Synthesis of novel 4,4'-bis(2,4-pyrimidinyl)-diaminostilbene-2,2'-disulfonic acid derivatives and their whitening effect on cotton fiber as fluorescent whitening agents

Abstract

Some novel 4,4'-bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d were designed and synthesized. Their structures were confirmed by proton nuclear magnetic resonance, elemental analysis and ultraviolet absorption spectra. All newly synthesized compounds were interrogated on their whitening effect on cotton fiber as fluorescent whitening agents. Their optical properties were evaluated by the degree of whiteness, color index, light fastness, wet rubbing fastness and dry rubbing fastness. The results indicated that compounds 4a-d showed an obvious whitening effect on cotton fiber and higher fastness than that of C186. The reflectivity of cotton fiber treated with compounds 4a-d at the critical concentration was up to 120.76%. In contrast, the reflectivity of untreated cotton fiber was only 87.23%. The ultraviolet absorption properties of compound 4d were tested under diverse pH values. The results showed that the ultraviolet absorption intensity of compound 4d decreased significantly and the absorption curve had blue shift under acidic conditions. Compound 4d was found to be the most effective molecule as a fluorescent whitening agent with a higher degree of whiteness (CIE Whiteness 135.04) compared to untreated cotton fiber (CIE Whiteness 73.02) at the critical concentration (0.1%).

Keywords

4 4'-Bis(2 4-pyrimidinyl)-diamino stilbene-2 2'-disulfonic acid derivatives synthesis fluorescent whitening agents optical properties

Fluorescent whitening agents (FWAs) are complex organic compounds that can absorb ultraviolet (UV) light and emit blue fluorescence,¹ which dramatically improves the whiteness and brightness of the treated matrix^2,3; they are widely used in the textile, detergent, papermaking, pigment, paint and plastic areas. FWAs also significantly increase the UV-blocking properties of the medium to which they are applied.⁴ FWAs are also used as special devices, such as pH chemo-sensing materials,⁵ chemo-sensors,⁶ photo-induced electron transfer sensors,⁷ light emitting diodes,⁸ polyurethane fluorescent brightener dispersions⁹ and luminescent materials.¹⁰ The molecular structures of FWAs have large conjugated systems, which are mainly derived from stilbene,¹¹ pyrazole,¹² quinolone,¹³ oxadiazole¹⁴ and benzoxazole derivatives.¹⁵

Stilbene derivatives, especially triazine-stilbene compounds, represent the first-generation FWAs, which account for more than 80% of the FWAs. Although triazine-stilbene FWAs have a good whitening effect on the treated substrate, they also showed some disadvantages, which limited their application in some fields because of the inherent properties of the stilbene-triazine skeleton. Therefore, the design, synthesis and screening of novel stilbene FWAs with a special skeleton will be a feasible means to improve the performance of such FWAs.

Pyrimidine is an important heterocyclic compound, which is formed by two nitrogen atoms substituted for two carbon atoms in the intermolecular position of benzene. Pyrimidine derivatives, such as cytosine, uracil and thymine, are important components of nucleic acid and deoxyribonucleic acid. Pyrimidine rings are often introduced into other matrix molecules as functional groups to enhance their solubility and binding affinity to targeted proteins. Therefore, compounds containing a pyrimidine ring have been developed as anti-cancer drugs,^16–18 antibacterial drugs,^19,20 antioxidant agents²¹ and antiplasmodial agents.²²

In this work, we integrated the pyrimidine ring into stilbene-2,2'-disulfonic acid and synthesized some novel 4,4'-bis(2,4-pyrimidinyl)-diaminostilbene-2,2'-disulfonic acid derivatives. These compounds were applied to cotton fiber as FWAs and their dyeing properties, including the whiteness, color index and fastness, were evaluated. The reflectivity of the treated cotton fiber with compounds 4a-d at the critical concentration (0.1%) and the UV absorption properties of compound 4d under various pH values were investigated.

Result and discussion

Synthesis of compounds 4a-d

Reaction conditions: (i) Na₂CO₃ (aq), H₂O, DMF, 20–25℃, 10 h; (ii) then amino compounds, reflux, 7 h.

