N-Pyrrylarylsulfones with High Therapeutic Potential

This review illustrates the various studies made to investigate the activity of N-pyrrylarylsulfone containing compounds as potential antiviral, anticancer and SNC drugs. A number of synthetic approaches to obtain tetracyclic, tricyclic and non-cyclic compounds, and their biological activity with regard to structure–activity relationships (SARs) have been reviewed. The literature reviewed here may provide useful information on the potential of N-pyrrylarylsulfone pharmacophore as well as suggest concepts for the design and synthesis of new N-pyrrylarylsulfone based agents.


Introduction
Sulfonamide is the basis of several groups of drugs [1]. Intense interest focused on sulfonamide drugs after the discovery in 1935 that the activity of red dye Prontosil [2,3] was attributed to breakdown product sulfanilamide (1). The antibacterial sulfonamides work as competitive inhibitors of the dihydropteroate synthase, an enzyme involved in folate synthesis [4]. The simple 1 was cheaper than Prontosil, had fewer unwanted effects and did not impart the typical red color to the skin. Nowadays, sulfonamides have been replaced by other antibacterial drugs such as β-lactam antibiotics, with some important exception; for example, sulfamethoxazole (2) is used for treatment of urinary and respiratory-tracts infections [5]. Sulfa molecules have been chemically manipulated to obtain drug for the treatment of leprosy, fluid accumulation and diabetes. The modern era of drug treatment of leprosy began in 1937 when the sulfa drug dapsone (3) [6] proved to be highly effective. For more than six decades, 3 remained first line drug to treat leprosy. Since 1980s, 3 has been administered in combination with rifampicin and clofazimine for treatment of leprosy [7]. Chlorothiazide (4) is a carbonic anhydrase inhibitor which was introduced in 1958 as a diuretic drug and is used to treat hypertension and edema [8,9]. Before the discovery of 4, mercurial drugs associated with severe toxicity were the only available drugs to treat fluid retention. Few years later, in 1962, another sulfonamide, furosemide (5), was discovered as diuretic drug and is used to treat fluid retention and for the treatment of high blood pressure [10]. Tolbutamide (6), the first sulfonylurea anti-diabetic drug, was approved in the United States in 1957 for the treatment of type 2 diabetics [11]. Even though since 1964 there were concerns that sulfonylurea antidiabetic drugs may increase cardiovascular risk, the current literature does not confirm the detrimental risk profile of sulphonylureas compared with other anti-diabetic drugs [12]. Ethoxyzolamide (7) is a carbonic anhydrase inhibitor used in the treatment of glaucoma and duodenal ulcers, and as a diuretic [13]. Other sulfa drug examples include antivirals agents, such HIV-1 non-nucleoside reverse transcriptase and protease inhibitors [14][15][16][17][18], HCV NS3/4A protease [19] and NS5B polymerase inhibitors [20]; antibiotics, such mafenide, approved by the FDA in 1948 [21]; and nonsteroidal anti-inflammatory drug such celecoxib, a COX-2 selective inhibitor [22] (Chart 1). NS5B polymerase inhibitors [20]; antibiotics, such mafenide, approved by the FDA in 1948 [21]; and nonsteroidal anti-inflammatory drug such celecoxib, a COX-2 selective inhibitor [22] (Chart 1).

Chart 1. Examples of sulfa and pyrrole containing drugs.
Pyrrole ring is a well-known privileged scaffold that exhibits a wide variety of biological activities [23]. Including the pyrrole into different pharmacophores has resulted in non-cyclic and polycyclic pyrrole-containing systems with potential therapeutic effects such as anticancer (leukemia and lymphoma), anti-microbic (bacteria, malaria, protozoa, and fungi) and central nervous system agents (antipsychotic and anxiolytic) (for example, pyrrolnitrin (8) [24], tolmetin (9) [25], isamoltane (CGP-361A) (10) [26], porphobilinogen (11) [27], and atorvastatin (12) [28]). Recently, VU0410150 (13), a pyrrylarylsulfone containing compound, has been discovered as mGluR4-positive allosteric modulator and evaluated as potential drug for treatment for Parkinson's disease [29,30]. In the past decades, numerous N-pyrrylarylsulfones have been synthesized by our research group in several drug discovery projects. In this work, attempt has been made to review various N-pyrrylarylsulfone based compounds to discuss the synthetic approaches and the biological activity with regard to structureactivity relationships (SARs).

