Development of new epoxy resin monomers â A delicate balance

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Development of new epoxy resin monomers – A delicate balance between skin allergy and polymerization properties David J. Ponting, Miguel A. Ortega, Ida B. Niklasson, Isabella Karlsson, Tina Seifert, Johanna Steen, Kristina Luthman, and Ann-Therese Karlberg Chem. Res. Toxicol., Just Accepted Manuscript • DOI: 10.1021/acs.chemrestox.8b00169 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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Chemical Research in Toxicology

Development of new epoxy resin monomers – A delicate balance between skin allergy and polymerization properties

David J. Ponting,† Miguel A. Ortega,† Ida B. Niklasson,† Isabella Karlsson,†§ Tina Seifert,‡ Johanna Stéen,† Kristina Luthman,‡ Ann-Therese Karlberg†,* †Department

of Chemistry and Molecular Biology, Dermatochemistry, University of Gothenburg, SE-412 96 Gothenburg, Sweden

‡Department

of Chemistry and Molecular Biology, Medicinal Chemistry, University of Gothenburg, SE-412 96 Gothenburg, Sweden

§Present

address: Department of Environmental Science and Analytical Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden * Corresponding author. E-mail: [email protected]

Keywords: contact allergy, DFT calculations, DGEBA, epoxy resins, epoxy resin monomers, in vivo, LLNA, occupational contact dermatitis, sensitizing potency, skin, synthesis

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Chemical Research in Toxicology 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table of Contents Graphic

Remove aromaticity and ether oxygen

Considerably reduced sensitizing potency but unsatisfactory technical properties

Replace isopropyl group and remove ether oxygen

No reduction in sensitizing potency


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ABSTRACT: Epoxy resin monomers (ERMs) are used as building blocks for thermosetting polymers in applications where strong, flexible, and light-weight materials are required. Most epoxy resins are polymers of diglycidyl ether of bisphenol A (DGEBA). It is highly allergenic and causes occupational allergic contact dermatitis and contact allergy in the general population. Thus, measures to prevent exposure by protective clothing and education are not enough. The work describes a continuation of our research aiming at reducing the skin sensitizing potency of ERMs while maintaining the ability to form polymers. Alternative ERMs were designed and synthesized whereafter the sensitizing potency was determined using the murine local lymph node assay (LLNA). The reactivity of the diepoxides towards a nucleophilic peptide was investigated and the differences in reactivity explained using computational studies. The diepoxides were reacted with triethylenetetramine and the formed polymers were tested for technical applicability using thermogravimetric analysis. We had previously shown that the absence of an oxygen atom in the side chains or removal of aromaticity reduced the sensitizing potency compared to that of DGEBA. Thus, a cycloaliphatic analogue 1 of DGEBA without ether oxygen in the side chains was considered promising and was synthesized. As predicted the sensitizing potency was considerably reduced (10 times) compared to that of DGEBA. However, the technical properties of the polymer of this compound were not considered sufficient. More polar aromatic analogues were investigated, but they could not compete with our previously described ERMs regarding polymerization properties and with 1 regarding low skin sensitization properties. Development of alternative epoxy materials is a delicate balance between allergenic activity and polymerization properties. Tuning of structural properties together with investigation of polymerization conditions combined with skin sensitization studies should be used in industrial research and development. ERM 1 could be used as a lead compound for further studies of aliphatic ERMs.


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INTRODUCTION The focus of our work is to reduce the skin-sensitizing potency and the cause of allergic contact dermatitis (ACD) from epoxy resin systems (ERSs). Measures taken to prevent from occupational exposure to ERSs involving education, medical examination and voluntary agreements between employers and workers are not effective enough to protect from skin sensitization.1-3 ERSs are multi-component systems comprised of epoxy resin monomers (ERMs), reactive diluents, hardeners and modifiers. ERMs are polymer precursor units which are reacted with hardeners to give thermostable and insoluble polymers for applications where strong, flexible, and light-weight materials are required.4 ERMs are among the most common causative agents of occupational ACD and also a common cause of contact allergy in the general population. Of all epoxy resins 75% - 90% are polymers of diglycidyl ethers based on bisphenol A (DGEBA or BADGE) while 1% are polymers of diglycidyl ethers based on bisphenol F (DGEBF) (Figure 1). Most cases of ACD are attributed to contact with DGEBA and DGEBF as they are highly allergenic. DGEBA can sensitize upon first contact.5 The polymers are non-sensitizing since the large molecules cannot penetrate into epidermis as easily as the monomers. However, non-cured monomers are always present in the commercial resins and therefore sensitization occurs not only to the monomers before curing but also from contact with the cured resins. DGEBA epoxy resins (MW of 350–400) are liquids with a relatively high viscosity, containing monomeric DGEBA up to more than 90%. Resins with an average MW > 900 are solids but may contain more than 15% DGEBA.6 It should be observed that also other ERS chemicals are skin sensitizers.7 Overall, the reported prevalence of cases of ACD attributable to epoxy chemicals in occupational settings is high (11.7 - 12.5%). A comprehensive review of ERS and contact allergy is given by Higgins et al.4 DGEBA is included in the European baseline series for standardized patch testing of dermatitis patients with suspected contact allergy. The reported prevalence of contact allergy to DGEBA among consecutive dermatitis patients ranges from 0.9 to 2.3%.1 The skin sensitization potential for ACS Paragon Plus Environment

