Synthesis and In Vitro Antiproliferative Activity of Novel Androst-5-ene Triazolyl and Tetrazolyl Derivatives

A straightforward and reliable method for the regioselective synthesis of steroidal 1,4-disubstituted triazoles and 1,5-disubstituted tetrazoles via copper(I)-catalyzed cycloadditions is reported. Heterocycle moieties were efficiently introduced onto the starting azide compound 3β-acetoxy-16β-azidomethylandrost-5-en-17β-ol through use of the “click” chemistry approach. The antiproliferative activities of the newly-synthesized triazoles were determined in vitro on three human gynecological cell lines (HeLa, MCF7 and A2780) using the microculture tetrazolium assay.


Introduction
In the past few years, the Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes to form triazoles has received revived attention. Since the independent reports of Sharpless [1] and Meldal [2], this process has become the most extensively studied "click" reaction, as evidenced by a nearly exponential growth in the number of related publications. Compared with the non-catalyzed version OPEN ACCESS the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) has certain advantageous properties, such as regioselectivity, versatility, high conversions and the lack of by-products [3][4][5]. Moreover, this process performs well in most common laboratory solvents and usually does not require protection from oxygen and water, making it an ideal tool for the synthesis of libraries for initial screening and structure-activity profiling.
In contrast, other 1,3-dipolar cycloadditions between nitriles and organic azides to afford tetrazoles generally requires highly electrophilic nitrile carbon atoms and harsh conditions [6]. Demko and Sharpless recently reported the syntheses of some 1,5-disubstituted tetrazoles [7,8] under solvent-free conditions at 100-120 °C. Furthermore, a series of potential catalysts for these reactions were investigated by Vilarrasa et al. [9], with the aim of achieving milder conditions. The commercially available or easily prepared [10] copper(I) triflate was observed to be the most efficient catalyst.
To the best of our knowledge, only a few examples are to be found in the literature in which 1,3-dipolar cycloadditions have been applied to steroidal azides [11][12][13][14][15]. Thus, in continuation of our program on the synthesis of steroidal heterocycles [16][17][18][19], we set out to develop an effective route for the production of novel steroidal triazoles and tetrazoles through use of the "click" chemistry approach. The present paper reports the syntheses of D-ring-substituted androst-5-ene derivatives containing a 1,4-disubstituted triazole (compounds 6a-j, 7a-j) or a 1,5-disubstituted tetrazole moieties (compounds 9a-e, 11a-e).
Five-membered nitrogen heterocycles play an important role in biological systems. Not surprisingly, a number of compounds containing 1,2,3-triazoles are found to exhibit a broad spectrum of biological activities, including antimicrobial [20], anti-HIV [21], antiallergic [22] and antiviral [23] effects. A set of 1,2,3-triazol-1-yl podophyllotoxin derivatives were synthesized and some of them proved to be more potent in inhibiting the growth of human cancer cells than etoposide [24]. Several benzotriazoles proved to be novel and potent antiproliferative agents and some of them exhibited nanomolar IC 50 values against human adherent cancer cell lines [25]. A series of substituted tetrazol-5ones have been synthesized and three of them were found to inhibit leukemia and breast cancer growth in vitro [26]. On the basis of these reports, the triazole and tetrazole ring systems can therefore be regarded as structural blocks suitable for improvement of the anticancer properties of potential pharmacons. Since we reported a set of androstene-fused arylpyrazolines as antiproliferative compounds, it appeared rational to improve the pharmacological profile of the skeleton by means of the introduction of a triazole or tetrazole moiety [19]. Moreover, some 21-triazolyl derivatives of pregnenolone were recently reported as potential anticancer agents by Banday et al. [15]. Thus, the newly-prepared triazolyl derivatives were screened in vitro for their activities against a panel of three human malignant cell lines.
Several D-ring-substituted androst-5-ene derivatives containing a 1,2,3-triazole ring (compounds 6a-j) were synthesized by the reaction of 5 with various terminal alkynes through use of the "click" chemistry approach (Table 1). Although there are a number of methods for generation of the active catalyst [29], one of the most common techniques was chosen. Thus, the Cu(I) species was generated in situ by the reduction of CuSO 4 ·5H 2 O with sodium ascorbate to minimize the formation of by-products. Furthermore, a mixture of CH 2 Cl 2 as solvent and water as co-solvent was employed to eliminate the need for ligands and to simplify the reaction protocol [30].
In all cases, total consumption of the starting compound was observed within 1-4 h at room temperature. The reactions were very selective, and triazole products could be isolated in 78-93% yields. The trace quantities of copper and reagents remaining in the reaction mixtures were removed by flash chromatography. Treatment of 6a-j containing a 3β-acetyl group with KOH in MeOH at 50 °C resulted in the corresponding 3β-hydroxy compounds 7a-j in good yields (Table 1). These outstanding results encouraged us to investigate another example of "click" reactions. The intermolecular [3+2] cycloadditions between the steroid azides 5 and 10 and several nitriles 8a-e containing an electron-withdrawing group (EWG) afforded the desired 1,5-disubstituted steroidal tetrazoles 9a-e and 11a-e. As mentioned earlier, highly electrophilic nitrile carbon atoms are required for successful addition [9]; some commercially available acyl cyanides and cyanoformates were therefore chosen as reagents. In all cases, the reactions were carried out at room temperature, with stirring for 2 days, 10 mol % copper(I) complex Cu 2 (OTf) 2 ·C 6 H 6 (OTf = O 3 SCF 3 ) being used as catalyst. The newly-synthesized tetrazolyl compounds could be isolated in 45-72% yields after purification by column chromatography (Table 2). The structures of all synthesized compounds were confirmed by 1 H-and in some cases, 13 C-NMR measurements. The 1 H-NMR spectra of 6a-i and 7a-i revealed the appearance of the new signals of the incorporated aryl groups at 6.9-8.5 ppm as compared with the spectra of the starting azide 5, while the 5'-H singlet of the newly-formed heterocycle was identified at 7.8-8.5 ppm. Compounds 6j and 7j containing a cycloalkyl substituent were exceptions, with a chemical shift of 7.28 ppm (5'-H). As far as the tetrazolyl derivatives are concerned, the newly-formed heterocycle does not contain any protons, but the signal of 5'-C can be identified at 145-149 ppm in the corresponding 13 C-NMR spectra. Furthermore, in the cases of 9c, 11c and 9e, 11e the new signals of the incorporated Ph ring appeared at 7.3-8.3 ppm in the 1 H-NMR spectra.