Pyrrolo[1,2-b][1,2,5]benzothiadiazepine Acetic Acid 5,5,-dioxide
The PBTD scaffold has been exploited in several antiviral research programs.  Crystal structure [40] of 37a showed that the aromatic moieties adopted a dihedral angle of 114.4 • , a value that was very near to the optimal value of the butterfly-like conformation reported by the Schaefer's model [41].

PBTDs as Chronic Myelogenous Leukemia (CML) Agents
The antitumor activity of pyrrolo[2,1-c] [1,4]benzodiazepines (PBDs, e.g., 48) related to anthramycin was extensively studied, as it was documented in Thurston's review [46]. Given the high structural similarity between PBTD and PBD compounds, two PBTDs, 23 and its 10-(4-methylbenzoyl) derivative 49, were selected for screening of pro-apoptotic and anti-leukemia activity (Chart 3, Tables 2 and 3) [47]. PBTD 23 was prepared by an improved procedure using dimethoxyacetal of ethyl glyoxylate in absolute ethanol in the presence of 4-toluenesulfonic acid (PTSA). Compound 23, prepared as described in Scheme 2, was N-acylated to 49 with 4-methylbenzoyl by refluxing in 1-bromo-3-chloropropane in the presence of sodium hydrogen carbonate. PBTDs 23 and 49 induced apoptosis in K562 cells and caused cell death in BCR-ABL-positive leukemia cells obtained from chronic myeloid leukemia patients who were at onset or were IM-resistant. Apoptotic mechanism studies showed that PBTDs 23 and 49 activated the caspase activity through two different pathways: both compounds activated caspase-3; 23 significantly reduced the procaspase-8; in contrast 49 evidenced a decrease of procaspase-9 band. The apoptosis was observed before the expression of BCR-ABL protein and the tyrosine phosphorylation. PBTDs-mediated suppression of K562 cell proliferation was characterized by the appearance of DNA fragmentation and was associated with the poly(ADPribose) polymerase (PARP) cleavage. PBTDs 23 and 49 treatment resulted in caspase-3 activation through down-regulation of Bcl-2 and up-regulation of Bax [48]. PBTDs possessed inhibitory activity against mTOR and impeded hyper-phosphorylation of Akt as a feedback of inhibition of mTOR by rapamycin [49]. These findings highlighted PBTDs as potential agents for the treatment of CML [50,51]. Replacement of the pyrrole ring of PBTD with the indole (43) resulted in weaker antiretroviral compounds [39]. On the other hand, the 5H-indolo[3,2-b] [1,5]benzothiazepine isomers (e.g., 44), were endowed with anti-HIV-1 activity in the low micromolar range of concentrations [43]. In addition, 1H-pyrrolo [2,3-

PBTDs as Chronic Myelogenous Leukemia (CML) Agents
The antitumor activity of pyrrolo[2,1-c] [1,4]benzodiazepines (PBDs, e.g., 48) related to anthramycin was extensively studied, as it was documented in Thurston's review [46]. Given the high structural similarity between PBTD and PBD compounds, two PBTDs, 23 and its 10-(4-methylbenzoyl) derivative 49, were selected for screening of pro-apoptotic and anti-leukemia activity (Chart 3, Tables 2 and 3) [47]. PBTD 23 was prepared by an improved procedure using dimethoxyacetal of ethyl glyoxylate in absolute ethanol in the presence of 4-toluenesulfonic acid (PTSA). Compound 23, prepared as described in Scheme 2, was N-acylated to 49 with 4-methylbenzoyl by refluxing in 1-bromo-3-chloropropane in the presence of sodium hydrogen carbonate. PBTDs 23 and 49 induced apoptosis in K562 cells and caused cell death in BCR-ABL-positive leukemia cells obtained from chronic myeloid leukemia patients who were at onset or were IM-resistant. Apoptotic mechanism studies showed that PBTDs 23 and 49 activated the caspase activity through two different pathways: both compounds activated caspase-3; 23 significantly reduced the procaspase-8; in contrast 49 evidenced a decrease of procaspase-9 band. The apoptosis was observed before the expression of BCR-ABL protein and the tyrosine phosphorylation. PBTDs-mediated suppression of K562 cell proliferation was characterized by the appearance of DNA fragmentation and was associated with the poly(ADPribose) polymerase (PARP) cleavage. PBTDs 23 and 49 treatment resulted in caspase-3 activation through down-regulation of Bcl-2 and up-regulation of Bax [48]. PBTDs possessed inhibitory activity against mTOR and impeded hyper-phosphorylation of Akt as a feedback of inhibition of mTOR by rapamycin [49]. These findings highlighted PBTDs as potential agents for the treatment of CML [50,51].