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DGEBA, DGEBF and others has been investigated experimentally in vivo.8-10 DGEBA and DGEBF are equally sensitizing and classified as strong sensitizers in both mice and guinea pigs according to regulatory guidelines.11 It would be most advantageous to replace these highly sensitizing ERMs with less hazardous alternatives. It is a challenging task to reduce the adverse skin sensitizing effects of ERMs while maintaining their ability to form thermosetting polymers. Skin sensitization is caused by low-molecular weight chemicals that can pass the skin barrier but are too small to be recognized by the immune system. However, electrophilic compounds like DGEBA can react with nucleophilic moieties in endogenous skin proteins thereby forming immunogenic complexes. Thus, reducing the reactivity of a compound is a way of reducing its sensitizing potency, but for ERMs it is vital to keep a certain level of reactivity in order to enable polymerization. We have in previous work elucidated the most important sensitizing structural features of DGEBA and DGEBF.12-15 Our approach recognized that skin sensitization depends on the inherent reactivity of the terminal epoxy groups. As these groups are vital for the polymerization process, we designed compounds containing terminal epoxy groups with reduced reactivity by alteration of the total chemical structure of the ERMs (A-F in Figure 1).15 Each novel ERM contained structural modifications compared to DGEBA and DGEBF, and were designed to be capable of polymerization without excessive reactivity and skin sensitization. The alternative ERMs were synthesized, their peptide reactivity determined, the skin sensitizing potency assessed according to the murine local lymph node assay (LLNA),16 and the polymerization potential evaluated.15 It was demonstrated that the sensitizing potency was reduced with maintained polymerization properties when the length of the side chains was increased (A and D), alternatively when the aromaticity (B and E) or the oxygen atom (C and F) in the side chains were omitted (Figure 1). The ERMs C and F emerged as the best candidates for further development when combining the results from the investigations of skin sensitization and the technical properties.15 In clinical studies a potential cross-reactivity between DGEBA and analogues B and C (Figure 1) was investigated by patch testing with these three compounds in parallel in individuals with known contact allergy to DGEBA.17 The results clearly indicated that the two alternative compounds have ACS Paragon Plus Environment

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low (B) or no (C) cross-reactivity with DGEBA. The presence of the ether oxygen in the carbon chain (as in B) was shown to be more important than the aromaticity of the ring (as in C), for the cross-reactivity. The aim of the present study was to investigate the skin sensitizing and technical properties of compound 1 (Figure 2), the saturated analogue of C, which according to our previous results would be optimal from the point of skin sensitization. However, as the technical properties of 1 were found less satisfying we continued our investigations with chemicals more prone to polymerization. Thus, more polar aromatic compounds were synthesized (2-4, Figure 2) and their technical and sensitizing properties were investigated analogously to our previous work.


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EXPERIMENTAL PROCEDURES Caution: This study involves skin sensitizing compounds which should be handled with particular care. Instrumentation and Mode of Analysis. 1H and 13C NMR spectroscopy was performed on a Varian 400 MR spectrometer at 400 MHz and 100 MHz, respectively, using CDCl3 (residual CHCl3 δ 7.26 and δ 77.0 as internal standards) or DMSO-d6 solutions (residual (CH3)2SO δ 2.54 and δ 40.45 as internal standards). All NMR experiments were performed at ambient temperature. Electronionization mass spectrometry analysis (70 eV) was performed on a Hewlett-Packard 5973 mass spectrometer connected to a gas chromatograph (Hewlett-Packard 6890). The GC was equipped with a cool on-column capillary inlet and an HP-5MSi fused silica capillary column (30 m × 0.25 mm i.d., particle size 0.25 µm, Agilent Technologies, Palo Alto, CA). Helium was used as carrier gas, and the flow rate was 1.2 mL/min. The temperature program started at 70 °C for 1 min, increased by 10 °C/min for 20 min, and ended at 270 °C for 20 min. For mass spectral analysis, the mass spectrometer was used in scan mode detecting ions with m/z values from 70 to 700. High performance liquid chromatography/mass spectrometry (LC/MS) analyses were performed using electrospray ionization (ESI) on a Hewlett-Packard 1100 HPLC/MS. The system included a vacuum degasser, a binary pump, an autoinjector, a column thermostat, a diode array detector, and a single quadrupole mass spectrometer. The HPLC was equipped with a HyPURITY C18 column (150 × 3 mm i.d., particle size 3 µm, Thermo Hypersil-Keystone, Thermo Electron Corp., Bellafonte, PA, USA). Mobile phase A consisted of 0.005% pentafluoropropanoic acid, 0.1% acetic acid, and 5% acetonitrile in water, and mobile phase B consisted of 0.005% pentafluoropropanoic acid, 0.1% acetic acid, and 50% water in acetonitrile. The gradient conditions for separation of compounds were: 0 min 0% B, 3 min 8% B, 15 min 10% B, 18 min 100% B, 30 min 100% B. The column was equilibrated with 0% B for 10 min between each run. Aliquots of 3 µL were injected onto the column and eluted with a flow rate of 0.40 mL/min and a column temperature of 40 °C. The ESI interface was used in positive ionization mode with the following spray chamber settings: nebulizer ACS Paragon Plus Environment

