bromo-1,2-dihydropyridazine-3,6-dione (2) [6]. Chlorination of 2 with phosphoryl chloride yielded 4-bromo-3,6-dichloropyridazine (3) [6]. This compound was converted to 3-[(3,6-dichloropyridazine-4-yl)thio]analine (4) by selective displacement of the 4-bromine atom with 2-aminothiophenol in acetonitrile at room temperature [5]. The reaction time was reduced from 4 h to 20 min when 4-bromo-3,6-dichloropyridazine (3) instead of 3,4,6-trichloropyridazine was used. This modification resulted in an increased yield of 4 from 70% to 90% [5]. Then the key intermediate products, 2-{[3-chloro-6-(4-substituted-1-yl)pyridazine-4-yl]thio}anilines 5a–g were obtained by the reaction of compound 4 with secondary amines in ethanol at 80°C [1]. Treatment of compounds 5a–g with sodium amide in N,N-dimethylformamide furnished a host of 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines 6a–g in good yields (39–70%). The structural assignments of compounds 4, 5a–g, and 6a–g were based upon their spectral and microanalytical data.

Scheme 1

The inhibitory property of compounds 6a–g on 15-LO was assessed according to our previously reported procedure [1, 7]. The compounds show low inhibitory activity. A notable exception is compound 6d with the inhibitory activity IC₅₀ of 287 μm. By comparison with the activity of 4-methyl-2-(4-methylpiperazinyl)pyrimido[4,5-b]benzothiazine (4-MMPB, IC₅₀=20.7 μm) [1], one can come to the conclusion that replacement of the pyrimidine ring with the pyridazine moiety in the fused benzothiazine molecule has a profound diminishing effect on the enzyme inhibitory activity.

Conclusion

The sequential treatment of 4-bromo-3,6-dichloropyridazine with 2-aminothiophenol and then a secondary amine, followed by heterocyclization by treatment with NaNH₂ in DMF is a new and general route to 10H-benzo[b]pyridazino[3,4-e][1,4]thiazines. The synthesized compounds are less active than 4-MMPB as inhibitors of the soybean 15-LO enzyme.

Experimental

Melting points were recorded on an Electrothermal 9100 melting point apparatus. The IR spectra were obtained in KBr pellets on an AVATAR 370 FT-IR Thermo Nicolet spectrometer. The ¹H NMR (100 MHz) spectra were recorded on a Bruker AC 100 spectrometer with TMS as internal reference. The mass spectra were scanned on a Varian Mat CH-7 instrument at 70 eV. Elemental analysis was performed on a Thermo Finnigan Flash EA 1112 instrument.