PBTDs as Chronic Myelogenous Leukemia (CML) Agents
The antitumor activity of pyrrolo[2,1-c] [1,4]benzodiazepines (PBDs, e.g., 48) related to anthramycin was extensively studied, as it was documented in Thurston's review [46]. Given the high structural similarity between PBTD and PBD compounds, two PBTDs, 23 and its 10-(4-methylbenzoyl) derivative 49, were selected for screening of pro-apoptotic and anti-leukemia activity (Chart 3, Tables 2 and 3) [47]. PBTD 23 was prepared by an improved procedure using dimethoxyacetal of ethyl glyoxylate in absolute ethanol in the presence of 4-toluenesulfonic acid (PTSA). Compound 23, prepared as described in Scheme 2, was N-acylated to 49 with 4-methylbenzoyl by refluxing in 1-bromo-3-chloropropane in the presence of sodium hydrogen carbonate. PBTDs 23 and 49 induced apoptosis in K562 cells and caused cell death in BCR-ABL-positive leukemia cells obtained from chronic myeloid leukemia patients who were at onset or were IM-resistant. Apoptotic mechanism studies showed that PBTDs 23 and 49 activated the caspase activity through two different pathways: both compounds activated caspase-3; 23 significantly reduced the procaspase-8; in contrast 49 evidenced a decrease of procaspase-9 band. The apoptosis was observed before the expression of BCR-ABL protein and the tyrosine phosphorylation. PBTDs-mediated suppression of K562 cell proliferation was characterized by the appearance of DNA fragmentation and was associated with the poly(ADPribose) polymerase (PARP) cleavage. PBTDs 23 and 49 treatment resulted in caspase-3 activation through down-regulation of Bcl-2 and up-regulation of Bax [48]. PBTDs possessed inhibitory activity against mTOR and impeded hyper-phosphorylation of Akt as a feedback of inhibition of mTOR by rapamycin [49]. These findings highlighted PBTDs as potential agents for the treatment of CML [50,51].

Pyrryl Aryl Sulfones
Diarylsulfones emerged as a chemical class of HIV-1 NNRTIs. The presence of the nitro group at position 2 of the phenyl ring and the sulfur bridging atom as sulfur dioxide are fundamental structural characteristics for their activity. The antiviral activity of 2-nitrophenyl phenyl sulfone (50, NPPS) [52] prompted the synthesis of a series of 41 pyrryl aryl sulfones (PAS) and some related derivatives [53]. Pyrryl 2-nitrophenyl sulfone (51) was straightforwardly prepared by nucleophilic substitution reaction between 2-nitrobenzenesulfonyl chloride and pyrrole in the presence of n-tetrabutylammonium hydrogen sulfate (TBAS) as a phase transfer catalyst. On the other hand, alkaline hydrolysis of 2ethoxycarbonylpyrrole (16) [34] afforded the acid 52 which was transformed into 53 by reaction with ethyl chloroformate in the presence of 4-methylmorpholine followed by treatment of the intermediate mixed anhydride with glycine ethyl ester (Scheme 6).
Ester 55 was prepared by treating the corresponding acid 54 [54] with oxalyl chloride and then with anhydrous ethanol. Reaction of 1-(2-aminobenzenesulfonyl)pyrrole [35] with methyl malonyl chloride in the presence of triethylamine led to amide 56 which in turn was methylated to 57 or 58 with one or two equivalents of methyl chloride, respectively, in the presence of potassium carbonate