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pressure, 35 psi; capillary voltage, 3000 V; drying gas temperature, 350 °C; and drying gas flow rate, 12 L/min. The fragmentor voltage was set to 120 V. The mass spectrometer was used in scan mode detecting molecular ions with m/z values ranging from 50 to 1800. The HPLC-ESI-HRMS system consisted of a Dionex UltiMate 3000 LC system interfaced to an Orbitrap Q Exactive HF mass spectrometer (Thermo Fisher Scientific, MA, USA). Mobile phase A consisted of 0.1% formic acid and 5% ACN in H2O, whereas mobile phase B consisted of 0.1% formic acid and 5% H2O in ACN. Chromatographic separation was performed using an Aquity UPLC HSS C18 column (2.1 x 100 mm, 1.8 µm) from Waters and a flow rate of 0.30 mL/min. The gradient started at 20% B for 0.5 min, followed by an increase to 100% B in 8 min, and finally the solvent composition was held at 100% B for 3.5 min before re-equilibration for 2.5 min. The MS was used in full scan mode detecting positive ions ranging from 50 to 750 m/z. The resolution was set to 120 000, the AGC target to 3106, and the maximum IT to 200 ms. The tune parameters were as follows: spray voltage, 4kV; capillary temperature, 275 C; sheath gas, 20 arbitrary units (au); auxiliary gas, 10 au; S-Lens RF level, 60%; probe heater temperature, 240 C. Chemistry. DGEBA was purchased from Sigma-Aldrich (Steinheim, Germany). Acetone was purchased from Merck (Darmstadt, Germany) and olive oil from Apoteket AB (Gothenburg, Sweden). Ac-Pro-His-Cys-Lys-Arg-Met-OH (AcPHCKRM, 98%) was obtained from Peptide 2.0 Inc. (Chantilly, Virginia, USA). All reactions were monitored by thin-layer chromatography (TLC) on silica plated aluminum sheets (Silica gel 60 F254, E. Merck). Spots were detected by UV light (254 or 365 nm), or developed with heating or anisaldehyde or potassium permanganate staining. All reactions were carried out using magnetic stirring under ambient atmosphere if not otherwise noted. All starting materials and reagents were obtained from commercial producers and were used without prior purification. THF and dichloromethane used in reactions with dry conditions were distilled from Na and CaH2, respectively. Microwave reactions were carried out using a Biotage Initiator Sixty in 10–20 mL capped microwave vials with fixed hold time, normal absorption and 10 sec prestirring. Purification by flash column chromatography was performed using Merck silica gel Geduran Si 60 (0.063-0.200 mm) or by using an automatic Biotage SP4 Flash+ instrument with ACS Paragon Plus Environment

