An Improved Synthesis of 4-Chlorocoumarin-3-sulfonyl Chloride and Its Reactions with Different Bidentate

An improved synthetic method affording 4-chlorocoumarin-3-sulfonyl chloride (4) in very good yield (ca. 85%) is reported. This compound was reacted with various bidentate nucleophiles such as 2-aminopyridines and 2-aminothiazoles in order to obtain substituted pyrido- and thiazino-1,2,4-thiadiazino-benzopyranone dioxides (potential anticancer and anti-HIV agents). These reactions occurred rapidly at room temperature giving yellowish precipitates, which are insoluble in common organic solvents, making the purification process challenging. Further investigation has shown that these fused heterocycles are not stable and decompose with opening of the 1,2,4-thiadiazine ring.


Introduction
Heterocyclic chemistry is one of the largest areas of research in organic chemistry and it is growing rapidly.Of all published organic chemistry literature, papers on heterocyclic synthesis accounted for around 60 % in 1998 [1], but nowadays the fraction is much larger considering that novel heterocyclic compounds are published in different fields such as biochemistry, pharmaceuticals, materials and others.A similar trend is seen for coumarin, a heterocyclic system with a very large number of different derivatives.Coumarin is a compound with varied biological activities and in 1954 it was classified as a carcinogenic substance [2,3].Main representatives of the class are its hydroxy derivatives 4-hydroxycoumarin (1) and 7-hydroxycoumarin (umbeliferone), also biologically active and very important for synthesis of other coumarin derivatives.Until now, an enormous number of compounds with coumarin systems in their structure have been synthesized.Those derivatives have shown a remarkably broad spectrum of pharmacological and physiological activities and they are used as anticoagulant [4][5][6], antibacterial [7,8], antiviral [9,10], antitumor [11][12][13][14], bactericidal [15], fungicidal [16], and anti-inflammatory agents [17].Also, in recent times there are references to derivatives with anti-HIV activity [18][19][20][21].
On the other hand, the nitrogen and sulfur heterocyclic system families are very interesting because of their physicochemical properties with relevance to the design of new drugs and new materials, especially those relating to molecular conductors and magnets [22].1,2,4-Thiadizines are also used for treatment of HIV infection [23,24], as cardiovascular agents [25], for their antimicrobial effects [26], and as diuretics and antihypertensive drugs [27,28].Based on these facts we wanted to combine the coumarinic system with 1,2,4-thiadiazines in the hope that the resulting novel heterocycles would be biologically active, especially as anticancer and anti-HIV agents.Also, we designated positions 3 and 4 of coumarin for annellation, because these positions are mainly attacked by electrophiles and nucleophiles, respectively [29].

Results and Discussion
Checchi et al. had previously reported one of the target compounds, pyrido[1',2':2,3]-1,2,4thiadiazino [6,5-c]benzopyran-6-one 7,7-dioxide (7a), as one of two possible structures [30].It was obtained by reaction of 4-chlorocoumarin-3-sulfonyl chloride (4) with 2-aminopyridine (5a) in dry benzene.In our attempts to obtain the same compound 7a and its derivatives, unsatisfactory results were obtained and difficulties followed the synthesis of 4 as the key substrate.These problems were avoided by modification of the reaction route used, simplifying the preparation of the sodium salt 3 of 4-hydroxy-coumarin-3-sulfonic acid (2).Thus, considering that 2 is very soluble and stable in aqueous solution, 3 was obtained by simple mixing aqueous solutions of 2 and sodium chloride.Contrary to the abundant literature mentioning the good solubility of alkali-metal salts of sulfonic acids [31], the salts we prepared are not soluble in water and can be easily isolated in nearly quantitative yields from aqueous solutions by simple filtration (Scheme 1).Compound 3 was reported once by Huebner and Link [32], but the product was only characterized by its elemental analysis for sodium.This paper presents the full characterization of 3. In the next step 3 was chlorinated by refluxing with POCl 3 for approximately three hours.Afterwards, instead of removing POCl 3 by vacuum distillation, the suspension was poured onto a mixture of crushed ice and water to afford a nice precipitate of 4 in 85 % yield (lit. [30]: ca. 25 %) (Scheme 2).Modification of this step was based on the report that the chlorine atom at position 4 cannot be attacked by water [29].Also, it was found that in general the sulfonyl chlorides are insoluble and stable in water [31].Structural assignment of 4 was based on 1 H-and 13 C-NMR, IR and MS spectral analysis.The signals of four aromatic protons at 7.34-7.85ppm could be observed in the 1 H-NMR spectrum, as well as nine signals in the expected regions of the 13 C-NMR spectrum.In the mass spectrum (ESI, +ve mode) there was a peak at m/z 302 corresponding to the [M+Na] + species.These data confirmed that the product isolated from water is identical with that isolated by vacuum distillation.Substrate 4 is very soluble in organic solvents and it was used without further purification.