4,4'-Bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d were synthesized in the yields of 82–91% by the reaction of compound 3 with four amino compounds at reflux temperature for 7 h. Compound 3 was obtained by the reaction of 2,4-dichloro-4,5-dihydropyrimidine with 4,4’-diaminostilbene-2,2’-disulfonic acid at 20–25℃ for 10 h. The synthetic route for compounds 4a-d is shown in Scheme 1.

Scheme 1.

Synthesis of 4,4'-bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d.

Ultraviolet absorption spectra of compounds 4a-d

The concentration of compounds 4a-d is 0.02 g/L in deionized water. UV absorption data for compounds 4a-d were collected on a U-4100 spectrophotometer. As shown in Figure 1, the UV absorption values of compounds 4a-d were in the range of 300–400 nm and their maximum absorption peaks located at 352, 352, 352 and 354 nm, respectively. The absorption intensity of these compounds is from 0.7817 to 0.8689. Since the parent structures of compounds 4a-d are identical, their UV absorption peaks are similar. The difference in substituents and molar concentrations of compounds 4a-d results in a slight difference in their absorption properties.

Figure 1.

Ultraviolet absorption spectra of compounds 4a-d in deionized water.

Ultraviolet absorption properties of compound 4d under diverse pH values

Stilbene-disulfonic acid derivatives as FWAs are usually soluble sulfonates. However, in acid systems, stilbene-disulfonic acid derivatives usually form precipitates, resulting in reduced concentrations and decreased fluorescence efficiency. As a result, stilbene-disulfonic acid FWAs are usually used in weak alkaline solutions. The concentration of compound 4d under various pH values is 0.02 g/L in deionized water. The UV absorption data were collected on a U-4100 spectrophotometer and the pH values were adjusted by 1 M HCl and 1 M NaOH solutions. As shown in Figure 2, when the pH values were 7.9, 9.5, 10.2 and 11.0, the maximum UV absorption peak of compound 4d was located at 354 nm, and the values of the absorbance intensities were 0.8285, 8424, 0.8691 and 0.8761, respectively. It can be seen that the absorption strength of compound 4d increased slightly with increasing alkalinity. In the acidic system, the UV absorption peaks of compound 4d showed a large blue shift phenomenon. When pH values were 5.5, 4.6, 3.0 and 1.8, the maximum absorption peaks of compound 4d appeared at 350, 348, 342 and 342 nm, respectively, and their absorption intensities were 0.7558, 0.7401, 0.6968 and 0.6931, respectively. The UV absorption data for compound 4d under diverse pH values are shown in Table 1.

Figure 2.

Changes of the ultraviolet spectra of compound 4d in deionized water under diverse pH values.

Table 1.

Maximum ultraviolet absorption and absorbance intensity values of compound 4d in deionized water under diverse pH values

pH	Maximum ultraviolet absorption (λ max nm)	Absorbance intensity
1.8	342	0.6931
3.0	342	0.6968
4.6	348	0.7401
5.5	350	0.7558
7.9	354	0.8285
9.5	354	0.8424
10.2	354	0.8691
11.0	354	0.8761

Apparently, compound 4d showed better UV absorption performance in the alkaline system than that in the acid condition. These results demonstrated that compound 4a-d showed excellent stability in the weak alkaline system. However, in the acidic system, the UV absorption peak of compound 4d shifted blue and the absorption intensity decreased. When the structure of the organic compound or the medium of the solution changes, the wavelength of the maximum absorption peak of the organic compound will move to a short wavelength and this phenomenon is called blue shift. The blue shift phenomenon can also be attributed to the influence of substituents or solvents, such as the change of solvent polarity, acid–base property and conjugation, can cause the change of UV spectrum. We believed that in an acidic medium, the nitrogen atom on the morpholine ring in compound 4d was protonated, which weakened the conjugation between the nitrogen atom and stilbene-pyrimidine matrix, resulting in blue shift of the UV absorption peak. At the same time, in the acidic medium, compound 4d was precipitated from the soluble sulfonate to the less soluble sulfonic acid, resulting in a decrease in the concentration and a decrease in the absorption strength. The possible structural transformation of compound 4d in an acid–base medium is shown in Figure 3.

Figure 3.

