Synthesis of new 4-(phenylamino)thieno[2,3-b ]pyridines and derivatives of the novel benzo[ b ]thieno[3,2-h ]-1,6-naphthyridine tetracyclic system

Recently 1-hydroxyacridone derivatives were described as a new class of non-nucleoside inhibitors of Herpes Simplex Virus-1 (HSV-1), the agent of the most common human diseases. Based on these molecules, we applied rigid analogue and isosteric replacement approaches to design and synthesize five new benzo[ b ]thieno[3,2-h ]-1,6-naphthyridines derivatives 8a-e as feasible anti-HSV prototypes. Herein we described the synthesis of this series of compounds and a theoretical evaluation of the drug score and drug likeness of these compounds by using an in silico ADMET screening.


Figure 2
To investigate the impact of replacing the isosteric aromatic ring and the influence of the restricted flexibility imposed by the new benzo[b]thieno[3,2-h]-1,6-naphthyridine system, the compounds were submitted to an in silico ADMET screening to theoretically evaluate the drug score and drug likeness compared to acyclovir, a commercial antiviral drug, and 5-chloro-1,3dihydroxyacridone.

In silico ADMET screening
The five benzo[b]thieno[3,2-h]-1,6-naphthyridine derivatives 8a-e were submitted to an in silico ADMET screening (http://www.organic-chemistry.org/prog/peo/and http://www.molinspiration.com/cgi-bin/properties) to analyze their overall potential drug likeness and drug score compared to the 5-chloro-1,3-dihydroxyacridone and the commercial antiviral drug, acyclovir.Therefore, we evaluated some parameters such as: a) the cLogP value, which refers to the molecular hydrophobicity and that affects drug absorption, bioavailability, hydrophobic drug-receptor interactions, metabolism of molecules, as well as their toxicity, b) the Lipinski "Rule of 5", which states that most "drug-like" molecules have logP <= 5, molecular weight <= 500, number of hydrogen bond acceptors <= 10, and number of hydrogen bond donors <= 5; c) the toxicity risks (mutagenic, tumorigenic, and reproductive effects), d) the drug likeness value determined within the Fluka traded drugs collection, and e) the drug score that combines drug likeness, c log P, log S, molecular weight, and toxicity risks, and theoretically may be used to evaluate the drug potential of a compound 29 (Figure 4).
Two of the benzo[b]thieno[3,2-h]-1,6-naphthyridines 8a and 8d show positive drug likeness values even higher than acyclovir and acridone, which suggests the presence of active fragments frequently present in the commercial drugs.Interestingly, the 8-chlorobenzo[b]thieno[3,2-h]-1,6naphthyridin-6(11H)-one 8d derivative is analogous to 5-chloro-1,3-dihydroxyacridone, a point that could be well is favour of finding antiviral activity in this compound.In addition, naphthyridine derivatives show equivalent values in the drug score compared with the antiviral acyclovir and acridone, which reinforces their overall drug-like structure.
According to our theoretical study, the naphthyridine derivatives 8a-e show low tumorgenicity, mutagenicity and reproductive effects profiles (green scale) similar to acyclovir and the 5-chloro-1,3-dihydroxyacridone (not shown).It is important to notice that the toxicity predicted herein neither is a fully reliable toxicity prediction, nor guarantees that these compounds are completely free of any toxic effect.However, it reinforces the promising profile of these compounds for further experimental investigation 27,28 .

Conclusions
In acridones, the variation of the functional groups has impact on the activity as described in the literature.Due to the isosteric replacement and rigid analogue structure, the new series of benzo[b]thieno[3,4-h]-1,6-naphthyridines reported herein may have the potential to display an anti-HSV-1 profile.Although the Osiris risk alerts are not a fully reliable toxicity prediction, the theoretical low-toxicity profile of these compounds and their analogous drug likeness and drug score values compared to acyclovir and 5-chloro-1,3-dihydroxyacridone may suggest the potentiality of these compounds as new antiviral prototypes to be further investigated.ARKAT USA, Inc.

Experimental Section
General Procedures.The 1 H Nuclear Magnetic Resonance (NMR) spectra obtained from Varian model Unity Plus spectrometer operating at 300.00 MHz, using tetramethylsilane as internal standard.The chemical shifts (δ) reported in ppm and the coupling constants (J) in Hertz.Fourier transform infrared (FT-IR) absorption spectra were recorded in a Perkin-Elmer mode Spectrum One FT-IR spectrophotometer.The solid samples were measured using potassium bromide pellets.Melting points (m.p.) were determined with a Fisher-Johns apparatus.TLC was carried out using silica gel F-254 Glass Plate (20 x 20 cm).All other reagents and solvents used were analytical grade.The EIMS spectra were recorded using a Finingan MAT 711 A instrument.The ionization energy was 70 eV with the source 200 o C and an accelerative voltage of 8 KV.The standard direct insertion probe introduced the samples.High-resolution data were obtained with the instrument using 10 000 resolution.

General procedure for synthesis of 4-(phenylamino)thieno[2,3-b]pyridine-5-carbonitriles (6
a-e) 26 An equimolar (5 mmol) amounts of 4-chlorothieno[2,3-b]pyridine-5-carbonitrile and the appropriate aniline were heated in a silicone oil bath at 140° C for 2 hours.The mixture was diluted with CH 2 Cl 2 and purified by preparative silica gel plates (Glass Backed TLC Silica Gel, Hard Layer 250µm, F-254), eluent CH 2 Cl 2 .(2 mmol) in 6 N HCl (5 mL) was heated under reflux for 24 h.On cooling, the mixture was alkalinized with 10 % aq.NaOH solution and the precipitated was filtered and recrystallized from a mixture of ethanol and water.Yield (7 a-e) 75, 70, 67, 75, 70%, respectively.(b) Alkaline hydrolysis.A solution of 4-(phenylamino)thieno[2,3-b]pyridine-5-carbonitriles (6 a- e) (2 mmol), 2 g of potassium hydroxide pellets, 4 mL of ethyleneglycol, was heated under reflux for 24 h.On cooling mixture was acidified with diluted HCl (1:3), and the precipitated was filtered and recrystallized from a mixture of ethanol and water. mixture of the acids derivatives (7 a-e) (2 mmol) and phosphorus oxychloride (5 mL) was heated under reflux for 3 hours.The reaction mixture was poured over crushed ice.In some cases the excess of phosphorus oxychloride was removed under reduced pressure before invert crushed ice and neutralized with water.The resulting precipitate was collected and purified by flash