Biological Activity
Compounds 7a-j and 11a-e were screened for anticancer activity against a panel of three human cancer cell lines (Table 3). Although there is no generally accepted threshold for efficacy, when the inhibition of cell growth is less than 25% at 30 µM, such a substance may be considered ineffective. No clear structure-activity relationships could be concluded, but triazole-containing androst-5-enes exhibit substantial antiproliferative activity, for which a substituted aromatic group of the triazole ring is preferred. The antiproliferative action of a compound with an unsubstituted phenyl group on the triazole ring (compound 7a) could be maintained or moderately increased by substitution in the para or meta position (compounds 7b-e, 7g), while ortho-OMe (compound 7f) was less effective. Nevertheless, an amino group in the meta position offers no advantage (compound 7h). A pyridyl, but not a cyclopropyl group, instead of phenyl (compounds 7i-j) could be beneficial. Although inactive on HeLa cells at 10 µM, 7c is considered the most effective of the presented compounds. In contrast, tetrazoles substituted on the D-ring of the steroidal skeleton proved to be ineffective with the exception of 11e, which has a moderate effect.

General
Melting points (mp) were determined on a Kofler block and are uncorrected. The reactions were monitored by TLC on Kieselgel-G (Merck Si 254 F) layers (0.25 mm thick); solvent systems (ss): (A) CH 2 Cl 2 /EtOAc (95:5 v/v), (B) CH 2 Cl 2 /EtOAc (80:20 v/v), (C) CH 2 Cl 2 /EtOAc (50:50 v/v). The spots were detected by spraying with 5% phosphomolybdic acid in 50% aqueous phosphoric acid. The R f values were determined for the spots observed by illumination at 254 and 365 nm. Flash chromatography: Merck silica gel 60, 40-63 μm. All solvents were distilled prior to use. Reagents and materials were obtained from commercial suppliers and were used without purification.
Elementary analysis data were determined with a PerkinElmer CHN analyzer model 2400 and IR spectra were recorded on a BioRad FTS 60A FTIR spectrometer. NMR spectra were obtained at room temperature with a Bruker DRX 500 instrument. Chemical shifts are reported in ppm (δ scale), and coupling constants (J) in Hz. For the determination of multiplicities, the J-MOD pulse sequence was used.
Automated flow injection analyses were performed by using an HPLC/MSD system. The system comprised an Agilent 1100 micro vacuum degasser, a quaternary pump, a micro-well plate autoinjector and a 1946A MSD equipped with an electrospray ion source (ESI) operated in positive ion mode. The ESI parameters were: nebulizing gas N 2 , at 35 psi; drying gas N 2 , at 350 °C and 12 L/min; capillary voltage (VCap) 3000 V; fragmentor voltage 70 V. The MSD was operated in scan mode with a mass range of m/z 60−620. Samples (0.2 μL) with automated needle wash were injected directly into the solvent flow (0.3 mL/min) of CH 3 CN/H 2 O, 70:30 (v/v) supplemented with 0.1% formic acid. The system was controlled by Agilent LC/MSD Chemstation software.

3β-Acetoxy-16β-p-toluenesulfonyloxymethylandrost-5-en-17β-ol (4)
Compound 3a (7.25 g, 20 mmol) was dissolved in pyridine (50 mL), and a solution of p-toluenesulfonyl chloride (7 g, 35 mmol) in pyridine (10 mL) was then added dropwise while cooling in ice. The reaction mixture was allowed to stand overnight, and was then poured into a mixture of ice and H 2 SO 4 (20 mL). The precipitate that formed was filtered off and washed with water to neutrality. This substance was used in the subsequent step without further purification and characterization. (5) Sodium azide (1.8 g, 28 mmol) was added to a solution of 4 (5.8 g, 11 mmol) in DMF (80 mL). The reaction mixture was stirred at 70 °C for 6 h and was then poured into water. The precipitate that formed was allowed to stand overnight, and then filtered off and washed with water. Purification of the resulting crude product by column chromatography with CH 2 Cl 2 as eluent afforded 5 as a white solid

Conclusions
In summary, the efficient syntheses of several D-ring-substituted steroidal triazoles and tetrazoles were achieved by means of 1,3-dipolar cycloadditions. The simple and fast reactions were carried out under mild conditions that furnished the desired compounds in good yields. The novel synthesized compounds were screened for their activities against a panel of three human gynecological cancer cell lines (HeLa, MCF7 and A2780). The application of "click" chemistry to further sterane skeletons was encouraged by these promising results.