Possible structural changes of compound 4d in an acid–base medium.

Whiteness test

The whiteness values and CIE L* a* b* coordinates were measured using a SF600 data color, illuminant D65 and 10° observer. Each treated cotton fiber was measured three times at different locations on the fabric surfaces and the average value of whiteness values and CIE L* a* b* coordinates were recorded. The obtained data are presented in Table 2. Compared with the untreated samples, compounds 4a-d showed a significant whitening effect on cotton fiber at five different concentrations. As shown in Table 2, compounds 4a-d imparted the highest degree of whiteness at 0.1% (o.w.f.). The abilities of their whitening effect on cotton fiber are 4d > 4a > 4b > 4c at the critical concentration (0.1%). Among them, compound 4d turned out to be the most active compound as a FWA with imparting a higher degree of whiteness (CIE Whiteness 135.04) compared to untreated cotton fiber with the application of the critical concentration.

Table 2.

CIE Whiteness and L* a* b* coordinates for cotton fiber treated with compounds 4a-d

	o.w.f. (%)	Whiteness	L*	a*	b*
Compounds	Untreated	73.02	94.03	–0.33	2.69
4a	0.05	123.86	96.31	–1.27	–7.41
	0.1	131.97	96.92	–1.97	–9.02
	0.15	127.74	96.71	–1.68	–8.54
	0.2	128.22	96.93	–2.66	–8.08
	0.3	127.06	96.67	–1.66	–8.14
4b	0.05	105.50	95.41	–0.61	–3.75
	0.1	124.82	96.47	–1.76	–7.54
	0.15	103.15	96.48	–3.49	–2.16
	0.2	90.06	96.85	–5.99	0.41
	0.3	80.90	96.84	–7.23	2.42
4c	0.05	107.42	96.44	–3.03	–3.66
	0.1	113.71	96.68	–2.9	–4.94
	0.15	101.01	96.54	–4.74	–2.18
	0.2	94.97	96.23	–4.07	–1.0
	0.3	88.70	96.67	–6.14	0.61
4d	0.05	126.73	96.53	–1.35	–7.48
	0.1	135.04	97.47	–1.99	–9.88
	0.15	132.47	97.18	–1.80	–9.29
	0.2	129.94	96.96	–1.84	–8.56
	0.3%	127.52	96.24	1.73	–8.07

o.w.f. (%): percent concentration of the fluorescent whitening agents (FWAs) on the total weight of fiber; L*: the lightness value; a*: the red/green value; b*: the yellow/blue value.

Fastness evaluation

Fastness is one of the important indexes to evaluate FWAs. Excellent FWAs should not only have good whitening and brightening effects, but also have good fastness performance. We used the ISO 105-B02 standard to evaluate light fastness and used ISO105-X12 standards to evaluate friction fastness. As shown in Table 3, in comparison with C186 (Figure 4), compounds 4a-d showed higher light fastness, wet rubbing fastness and dry rubbing fastness, which indicated that novel 4,4'-bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d showed excellent dyeing properties.

Table 3.

Fastness data of the treated cotton fiber with compounds 4a-d

Compounds	Light fastness	Wet rubbing fastness	Dry rubbing fastness
C186	2	2	3–4
4a	3	3–4	3–4
4b	2–3	3–4	4–5
4c	2–3	3–4	3–4
4d	3	3–4	4–5

Figure 4.

The structure of C186.

Reflectivity

The reflectivity of the treated fabric depends on the intensity of UV light, the number of FWAs adsorbed on the fabric and the quantum efficiency of fluorescence of the FWAs. Generally, when the intensity of the UV light is the same, the reflectivity gradually increases with increasing dye concentration. Different FWAs at the same concentration usually show different reflectivity, which may be related to the dye structure and the fluorescence quantum efficiency of the FWAs. When the concentration of the FWAs on the fabric reaches the critical concentration, the reflectivity reaches its maximum. When the dye on the fabric exceeds this concentration, the reflectivity will decrease. As shown in Figure 5, the cotton fiber treated with compounds 4a-d had obvious radiation in the visible light region and the reflectivities of the treated cotton fiber were 119.78%, 117.69%, 110.76% and 120.76% at the critical concentration (0.1%), while the reflectivity of untreated cotton fiber was no more than 88%.