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prefabricated columns (surface area 500 m2/g, porosity 60 Å, particle size 40–63 μm). The synthesis of compound C (Figure 1) was performed according to O’Boyle et al.15 4,4´-Bis(2,3-epoxypropyl)isopropylidenedicyclohexane (1) (Scheme 1). 4,4′-Bis(tertbutyldimethylsilyloxy)isopropylidenedicyclohexane (1a). Imidazole (1.94 g, 28.5 mmol) and TBDMSCl (4.30 g, 28.5 mmol) were added to an ice-cooled solution of the diol (2.86 g, 11.9 mmol) in dry DMF (25 mL). The ice bath was removed and the mixture was stirred overnight at room temperature. The reaction mixture was suspended in Et2O (400 mL) and the organic phase was washed with water (5x) and brine (2x), dried over MgSO4, filtered and concentrated to give 1a (5.49 g, 97%) as a white solid. The crude product (mixture of isomers, 2:1) showed sufficient purity and was used in the next step without further purification. 1H NMR (400 MHz, CDCl3) (major isomer) δ 3.47 (tt, J = 10.8, 4.4 Hz, 2H), 1.95 – 1.80 (m, 4H), 1.70 – 1.58 (m, 4H), 1.32 – 1.14 (m, 6H), 1.07 – 0.92 (m, 4H), 0.88 (s, 18H), 0.69 (s, 6H), 0.05 (s, 12H). 4,4'-Dibromo-isopropylidenedicyclohexane (1b). Compound 1a (2.78 g, 5.93 mmol, crude product) was dissolved in dry CH2Cl2 (30 mL) and the mixture was cooled to 0 °C. BBr3 (1 M in hexane, 13.1 mL, 13.1 mmol, 2.2 eq.) was added dropwise. The mixture was stirred for 30 min at 0 °C before it was allowed to reach room temperature and stir for 16 h. The intense red colored mixture was cooled to 0 °C and quenched by a slow addition of saturated aqueous NaHCO3 (25 mL). The organic phase was washed with water (1x), brine (2x), dried over Na2SO4, filtered and concentrated. The crude product was purified with automated flash column chromatography using pentane as eluent to give 1b (1.88 g, 87%) as a colorless oil that crystalizes over time. 1H NMR (400 MHz, CDCl3) (mixture of isomers) δ 4.85-3.84 (m, 2H), 2.45-0.88 (m, 18H), 0.76, 0.732, 0.726, 0.70 (4s, 6H). 4,4' Diallyl-isopropylidenedicyclohexane (1c). Allylmagnesium bromide (1 M in Et2O, 6.7 mL, 6.74 mmol) was added dropwise to a 0.25 M solution of 1b (617 mg, 1.68 mmol) and AgNO3 (29 mg, 0.16 mmol) in dry Et2O (7 mL) at room temperature under N2 atmosphere. The mixture was stirred for 5 h before it was partitioned between aqueous NH4Cl (sat.) and pentane. The combined organic phases were washed with water and brine, dried over MgSO4, filtered and concentrated under reduced pressure. The crude product was purified by flash column chromatography using pentane as ACS Paragon Plus Environment

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eluent to afford 1c (403 mg, 83%) as a colorless oil. 1H NMR (CDCl3) (major isomer) δ 5.87–5.69 (m, 2H), 5.06–4.91 (m, 4H), 2.10 (tt, J = 7.3, 1.2 Hz, 2H), 1.94 (tt, J = 6.9, 1.3 Hz, 2H), 1.86–1.72 (m, 3H), 1.72–1.61 (m, 4H), 1.50–1.35 (m, 3H), 1.33–1.07 (m, 6H), 1.05–0.81 (m, 4H), 0.75–0.65 (m, 6H). 4,4´-Bis(2,3-epoxypropyl)isopropylidenedicyclohexane (1). 3-Chloroperbenzoic acid (503 mg, 2.91 mmol) was added in one portion to a 0.1 M solution of 1c (210 mg, 0.73 mmol) in dry CH2Cl2 (5 mL) at 0 °C. The solution was stirred under N2 atmosphere overnight, while reaching room temperature. The mixture was treated with 1 M NaOH and diluted with CH2Cl2. The phases were separated and the aqueous phase was extracted with CH2Cl2. The combined organic phases were washed with water and brine, dried over MgSO4, filtered and concentrated under reduced pressure. The crude product was purified by flash column chromatography using EtOAc in pentane (0 → 5%) as eluent to afford 1 (216 mg, 93%) as a yellow oil. 1H NMR (CDCl3) (major isomer) δ 2.91 (m, 2H), 2.74 (td, J = 5.3, 3.9 Hz, 2H), 2.43 (ddd, J = 16.2, 5.1, 2.7 Hz, 2H), 2.00–1.92 (m, 1H), 1.90– 1.77 (m, 2H), 1.76–1.63 (m, 4H), 1.60–1.53 (m, 3H), 1.53–1.33 (m, 6H), 1.30–1.19 (m, 3H), 1.18– 1.04 (m, 2H), 1.03–0.90 (m, 3H), 0.70 (t, J = 1.4 Hz, 6H). 13C NMR (CDCl3) (major isomer) δ 52.1, 51.2, 47.42, 47.35, 43.9, 40.4, 37.0, 36.4, 34.4, 34.2, 33.9, 31.5, 30.9, 30.6, 26.8, 26.7, 21.24, 21.19, 20.7, 20.6. EI-MS (70 eV), m/z (%) 55 (36), 67 (49), 81 (62), 95 (48), 107 (54), 121 (35), 135 (12), 137 (12), 163 (100), 180 (20), 181 (23). ESI-MS m/z 321.1 [M+H]+ (100). Bis[4-(2,3-epoxypropyl)phenyl]methanol (2) (Scheme 2). 4,4´-Diallylbenzophenone (2a). A mixture of 4,4´-dibromobenzophenone (462 mg, 1.36 mmol), tetrakis(triphenylphosphine)palladium (314 mg, 0.272 mmol) and allyltributylstannane (1.35 mL, 4.35 mmol) in dry DMF (6 mL) was heated at 160 °C under microwave irradiation for 45 min. The reaction was monitored by TLC (pentane/ethyl acetate 95:5). The reaction mixture was filtered through a pad of Celite® using ethyl acetate. To the filtrate, a solution of KF (aq. sat.) (75 mL) was added and the two-phase system was stirred for 1 h. After filtration the organic layer was separated, washed with brine and dried over Na2SO4. The solvent was evaporated to give an ochre oily residue which was purified by flash column chromatography with ethyl acetate in pentane (3%) to give 2a (282 mg, 79%) as an oil. 1H ACS Paragon Plus Environment