Pyrryl Aryl Sulfones
Diarylsulfones emerged as a chemical class of HIV-1 NNRTIs. The presence of the nitro group at position 2 of the phenyl ring and the sulfur bridging atom as sulfur dioxide are fundamental structural characteristics for their activity. The antiviral activity of 2-nitrophenyl phenyl sulfone (50, NPPS) [52] prompted the synthesis of a series of 41 pyrryl aryl sulfones (PAS) and some related derivatives [53]. (Scheme 6). Compound 16, a 2-nitrophenyl 1-pyrryl sulfone bearing the 2-ethoxycarbonyl function, showed the highest anti HIV-1 activity (Table 4).  The importance of the diaryl sulfone moiety for the design of new anti-HIV-1 agents was further confirmed by the synthesis of new series of PAS and indolyl aryl sulfones [55,56]. The amino-PAS derivatives were synthesized as follows. Alkylation of the 2-amino group was achieved by reaction of 59a and 59b with the appropriate aldehyde in the presence of sodium cyanoborohydride; carboxamides were obtained by heating with an acyl chloride in pyridine (Scheme not shown). It was reported that the 4-chloroaniline moiety or the related 5-chloro-2-pyridylamine represented the key feature of highly potent HIV-1 NNRTIs, for example 8-Cl-TIBO [57], 7-Cl-PBTD (37) [39] (Table 1), 3,3-dialkyl-3,4-dihydroquinoxaline-2-(1H)thione [58], oxoquinoline [59], and PETT [60]. In the case of PAS Scheme 6. Synthesis of PAS 51-58.
Ester 55 was prepared by treating the corresponding acid 54 [54] with oxalyl chloride and then with anhydrous ethanol. Reaction of 1-(2-aminobenzenesulfonyl)pyrrole [35] with methyl malonyl chloride in the presence of triethylamine led to amide 56 which in turn was methylated to 57 or 58 with one or two equivalents of methyl chloride, respectively, in the presence of potassium carbonate (Scheme 6). Compound 16, a 2-nitrophenyl 1-pyrryl sulfone bearing the 2-ethoxycarbonyl function, showed the highest anti HIV-1 activity (Table 4). The importance of the diaryl sulfone moiety for the design of new anti-HIV-1 agents was further confirmed by the synthesis of new series of PAS and indolyl aryl sulfones [55,56]. The amino-PAS derivatives were synthesized as follows. Alkylation of the 2-amino group was achieved by reaction of 59a and 59b with the appropriate aldehyde in the presence of sodium cyanoborohydride; carboxamides were obtained by heating with an acyl chloride in pyridine (Scheme not shown). It was reported that the 4-chloroaniline moiety or the related 5-chloro-2-pyridylamine represented the key feature of highly potent HIV-1 NNRTIs, for example 8-Cl-TIBO [57], 7-Cl-PBTD (37) [39] (Table 1), 3,3-dialkyl-3,4dihydroquinoxaline-2-(1H)thione [58], oxoquinoline [59], and PETT [60]. In the case of PAS derivatives, the 4-chloroaniline moiety worked as a pharmacophore only when the sulfonyl group was near to the amino group. The nature of the pharmacophore could not be modified without affecting the anti-HIV-1 activity. The highest anti-HIV-1 activity of compounds 59a and 59b was also associated with the presence of the alkoxycarbonyl group at position 2 of the pyrrole ring. Alkylation of aniline nitrogen completely abolished the activity (data not shown), whereas acylation led to weakly active compounds ( Table 5). The ability to inhibit the recombinant reverse transcriptase (rRT) of HIV-1 is depicted in Table 6. When tested against the rRT form HIV-1 mutants resistant to nevirapine (Y181C) and TIBO (L1001I), the compounds showed activity at 10-fold higher concentrations. derivatives, the 4-chloroaniline moiety worked as a pharmacophore only when the sulfonyl group was near to the amino group. The nature of the pharmacophore could not be modified without affecting the anti-HIV-1 activity. The highest anti-HIV-1 activity of compounds 59a and 59b was also associated with the presence of the alkoxycarbonyl group at position 2 of the pyrrole ring. Alkylation of aniline nitrogen completely abolished the activity (data not shown), whereas acylation led to weakly active compounds ( Table 5). The ability to inhibit the recombinant reverse transcriptase (rRT) of HIV-1 is depicted in Table 6. When tested against the rRT form HIV-1 mutants resistant to nevirapine (Y181C) and TIBO (L1001I), the compounds showed activity at 10-fold higher concentrations.  Compound 59b was selected as lead compound for an antiviral project based on molecular modeling studies. Using the three-dimensional structure of HIV-1 RT cocrystallized with α-APA (alpha-anilinophenyl acetamide) derivative R95845, a model of RT/59b complex was derived using previously developed SARs. The experimentally determined RT bound conformations of α-APA  Compound 59b was selected as lead compound for an antiviral project based on molecular modeling studies. Using the three-dimensional structure of HIV-1 RT cocrystallized with α-APA (alpha-anilinophenyl acetamide) derivative R95845, a model of RT/59b complex was derived using previously developed SARs. The experimentally determined RT bound conformations of α-APA R90385 [61] served as basis to select conformations of 59b for docking studies. By scanning the rotatable bonds of the crystal structure of 59b, a low energy conformation was identified and this compound superimposable on α-APA R95845 about the aromatic rings and the COOEt/COMe and SO 2 /CONH 2 groups. Inspection of the RT/59b complex revealed a region of the HIV-1 NNBS (non-nucleoside binding site) delimited by Tyr181, Tyr188 and Trp229 side chains, which could be filed by substituents at position 4 of the pyrrole ring. Among the compounds synthesized, 60 (EC 50 = 42 nM; IC 50 = 50 nM) was the most potent PAS derivative (Table 7). Compared with 59b, it showed three-and eight-fold improvement in cell-based and enzyme assays, respectively [62].  (Table 7). Compared with 59b, it showed threeand eight-fold improvement in cell-based and enzyme assays, respectively [62]. Further studies were conducted to improve the activity of PAS 59b [63]. New PAS derivatives were synthesized by introduction of different alkyl, alkenyl or cycloalkyl substituents at the 2-ester function, along with a small series of 2-carboxamide derivatives, in order to explore the effects of substituents a position of the pyrrole ring. The new derivatives were less potent and sometimes more toxic than the previously reported 59a and 59b. This study confirmed the key role of the 4-chloroanilino moiety and the substituent at the ester function.
Compound 60 was synthesized as depicted in Scheme 7. 5-Chloro-2-nitrobenzenesulfonyl chloride was reacted with 2-ethoxycarbonyl-1H-pyrrole-4-carboxaldehyde in the presence of potassium tertbutoxide and 18-crown-6 to give 61. Sodium borohydride reduction of aldehyde 61 afforded alcohol 62, and the nitro group reduction with iron in glacial acetic acid to provide the amino derivative 60 (Scheme 7). Further studies were conducted to improve the activity of PAS 59b [63]. New PAS derivatives were synthesized by introduction of different alkyl, alkenyl or cycloalkyl substituents at the 2-ester function, along with a small series of 2-carboxamide derivatives, in order to explore the effects of substituents a position of the pyrrole ring. The new derivatives were less potent and sometimes more toxic than the previously reported 59a and 59b. This study confirmed the key role of the 4-chloroanilino moiety and the substituent at the ester function.
Compound 60 was synthesized as depicted in Scheme 7. 5-Chloro-2-nitrobenzenesulfonyl chloride was reacted with 2-ethoxycarbonyl-1H-pyrrole-4-carboxaldehyde in the presence of potassium tert-butoxide and 18-crown-6 to give 61. Sodium borohydride reduction of aldehyde 61 afforded alcohol 62, and the nitro group reduction with iron in glacial acetic acid to provide the amino derivative 60 (Scheme 7).  (Table 7). Compared with 59b, it showed threeand eight-fold improvement in cell-based and enzyme assays, respectively [62].  Further studies were conducted to improve the activity of PAS 59b [63]. New PAS derivatives were synthesized by introduction of different alkyl, alkenyl or cycloalkyl substituents at the 2-ester function, along with a small series of 2-carboxamide derivatives, in order to explore the effects of substituents a position of the pyrrole ring. The new derivatives were less potent and sometimes more toxic than the previously reported 59a and 59b. This study confirmed the key role of the 4-chloroanilino moiety and the substituent at the ester function.