% crude yield
Reactions of 4 with 2-aminopyridines 5a-h and 2-aminothiazoles 6a,b were performed in acetonitrile as a polar and aprotic solvent.Reactions were very fast at room temperature and after a few minutes crude yellowish precipitates of 7a-f and 8a,b were formed, respectively (Scheme 3).
Using single crystal X-ray diffraction the structure of 7a has been determined and the dilemma about its configuration was eliminated (Figure 1).The compound crystallizes in monoclinic space group P21/n with cell dimensions a=16.3541(5)Å, b=9.2246(2)Å, c=16.7064(5)Å, α=90.00°,β=110.802(4)°,γ=90.00°,V=2356.04(11)Å 3 , Z=8 [33].All atoms in the molecule are in one plane, except for the two oxygen atoms of the SO 2 group, which are arranged symmetrically out of plane.All products 7a-f and 8a,b are very insoluble in common organic solvents, with the exception of DMSO, which caused difficulties when purifying them.Purifications were performed on chromatographic columns with slow gradient elution using dry solvents.Further investigation confirmed that compounds 7a-f and 8a,b are not stable.Analysis carried out on 7d proved that those systems annulated on coumarin decompose with opening of the 1,2,4-thiadiazine ring.Decomposition in the solid state was slow, but when crystals were dissolved in DMSO, where complete elimination of traces of water is not possible, decomposition was rapid (within a few hours).Therefore, it was assumed that these heterocycles are sensitive to moisture.The reaction with water releases sulfuric acid.Since the decomposition is slow in the beginning it is possible that the reaction is autocatalytic, so when the amount of sulfuric acid increases decomposition is more rapid (Scheme 4).4-(5-Chloropyridin-2-yl-amino)-benzopyran-2-one ( 9) was isolated in pure form as white crystals after decomposition of 7d.In the IR spectrum one sharp band at 3328 cm -1 , corresponding to an NH vibrational mode and a C=O band at 1672 cm -1 , which is evidently lower than the band of the annulated heterocycles at 1710 cm -1 , were noticed.The 1 H-NMR spectrum showed a singlet at 9.5 ppm corresponding to the NH proton, seven aromatic protons ranging from 7.4 to 7.9 ppm and one singlet at 7.3 ppm, corresponding to the proton at the 3-position of the coumarin ring.In the 13  It was also observed that the substituent in the 2-aminopyridine moiety is very important because of its influence on the reaction, rate and the stability of derivatives.Substrate 4 did not react with any 2-aminopyridines possessing a nitro group in positions 3, 4 or 5 (10a-c, Scheme 5).This is understandable considering that the nitro group has a strongly negative inductive effect (-I), as well as a negative resonance effect (-M).As a result of both effects, the electron density on the nitrogen atoms is much lower, which consequently weakens the nucleophilicity [34].In view of these facts it was concluded that weak bidentate nucleophiles cannot substitute the chlorine atoms in 4, to form a 1,2,4thiadiazine ring, and that only strong nucleophiles are able to do so.To prove this, comparative reactions of 4 were performed with urea as a weaker and thiourea as a stronger bidentate nuclophile.A yellowish precipitate of 8H,10H-1,2,4-thiadiazino[6,5-c]benzopyran-9-thioxo-6-one 7,7-dioxide (11, Scheme 5) was formed within a few minutes after mixing an acetonitrile solution of 4 and thiourea crystals at room temperature, while all attempts to obtain a product from the reaction with urea failed.

General
Melting points were determined on a Reichert heating plate and are uncorrected.C, H elemental analysis was carried out on a Coleman Model 33 carbon-hydrogen analyzer.N elemental analysis was carried out by the Dümas method.NMR spectra were recorded on a Bruker 400 MHz instrument using DMSO-d 6 as solvent and tetramethylsilane as internal standard.Infrared spectra (KBr pellets) were measured on a Perkin-Elmer System 2000 FT IR.ESI-TOF mass spectra were measured using an LCT mass spectrometer (Waters) equipped with a lockspray dual-electrospray ion source combined with Waters Alliance 2695 HPLC unit.The X-ray structure of 7a was measured on a CCD Xcalibur S diffractometer.All non-hydrogen atoms were refined anisotropically and all hydrogen atoms were placed in ideal positions.All the reagents and solvents were obtained from commercial sources and were used without further purification.4-Hydroxycoumarin-3-sulfonic acid (2) was synthesized following a literature method [30,35], as described below in more detail.