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NMR (CDCl3) δ 7.76–7.73 (m, 4H), 7.32–7.28 (m, 4H), 6.04–5.94 (m, 2H), 5.16–5.14 (m, 2H), 5.13–5.10 (m, 2H), 3.48 (dt, J = 6.9, 1.5 Hz, 4H). Bis(4-allylphenyl)methanol (2b). Compound 2a (100 mg, 0.38 mmol) was dissolved in methanol (6 mL) and sodium borohydride (72 mg, 1.9 mmol) was added in portions while the mixture was cooled in an ice bath. After complete addition, the resulting mixture was stirred at room temperature until the reaction was completed (around 2 h). The reaction was monitored by TLC (toluene). The solvent was evaporated and the residue was partitioned between water and ethyl acetate. The organic phase was separated, dried over Na2SO4 and concentrated under reduced pressure to give 2b (88 mg, 87%) as an oil. 1H NMR (DMSO-d6) δ 7.28–7.23 (m, 4H), 7.12–7.07 (m, 4H), 5.90 (ddt, J = 16.8, 10.0, 6.8 Hz, 2H), 5.78 (d, J = 4.0 Hz, 1H), 5.62 (d, J = 4.1 Hz, 1H), 5.07–4.98 (m, 4H), 3.30 (dt, J = 6.9, 1.4 Hz, 4H). Bis[4-(2,3-epoxypropyl)phenyl]methanol (2). Compound 2b (698 mg, 2.64 mmol) was dissolved in chloroform (20 mL). 3-Chloroperbenzoic acid (911 mg, 5.28 mmol) was added and the mixture was stirred at room temperature for 90 min. Additional 3-chloroperbenzoic acid (911 mg, 5.28 mmol) was added and the mixture was stirred at room temperature overnight. The reaction was monitored by TLC (5% methanol in CH2Cl2). Aqueous NaOH (10% w/v) was added and the mixture was stirred for 10 min. CH2Cl2 was added and the organic phase was separated, washed with brine, dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by flash column chromatography with first ethyl acetate in hexane (0 → 27% ) to elute impurities, thereafter the eluent was changed to CH2Cl2 to elute the desired compound affording 2 (416 mg, 56%) as a colorless oil. 1H NMR (CDCl3) δ 7.33 (d, J = 8.1 Hz, 4H), 7.22 (d, J = 8.2 Hz, 4H), 5.82 (d, J = 3.5 Hz, 1H), 3.13 (tdd, J = 5.5, 3.9, 2.6 Hz, 2H), 2.92–2.76 (m, 6H), 2.54 (dd, J = 5.0, 2.7 Hz, 2H), 2.21 (d, J = 3.5 Hz, 1H). 13C NMR (CDCl3) δ 142.4, 136.7, 129.3, 126.8, 76.0, 52.5, 47.0, 38.6. EI-MS (70 eV), m/z (%) 103 (12), 105 (25), 117 (11), 118 (24), 131 (15), 161 (100), 239 (8), 265 (8), 296 (8) (M+). HRMS (Orbitrap ESI) calculated for C19H21O3+ 297.14907, found 297.14835.