Acylamino Pyrryl Aryl Sulfones
A series of PAS related compounds bearing acylamino moieties at position 2 of the benzene ring were synthesized as truncated analogs of PBTDs [39]. Furthermore, potent HIV-1 NNRTIs, such as PETT (67) [65] and truncated-TIBO (68) [66] compounds, were designed and synthesized based on the structure of 8-Cl-TIBO (66) [67,68] using a ring-opening strategy. Based on these findings, the same strategy was applied to 7-Cl-PBTD (37a) by breaking the 11,11b bond. The drug design strategy conceived a series of acylamino-PAS (APAS) derivatives, which were synthetized and characterized for their antiviral properties [69] (Chart 5).  [70]. Although structurally related to the previously reported PAS family, the APAS derivatives were investigated for binding mode in the non-nucleoside binding site of the HIV-1 RT [71]. Derivative 69, the most active among the test APASs, was modeled from the X-ray coordinates of 59b and docked into the HIV-1 NNBS of the RT using the 2-amino-6-[(3,5-dimethyl)sulfonylbenzonitrile/RT complex [70]. The binding mode of 69 shared similarities with previously reported PASs [62,64]: the ethoxycarbonyl filled the highly hydrophobic region of NNBS, and the 4-chloro-2-methoxycarbonyl moiety took up the H-bond region.

