Synthesis of 1,2-benzoxathiine 2,2-dioxide derivatives using aliphatic aldehydes and assessment of their antimicrobial activity

Nowadays the problem of the antimicrobial resistance promotes the search of new chemical core-structures with the antimicrobial properties. Aim. To study the interaction of 1,2-benzoxathiin-4(3H)-one 2,2-dioxide with active methylene nitriles and aliphatic aldehydes and assess the antimicrobial activity of the compounds obtained. Results and discussion. 1,2-Benzoxathiin-4(3H)-one 2,2-dioxide as a structural analog of 1,3-dicarbonyl compounds was used in the three-component interaction with aliphatic aldehydes and active methylene nitriles. In the case of malononitrile the target compounds were formed. When using ethyl cyanoacetate the only isolated product was triethylammonium salt that could be also obtained by the two-component reaction of 1,2-benzoxathiin-4(3H)-one 2,2-dioxide with aliphatic aldehydes. The study of the antimicrobial properties showed the higher activity of the compounds studied than in the reference drugs, especially against gram-positive strains. Experimental part. The series of 2-amino-4-alkyl-4,6-dihydropyrano[3,2-c][2,1]benzoxathiin-3-carbonitrile 5,5-dioxides and triethylammonium 3-[1-(4-hydroxy-2,2-dioxido-1,2-benzoxathiin-3-yl)alkyl]-1,2-benzoxathiin-4-olate 2,2-dioxides was synthesized. The antimicrobial activity of the compounds obtained was determined by the agar “well” diffusion method. Conclusions. It has been shown that 1,2-benzoxathiin-4(3H)-one 2,2-dioxide as a structural analog of 1H-2,1-benzothiazin-4-one 2,2-dioxide can be used in similar threeand two-component reactions, but its reactivity is less due to the replacement of the 1-N-R-group with an O-atom. The novel compounds obtained exceeded the antimicrobial activity of the reference drugs, and were more active against gram-positive bacteria in contrast to isosteric derivatives of 1H-2,1-benzothiazin-4-one 2,2-dioxide that were active against gram-negative strains and fungi.