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4,4´-Bis(2,3-Epoxypropyl)benzophenone (3) (Scheme 2). MnO2 (321 mg, 3.69 mmol) was added to a solution of 2 (73 mg, 0.246 mmol) in CH2Cl2 (6 mL) under N2 atmosphere. The resulting mixture was stirred at room temperature overnight. The reaction was monitored by TLC (2% methanol in CH2Cl2). The mixture was filtered through a pad of Celite® and the solvent was evaporated to give 3 (59 mg, 82%) as a pale yellow oil. 1H NMR (CDCl3) δ 7.78–7.74 (m, 4H), 7.40–7.36 (m, 4H), 3.20 (dddd, J = 6.2, 4.9, 3.9, 2.6 Hz, 2H), 3.01 – 2.90 (m, 4H), 2.84 (dd, J = 4.9, 3.9 Hz, 2H), 2.58 (dd, J = 4.9, 2.6 Hz, 2H). 13C NMR δ 196.1, 142.2, 136.3, 130.6, 129.1, 52.1, 46.9, 38.9. EI-MS (70 eV), m/z (%) 103 (15), 131 (13), 161 (100), 180 (9), 223 (10), 251 (10), 263 (10), 294 (52) (M+). HRMS (Orbitrap ESI) calculated for C19H19O3+ 295.13342, found 295.13259. 4,4´-Bis(2,3-epoxypropyl)diphenyl ether (4) (Scheme 3) 4,4´-Bis(trifluorosulfonyloxy)diphenyl ether (4a). 4,4´-Dihydroxydiphenyl ether (500 mg, 2.47 mmol) and CH2Cl2 (12 mL) were added to a round-bottomed flask containing pyridine (0.79 mL, 9.89 mmol) under N2 atmosphere. Trifluoromethanesulfonic anhydride (0.98 mL, 5.93 mmol) dissolved in CH2Cl2 (3 mL) was added dropwise at 0 °C, and the resulting mixture stirred overnight, while reaching room temperature. Additional trifluoromethanesulfonic anhydride (0.41 mL, 2.47 mmol) was added and the mixture was stirred overnight under N2 atmosphere. The reaction was monitored by TLC (5% methanol in CH2Cl2). The reaction mixture was diluted with Et2O (9 mL) and treated with 10% aqueous HCl (5 mL). The phases were separated and the organic phase was washed with sat. NaHCO3 and brine, dried over Na2SO4, filtered and concentrated under reduced pressure. The crude product was purified by flash column chromatography using CH2Cl2 as eluent to afford 4a (1.069 g, 93%) as a pale yellow oil. 1H NMR (CDCl3) δ 7.30–7.27 (m, 4H), 7.10–7.05 (m, 4H). 4,4´-Diallyldiphenyl ether (4b) A mixture of 4a (500 mg, 1.072 mmol), tetrakis(triphenylphosphine)palladium (248 mg, 0.214 mmol) and allyltributylstannane (1.06 mL, 3.43 mmol) in dry DMF (9 mL) was heated at 160 °C under microwave irradiation for 45 min. The reaction was monitored by TLC (10% ethyl acetate in hexane). The reaction mixture was filtered through a pad of Celite® using ethyl acetate. To the filtrate, a solution of KF (aq. sat.) (100 mL) was added and the mixture was stirred for 1 h. After filtration the organic layer was separated, washed ACS Paragon Plus Environment

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with water (×5) and brine, dried over Na2SO4 and concentrated to a brown slurry. The crude product was purified by flash column chromatography using pentane as eluent to afford 4b (110 mg, 41%) as a colorless oil. 1H NMR (CDCl3) δ 7.17–7.11 (m, 4H), 6.95–6.91 (m, 4H), 6.03–5.92 (m, 2H), 5.12– 5.05 (m, 4H), 3.37 (dt, J = 6.7, 1.5 Hz, 4H). 4,4´-Bis(2,3-epoxypropyl)diphenyl ether (4). Compound 4b (463 mg, 1.85 mmol) was dissolved in chloroform (30 mL). 3-Chloroperbenzoic acid (638 mg, 3.7 mmol) was added and the mixture was stirred at room temperature for 2 h. The reaction was monitored by TLC (5% ethyl acetate in pentane). Additional 3-chloroperbenzoic acid (0.638 g, 3.7 mmol) was added and the reaction mixture was stirred at room temperature for 72 h. NaOH (10% w/v) was added and the two-phase system was stirred for 10 min before addition of CH2Cl2. The organic layer was separated and washed with brine, dried over Na2SO4 and concentrated under reduced pressure. The crude product was purified by flash column chromatography using CH2Cl2 as eluent to afford 4 (299 mg, 57%) as a yellow oil. 1H NMR (CDCl3) δ 7.23–7.18 (m, 4H), 6.98–6.93 (m, 4H), 3.15 (tdd, J = 5.5, 3.9, 2.7 Hz, 2H), 2.91–2.79 (m, 7H), 2.55 (dd, J = 5.0, 2.7 Hz, 2H). 13C NMR δ 156.2, 132.1, 130.4, 119.0, 52.6, 47.0, 38.1. EI-MS (70 eV), m/z (%) 77 (17), 165 (7), 209 (12), 239 (100), 253 (7), 282 (78) (M+). HRMS (Orbitrap ESI) calculated for C18H19O3+ 283.13342, found 283.13271. Experimental Animals. Female CBA/Ca mice, 7 to 9 weeks of age, were purchased from NOVA SCB Charles River, Germany. The mice were housed in “hepa” filtered air flow cages and kept on standard laboratory diet and water ad lib. The regional ethics committee, Jordbruksverket, approved the protocol and the procedure was performed in accordance with the guidelines. Skin Sensitizing Potency of ERMs 1, 2 and 4 in Mice. The LLNA16 was used to assess sensitizing potency of compounds 1, 2 and 4 according to previous experience.15 The purity of all test compounds was >98% (GC/MS) before evaluation in the biological assays. Detailed information regarding the experimental procedure is given in Supporting Information Table S1. Peptide Depletion with AcPHCKRM. All solvents were degassed with argon prior to use. Solutions of DGEBA in dimethyl sulfoxide (DMSO) (40 mM, 75 µL) together with potassium ACS Paragon Plus Environment