Figure 5.

Reflectivity curves of cotton fiber before and after treatment with compounds 4a-d.

Conclusion

In summary, we designed and synthesized some novel 4,4'-bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d. These compounds were evaluated by measuring the degree of whiteness, color index and fastness to interrogate their potential as FWAs. Our results suggested that novel 4,4'-bis(2,4-pyrimidinyl)-diamino stilbene-2,2'-disulfonic acid derivatives 4a-d showed an efficient whitening effect on cotton fiber and higher rubbing fastness and light fastness than those of C186. By testing UV absorption under various pH values, it is found that compound 4d showed better UV absorption properties in the alkaline system than in the acid medium. The reflectivity of the treated cotton fiber with compound 4a-d at the critical concentration was up to 120.76%. Compound 4d turned out to be the most effective molecule as FWAs with imparting a high degree of whiteness.

Experimental details

Materials and methods

All drugs and reagents used in the synthesis of the target compounds were chemically pure and unrefined before use. All chemical reactions were determined by thin layer chromatography (TLC). Proton nuclear magnetic resonance (¹H NMR) data of the synthesized compounds were tested with a Bruker NMR spectrometer at 400 MHz and tetramethylsilane (TMS) was used as the internal standard. Elemental analysis was carried out on an Elementar elemental analyzer.

General procedure for the synthesis of compounds 4a-d

2,4-Dichloro-4,5-dihydropyrimidine (10 mmol) was added to a mixture of 4,4'-diaminostilbene-2,2'-disulfonic acid (5 mmol) and distilled water (100 mL) at 20–25℃. After the addition was completed, the reaction mixture was stirred for 10 h at 20–25℃ and the pH was maintained at 7–8 with 10% Na₂CO₃ solution. The disappearance of 2,4-dichloro-4,5-dihydropyrimidine was monitored by TLC. Then, the amino compound (10 mmol) was added to the reaction mixture and after the addition was completed, the reaction mixture was heated to 100℃ and stirred for 7 h, and the pH was maintained at 8–9 with 10% Na₂CO₃ solution. The disappearance of the amino compound was monitored by TLC. The reaction mixture was cooled and sodium chloride (15 g) was added and stirred for 30 min. The product was precipitated out and filtered by vacuum, and the precipitate was washed with distilled water (10 mL each time) three times and then washed with acetone three times (10 mL each time). The products were dried under vacuum overnight at 50℃ to obtain compounds 4a-d as light yellow solids.

Synthesis of sodium 6,6'-(ethene-1,2-diyl)bis(3-((2-(phenylamino)pyrimidin-4-yl)amino)benzenesulfonate) (4a)

Yield 89%; melting point > 250℃; maximum UV absorption 352 nm; ¹H NMR (400 MHz; DMSO-d₆, δ): 5.87 (d, 2H, pyrimidine ring), 7.47 (d, 4H, benzene ring), 7.55 (m, 6H, benzene ring), 7.57 (s, 2H, −CH=CH−), 7.77 (d, 2H, benzene ring), 7.94 (d, 2H, benzene ring), 8.01 (s, 2H, benzene ring), 8.12 (d, 2H, pyrimidine ring), 9.71 (s, 2H, −NH), 10.65 (s, 2H, −NH); anal. calcd for C₃₄H₂₆N₈Na₂O₆S₂: C 54.25, H 3.48, N 14.89, S 8.52; found: C 54.21, H 3.53, N 14.92, S 8.57.

Synthesis of sodium 6,6'-(ethene-1,2-diyl)bis(3-((2-(p-tolylamino)pyrimidin-4-yl)amino)benzenesulfonate) (4b)

Yield 82%; melting point > 250℃; maximum UV absorption 352 nm; ¹H NMR (400 MHz; DMSO-d₆, δ): 3.13 (s, 6H, −CH₃), 6.01 (d, 2H, pyrimidine ring), 7.56 (d, 4H, benzene ring), 7.86 (s, 2H, −CH=CH−), 7.88 (d, 4H, benzene ring), 7.89 (d, 2H, benzene ring), 7.99 (d, 2H, benzene ring), 8.13 (s, 2H, benzene ring), 8.14 (d, 2H, pyrimidine ring), 9.39 (s, 2H, −NH), 10.64 (s, 2H, −NH); anal. calcd for C₃₆H₃₀N₈Na₂O₆S₂: C 55.38, H 3.87, N 14.35, S 8.21; found: C 55.34, H 3.89, N 14.41, S 8.23.