Acylamino Pyrryl Aryl Sulfones
A series of PAS related compounds bearing acylamino moieties at position 2 of the benzene ring were synthesized as truncated analogs of PBTDs [39]. Furthermore, potent HIV-1 NNRTIs, such as PETT (67) [65] and truncated-TIBO (68) [66] compounds, were designed and synthesized based on the structure of 8-Cl-TIBO (66) [67,68] using a ring-opening strategy. Based on these findings, the same strategy was applied to 7-Cl-PBTD (37a) by breaking the 11,11b bond. The drug design strategy conceived a series of acylamino-PAS (APAS) derivatives, which were synthetized and characterized for their antiviral properties [69] (Chart 5).  [70]. Although structurally related to the previously reported PAS family, the APAS derivatives were investigated for binding mode in the non-nucleoside binding site of the HIV-1 RT [71]. Derivative 69, the most active among the test APASs, was modeled from the X-ray coordinates of 59b and docked into the HIV-1 NNBS of the RT using the 2-amino-6-[(3,5-dimethyl)sulfonylbenzonitrile/RT complex [70]. The binding mode of 69 shared similarities with previously reported PASs [62,64]: the ethoxycarbonyl filled the highly hydrophobic region of NNBS, and the 4-chloro-2-methoxycarbonyl moiety took up the H-bond region.
APAS derivatives 69 and 70 were prepared by reacting compound 59b with bromoacetyl bromide 1-bromo-3-chloropropane in the presence sodium hydrogen carbonate to give 2-bromoacetylamino derivative 71. Treatment of 71 with sodium methoxide or thiomethoxide afforded APASs 69 or 70, respectively (Scheme 8).  [70]. Although structurally related to the previously reported PAS family, the APAS derivatives were investigated for binding mode in the non-nucleoside binding site of the HIV-1 RT [71]. Derivative 69, the most active among the test APASs, was modeled from the X-ray coordinates of 59b and docked into the HIV-1 NNBS of the RT using the 2-amino-6-[(3,5-dimethyl)sulfonylbenzonitrile/RT complex [70]. The binding mode of 69 shared similarities with previously reported PASs [62,64]: the ethoxycarbonyl filled the highly hydrophobic region of NNBS, and the 4-chloro-2-methoxycarbonyl moiety took up the H-bond region.

Structurally Related Compounds
The highly potent anti-HIV-1 activity displayed by Merck carboxamide L-737,126 (78)