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phosphate buffer (100 mM, pH 7.4) (150 µL) were added to a vial purged with argon containing AcPHCKRM freshly prepared in DMSO (4 mM, 75 µL). Accordingly, final concentrations of DGEBA and the model peptide in the reaction mixture were 10 mM and 1 mM, respectively. The reaction was kept under argon at room temperature and was monitored with ESI-MS every 40 min for 24 h. Compounds 1 – 4 (Figure 2) and C (Figure 1) were treated identically to DGEBA and their reactions were investigated using the method described above. The stability of the peptide AcPHCKRM, under the experimental conditions used has previously been verified and no degradation was observed within 24 h.18 However, in the present study some dimerization was observed (2-10%) in the presence of a hapten, but this slow reaction was out-competed by peptide modification. The stability of DGEBA, 1 – 4 and C was explored as described18 to exclude any risk of hydrolysis of the epoxides, but no degradation was observed for any of the derivatives within 24 h under the conditions of the reactivity experiment. Polymerization Procedure. Polymerization reactions between DGEBA and compounds 1-4 with triethylenetetramine (TETA) were performed according to methods previously described.15 Detailed information on the polymerization procedure and the thermogravimetric analysis (TGA) is given in Supporting Information. Computational Techniques. All calculations were, unless otherwise indicated, carried out at the B3LYP-D3/6-31+G** 19-23 level of theory in Jaguar (Schrodinger LLC, N. Y. (2009) Jaguar, version 7.6). Implicit solvation was modelled using the Poisson Boltzmann finite (PBF) system; however in most cases at least one explicit water molecule was also included. Transition states were deduced initially using the linear synchronous transit (LST) method, however those that proved less easy to find were discovered using a variety of techniques including temporarily constraining the locations of reactive species, and variation of the initial guess. Initial preparation of structures was carried out using MacroModel (Schrodinger LLC, N. Y. (2009) Jaguar, version 9.7) and Maestro (Schrodinger LLC, N. Y. (2009) Jaguar, version 9.0). Calculations were performed on a mixture of the C3SE cluster (SNIC facility located at Chalmers University of Technology, Gothenburg) and standalone workstations running CentOS 6.6. These symmetrical molecules were in all cases truncated by ACS Paragon Plus Environment

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reducing the aromatic ring further from the site of interest to a simple methyl group, except in the case of the ketone, where the aromatic ring was truncated to a vinyl group to allow for the modelling of cross-conjugative effects.


15


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RESULTS Chemical Synthesis. Four novel DGEBA analogues (1-4, Figure 2) were successfully synthesized in an attempt to obtain ERMs with reduced skin sensitization potential. Compound 1 was synthesized according to Scheme 1. Commercially available 4,4′isopropylidenedicyclohexanol (mixture of isomers) was converted to the corresponding bis-tertbutyldimethylsilyl ether 1a and further to the dibromide 1b using BBr3 in CH2Cl2. A convenient silver(I)-mediated coupling24 with allylmagnesium bromide afforded 1c in 71% yield. The bisepoxidation using 3-chloroperbenzoic acid gave 1 (93% yield). Compound 2 was synthesized in three steps (Scheme 2) from commercially available 4,4´dibromobenzophenone which was reacted with allyltributylstannane in a Stille reaction to afford the bis-allyl intermediate 2a in 79% yield. The ketone was reduced to alcohol 2b in good yield (87%) using NaBH4. Epoxidation using 3-chloroperbenzoic acid afforded bis-epoxide 2 (56% yield). Synthesis of 3 (Scheme 2) was achieved by oxidation of 2 using MnO2 in high yield (82%). Compound 4 was prepared following a three-step procedure (Scheme 3). Commercially available 4,4´-dihydroxydiphenyl ether was reacted with trifluoromethanesulfonic anhydride in the presence of pyridine to afford bis-triflate 4a in a yield of 93%. A Stille reaction of 4a with allyltributylstannane gave the corresponding bis-allyl intermediate 4b in a lower yield (41%) compared to using the same conditions to afford 2a. Epoxidation with 3-chloroperbenzoic acid afforded bis-epoxide 4 (57% yield). It should be noted that due to the asymmetric centre in the oxirane rings, final diepoxides may be obtained as mixtures of isomers. For instance, in the 1H NMR spectrum of 1 signals from more than one isomer were observed. This should not have significant effects on the sensitizing potency or mechanical properties, since the epoxide moiety remains sterically unhindered in all cases. Skin Sensitizing Potency. The results in the LLNA are expressed as EC3 values, which is the estimated concentration of a compound required to induce a 3-fold increase in sensitizing potency ACS Paragon Plus Environment