Synthesis of sodium 4,4'-((((ethene-1,2-diylbis(3-sulfonato-4,1-phenylene))bis(azanediyl))bis(pyrimidine-4,2-diyl))bis(azanediyl))dibenzoate (4c)

Yield 87%; melting point > 250℃; maximum UV absorption 352 nm; ¹H NMR (400 MHz; DMSO-d₆, δ): 5.91 (d, 2H, pyrimidine ring), 6.80 (d, 2H, benzene ring), 7.57 (d, 2H, benzene ring), 7.64 (d, 4H, benzene ring), 7.86 (s, 2H, benzene ring), 7.94 (s, 2H, −CH=CH−), 8.03 (d, 4H, benzene ring), 8.20 (d, 2H, pyrimidine ring), 9.80 (s, 2H, −NH), 10.71 (s, 2H, −NH); anal. calcd for C₃₆H₂₄N₈Na₄O₁₀S₂: C 48.87, H 2.73, N 12.67, S 7.25; found: C 48.80, H 2.81, N 12.62, S 7.19.

Synthesis of sodium 6,6'-(ethene-1,2-diyl)bis(3-((2-morpholinopyrimidin-4-yl)amino)benzenesulfonate) (4d)

Yield 91%; melting point > 250℃; maximum UV absorption 354 nm; ¹H NMR (400 MHz; DMSO-d₆, δ): 3.04 (t, 8H, morpholine ring), 3.80 (t, 8H, morpholine ring), 6.45 (d, 2H, pyrimidine ring), 7.66 (d, 2H, benzene ring), 7.86 (d, 2H, benzene ring), 7.88 (s, 2H, −CH=CH−), 8.07 (s, 2H, benzene ring), 8.45 (d, 2H, pyrimidine ring), 9.64 (s, 2H, −NH); anal. calcd for C₃₀H₃₀N₈Na₂O₈S₂: C 48.65, H 4.08, N 15.13, S 8.66; found: C 48.58, H 4.18, N 15.06, S 8.61.

Dyeing process

We treated cotton fiber without FWAs using five different concentrations (0.05%, 0.10%, 0.15%, 0.20% and 0.30%) of compounds 4a-d for 30 min. The ratio of the bath to cotton fiber was 50:1. The temperature of the bath was gradually increased from 20℃ to 50℃ and the heating rate was 2℃/min. The cotton fibers were then dyed at 50℃ for a further 15 min. The treated cotton fibers were removed, washed in cold water and dried at room temperature.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Basic Ability Improvement Project for Young and Middle-aged Teachers in Guangxi Colleges and Universities, China (Grant Number KY2016YB459), the Doctor's Scientific Research Foundation of Hezhou University, China (Grant Number HZUbs201511) and the Project of Shandong Province Higher Educational Science and Technology Program, China (Grant Number J17KA101).

References

Lee

Kang

et al.

The synthesis and properties of triazine-stilbene fluorescent brighteners containing the phenolic antioxidant [II]. Dyes Pigm 2005; 64: 681–686.

Tutak

. Optical whitening of cationised cotton: effect on whiteness and whiteness tint. Color Technol 2011; 127: 340–345.

Wan

Zhou

Jiao

et al.

Synthesis, physical properties and cytotoxicity of stilbene-triazine derivatives containing amino acid groups as fluorescent whitening agents. J Fluores 2013; 23: 1099–1105.

Hussain

Shamey

Hinks

et al.

Synthesis of novel stilbene-alkoxysilane fluorescent brighteners, and their performance on cotton fiber as fluorescent brightening and ultraviolet absorbing agents. Dyes Pigm 2012; 92: 1231–1240.

Georgiev

Bojinov

Marinova

. Novel PAMAM light-harvesting antennae based on 1,8-naphthalimide: Synthesis, energy transfer, photophysical and pH sensing properties. Sens Actuat B Chem 2010; 150: 655–666.