Ключевые слова: 1,2-бензоксатиин-4(3H)-он 2,2-диоксид; 2-амино-4H-пиран; трехкомпонентное взаимодействие; триэтиламмониевая соль; антимикробная активность The results of the studies in different countries all over the world indicate the growth of the antimicrobial resistance (AR) and, particularly the multiple drug resistance in numerous microorganisms that are the main cause of growth of different infectious sickness rate [1,2,3]. The obvious consequences of this process include an increase in morbidity and mortality, prolongation of the disease time and a greater risk of complications [4]. AR becomes also the cause for the greater economic burden for the population due to decrease in their labor productivity and increase of the costs for diagnosis and treatment of such disease type [5]. Altogether the impact of AR on the health and economic system can be estimated as extremely negative. Therefore, the synthesis of new efficient biologically active compounds with the promising antimicrobial properties still remains one of the topical issues in development of new drugs [6]. The World Health Organization is also encouraging works in this direction. According to this the Global Strategy on Containment of Antimicrobial Resistance (2001) [7] was worked out. It contains a complete list of recommendations for AR combating. In particular, the strategy to promote the creation of new drugs and vaccines with the necessary properties was proposed, especially with novel chemical core-structures, which were not earlier utilized as antimicrobial substances.
In our previous works we used 1H-2,1-benzothiazin-4-one 2,2-dioxide as a core-structure to obtain its new pyran-annulated derivatives, as well as novel ammonium salts and consequently to assess their antimicrobial activity [8,9]. These studies allowed us to find the substances with a moderate activity against P. aeruginosa and C. albicans. In this regard, aiming to obtain new effective antimicrobial agents we continued our investigations in this field by modifying of the abovementioned core-structure via isosteric replacement of the 1-N-R group with an O-atom, resulting in 1,2-benzoxathiin-4(3H)-one 2,2-dioxide ( Fig.).
This idea has marked the beginning of new research of our scientific group dedicated to revealing the synthetic and pharmacological potential of 1,2-benzoxathiin-4(3H)-one 2,2-dioxide. It is also targeted on determination of the general regularities on the structure-bioactivity relationships in series of SO 2 -containing heterocycles and subsequently on purposeful construction of drugs with a desired activity.
It is well known that 2-amino-4H-pyrans can be easily obtained via three-component domino-type interaction of 1,3-dicarbonyl compounds with aldehydes and active methylene nitriles using a wide range of bases as a catalyst [12]. Since 1,2-benzoxathiin-4(3H)-one 2,2-dioxide 4 can be considered as a structural analog of 1,3-dicarbonyl compounds, according to the task set it was introduced into the three-component interaction with aliphatic aldehydes 5 and malononitrile 6 (Scheme 2) which resulted in formation of the target 2-amino-4-alkyl-4,6-dihydropyrano[3,2-c][2,1]benzoxathiin-3-carbonitrile 5,5-dioxides 7 as precipitates that did not require further purification. The reaction readily proceeded when using equimolar quantities of the initial reagents in ethanol in the presence of the catalytic amount of triethylamine to promote the interaction. As it was additionally determined, this three-component reaction did not require heating and proceeded smoothly under the room temperature. It is in a full agreement with the previously regularities found [8]. Performing the reaction under reflux affected neither the product obtained nor the yield.
The yields of 2-amino-4H-pyrans 7 (Tab. 1) in the range from formaldehyde to butyraldehyde were ave-rage. However, in the case of valeric aldehyde the yield of 7e turned out to be poor -only approximately 10 %. The solvent replacement of ethanol to methanol gave its increase up to 45 %, which might be due to lower solubility of the product. It is interesting that the fused derivative 7h was obtained for α-methylcinnamaldehyde, whereas despite of our efforts, involvement of cinnamaldehyde often used in the 2-amino-4H-pyrans synthesis [13] was not successful. Presumably, the α-methylgroup in this case may act as a steric hindrance and avoids formation of undesirable by-products.
In the three-component reaction glutaraldehyde 5i as a bifunctional representative was also introduced. This gives a chance to obtain two types of 2-amino-4H-pyrans depending on the number of aldehyde equivalents applying in the reaction, namely 2-amino-4H-pyran with a free aldehyde group and the corresponding bis-derivative of 2-amino-4H-pyran. Thus, using glutaraldehyde in the amount of 1 equiv in the three-component interaction did not result in the desired reaction product. At the same time, application of 0.5 equiv of 5i led to isolation of bis-2-amino-4Hpyran 7i, but unfortunately, in an extremely poor yield (Scheme 2).
In continuation of the current study we then applied other possible representative of active methylene nitrile -ethyl cyanoacetate 8 -in the three-component interaction with a view to introduce the ester group in position 3 of the 2-amino-4H-pyran ring. Nevertheless our efforts appeared to be unsuccessful since any desired ethyl 2-amino-4H-pyran-3-carboxylate was not isolated during these attempts. At the same time, the corresponding triethylammonium salt of bis-1,2-benzoxathiin-4(3H)-one 2,2-dioxide derivative 9g was isolated in 6% yield as the single product when isobutyric aldehyde was applied in the three-component interaction (Scheme 3). Taking into account the known mechanism of 2-amino-4H-pyran formation [14] and the regularities previously found Thereafter, enone A reacts as described above giving salt 9g. In our opinion, both of these routes are equiprobable. Triethylammonium salts similar to 9 are new derivatives of 1,2-benzoxathiine 2,2-dioxide. In this regard, we set the task of the purposeful obtaining of salts 9 based on the two-component approach described previously on the example of 1H-2,1-benzothiazin-4(3H)-one 2,2-dioxide [15]. According to this procedure the target salts 9 were obtained by the interaction of 1,2-benzoxathiin-4(3H)-one 2,2-dioxide 4 with aliphatic aldehydes in the molar ratio of 2:1 in the presence of the equimolar amount of triethylamine in i-PrOH for 1 hour (Scheme 4).
The data for the compounds obtained are presented in Tab. 1 and 2. The 1 H NMR data are given in Tab. 3.
The study of the antimicrobial properties of the compounds obtained was performed according to the international standards, 16] by the agar "well" diffusion method against the standard test-strains of gram-positive and gram-negative bacteria and fungi. The results revealed the higher antimicrobial activity than those of the reference drugs. The activity against the gram-positive strains was a little higher than moderate compared to gram-negative bacteria and fungi. The most active were samples with propyl, isopropyl and butyl substituents in position 4 of the pyran core, the activity increased along with the prolongation of the chain. Furthermore, triethylammonium salts corresponding to 2-amino-4H-pyran-3-carbonitriles showed higher antimicrobial properties. Thereby, utilization of long-chain aliphatic aldehydes along with the synthesis of the corresponding triethylammonium salts may be considered as a promising way for further construction of the narrow spectrum antibiotics.
The results of studying the antimicrobial properties are presented in Tab. 4.

Chemistry
The starting aldehydes and active methylene nitriles were obtained from commercial sources and used without further purification. Melting points were determined on a Gallenkamp melting point apparatus, Model MFB-595 in open capillary tubes. The 1 H NMRspectra were recorded on a Varian WXR-400 spectrometer using DMSO-d 6 as a solvent and TMS as an internal standard. Elemental analyses were carried out using a Carlo Erba CHNS-O EA 1108 analyzer.
The general procedure for the synthesis of triethylammonium 3-[(4-hydroxy-2,2-dioxido-2,1benzoxathiin-3-yl)alkyl]-2,1-benzoxathiin-5- olat 2,2-dioxides (9c,d,f,g). To the solution of 1,2-benzoxathiin-4(3H)-one 2,2-dioxide 4 (0.198 g, 0.001 mol) and the appropriate aliphatic aldehyde 3c,d,f,g (0.0005 mol) in propan-2-ol (10 mL) add triethylamine (0. 13   sions" (Kyiv) concerning innovations in the healthcare system. The inoculum density was 10 7 cells in 1 ml of the medium, and it was determined by comparing with McFarland standard. The 18 to 24-hour old culture of the microorganism was used for the test. For the antimicrobial evaluation the Mueller-Hinton agar was used, for Candida albicans strain the Sabouraud agar was taken. The compounds were introduced into agar by the "wells" method. The antibacterial activity was assessed by measuring the inhibition zones of the corresponding microorganism and was compared to those for the reference antimicrobial drugs.
2. Using ethyl cyanoacetate in the same interaction did not lead to the desired ethyl 2-amino-4H-pyran-3-carboxylates and in the case of isobutyric aldehyde resulted in formation of triethylammonium salt. It is explained by two equiprobable reaction pathways.
Conflict of Interests: authors have no conflict of interests to declare.