Bojinov

Konstantinova

. Fluorescent 4-(2,2,6,6-tetramethylpiperidin-4-ylamino)-1,8-naphthalimide pH chemosensor based on photoinduced electron transfer. Sens Actuat B Chem 2007; 123: 869–876.

Tian

Gan

Chen

et al.

Positive and negative fluorescent imaging induced by naphthalimide polymers. J Mater Chem 2002; 12: 1262–1267.

Zhu

Chen

et al.

Luminescent properties of copolymeric dyad compounds containing 1,8-naphthalimide and 1,3,4-oxadiazole. Synth Metal 1998; 96: 151–154.

Zhang

Dai

. Synthesis and characterization of a novel waterborne stilbene-based polyurethane fluorescent brightener. Chin Chem Lett 2011; 22: 997–1000.

10.

Tian

Yan

et al.

A luminescent ultrathin film with reversible sensing toward pressure. Chem Commun 2016; 52: 4663–4666.

11.

Modi

Naik

et al.

Synthesis of 4,4'-bis[(4-morpholino-6-arylureedo-s-traizin-2-yl)amino] stilbene-2,2 -disulphonic acid derivatives and their use as fluorescent brig. Dyes Pigm 1993; 23: 65–72.

12.

Wang

Zhang

et al.

A study on the synthesis and photophysical performances of some pyrazole and triazole fluorescent brightening agents. Dyes Pigm 2005; 64: 141–146.

13.

Rangnekar

Rajadhyaksha

. Synthesis of phthalimid-3-yl and -4-yl aminoethylenes and pyrroloquinolines and a study of their fluorescence properties. Dyes Pigm 1987; 8: 1–10.

14.

Rangnekar

Phadhe

. Synthesis of 2-hetaryl-5-phenyl-1,3,4-oxadiazole and bis-1,3,4-oxadiazole derivatives and their use as fluorescent whiteners for polyester fibres. Dyes Pigm 1985; 6: 293–302.

15.

. The synthesis and properties of benzoxazole fluorescent brighteners for application to polyester fibers. Dyes Pigm 2007; 75: 185–188.

16.

Ismail Kuthati

Thalari

et al.

Synthesis of novel spiro[pyrazolo[4,3-d]pyrimidinones and spiro[benzo[4,5]thieno[2,3-d]pyrimidine-2,3′-indoline]-2′,4(3H)-diones and their evaluation for anticancer activity. Bioorg Med Chem Lett 2017; 27: 1446–1450.

17.

Liu

Feng

Wang

et al.

Synthesis and in vitro evaluation of pyrimidine-fused 3-alkenyloxindoles as potential anticancer agents. Tetrahedr Lett 2016; 57: 4113–4118.

18.

Gregoric

Sedic

Grbcic

et al.

Novel pyrimidine-2,4-dione–1,2,3-triazole and furo[2,3-d]pyrimidine-2-one–1,2,3-triazole hybrids as potential anti-cancer agents: synthesis, computational and X-ray analysis and biological evaluation. Eur J Med Chem 2017; 125: 1247–1267.

19.

Suresh

Kumar

PSV

Chandramouli

GVP

. An efficient one-pot synthesis, characterization and antibacterial activity of novel chromeno-pyrimidine derivatives. J Mol Struct 2017; 1134: 51–58.

20.

Suresh

Kumar

PSV

Poornachandra

et al.

An efficient one-pot synthesis of thiochromeno[3,4-d]pyrimidines derivatives: Inducing ROS dependent antibacterial and anti-biofilm activities. Bioorg Chem 2016; 68: 159–165.

21.

Quiroga

Romo

Ortiz

et al.

Synthesis, structures, electrochemical studies and antioxidant activity of 5-aryl-4-oxo-3,4,5,8-tetrahydropyrido[2,3-d]pyrimidine-7-carboxylic acids. J Mol Struct 2016; 1120: 294–301.

22.

Kaur

Machado

De Kock

et al.

Primaquine–pyrimidine hybrids: synthesis and dual-stage antiplasmodial activity. Eur J Med Chem 2015; 101: 266–273.