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compared to a control. Compounds with lower EC3 values are more sensitizing.16 The DGEBA analogue 1 gave EC3 0.38 M whereas ERMs 2 and 4 gave EC3 0.027 M and 0.054 M, respectively (Figure 3, and Table S1 in Supporting Information). As 2 can be oxidized to 3 by metabolism in vivo we assumed that 3 would also show a strong sensitizing potency. Compound 3 was therefore not studied in the LLNA of ethical reasons. Peptide Reactivity. The reactivity towards AcPHCKRM of 1-4 and C was investigated and compared with that of DGEBF tested previously,14, 15 which can be assumed to be equivalent to that of DGEBA. All analogues except 1 gave complete depletion of the peptide after 24 h. Valuable information was obtained from the initial rate of reaction, which varied significantly (Figure 4, Table 1). The order is 3>DGEBF>2>4>C>1. This is in agreement with the results from the sensitization study. Cysteine-adducts were formed, as confirmed by the observation of characteristic peptide fragments. Fragments corresponding to b2, b3*, b4*, b5*, y2, y3 and y4* in the reactivity experiment with ketone 3 are shown in Supporting Information (Figures S1 and S2). Polymers from the ERMs. Epoxy resins were prepared from the ERMs with the commonly used curing agent triethylenetetramine (TETA) as co-reactant. The thermal properties of the new epoxy resins were assessed using TGA and the results were compared to epoxy resin based on DGEBA, prepared using the same procedure. TGA thermograms of the investigated epoxy resins are shown in Figure 5. Thermal stability and degradation data of the epoxy resins (i.e. IDT, Tmax, Rmax and Ea,) are given in Table 2. Initial decomposition temperature (IDT) indicates the apparent thermal stability of the epoxy resin, i.e. the failure temperatures of the resin in processing and molding, and is determined from the onset of weight loss of the sample in the TGA thermogram. The temperature at maximum rate of weight loss (Tmax) and the maximum weight loss rate (Rmax) were taken from the peak values of the differential thermograms. The activation energy (Ea) for the decomposition of the cured epoxy resins was calculated from the TGA thermogram using the Horwitz–Metzger equation.25 DGEBA epoxy resin was used as control.


17


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As shown in Table 2 and Figure 5, IDT values for the polymers formed from 1-4 are lower than that of the polymer formed from DGEBA, indicating lower thermal stability. The polymers formed from 1 and 4 have higher IDTs than those of 2 and 3, but polymers from 1 and 4 showed an earlier onset of degradation (alcohol>CH>vinyl ketone, following the decreasing electron density of the ring. Despite the fact that cross-conjugation has the greatest effect on 3, even a cross-conjugated species is nonetheless the most rapid at depleting the peptide (Figure 4 and Table 1). In addition to epoxide ring opening, the fast depletion rate for 3 might also be due to a rapid imine formation by reaction of the lysine residue in the peptide and the carbonyl group of 3 (∆Gǂ = 17 kJ mol-1) via the transition state shown in Figure S3G in the Supporting Information. This imine can then be decomposed by the excess water present, releasing the peptide again, or deliver the epoxide side chains to the cysteine residue. Secondly, in C and molecules 2-4, the water bridge previously discussed mediates the direct interaction between the C-O σ* orbital of the epoxide and the π-system orbitals of the aromatic ring. The interaction is of varying strength dependent on the relative electron density of the ring. The substituent on the ring (2 –CH(OH)CH3; 3 –(C=O)CH3; 4 –OMe) will contribute to the electron density of the aromatic ring and thereby to the change in strength of the O-H-ring bridging interaction. With 3, the ring orbitals are delocalized into the ketone, reducing the electron density in the aromatic ring and thus reducing the strength of the hydrogen bond, but the lack of direct interaction previously discussed predominates. The direct conjugation of the ether oxygen in 4 also allows stronger conjugation than that to the alcohol in 2, in which the oxygen is one atom further away. This means that the reaction with 4 is somewhat more favored than that with 2 (ΔG‡ of 43 as


22

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opposed to 51 kJ mol-1) when modeled, as a single-sided molecule (Figure 7), since the ring is electron-rich and can form a stronger hydrogen bond with the bridging water. However, it should be noted that analogously to the cross-conjugation effect observed for 3, the reaction with 4 is expected to be less favored than indicated by the single-sided model, as that strong interaction must be shared between the two rings and hence weakened. The observed in vivo reactivity of 2 could also be dependent on the easy oxidation to the more reactive ketone (3). Compound 1 is less reactive than any of the other compounds discussed, including C. This is due to the saturated ring not presenting any opportunity for the bridging water molecule to be kept in the necessary position, resulting in a strengthening of the targeted C-O bond, raising its σ* orbital further compared to the energy of the nucleophilic orbital.

CONCLUSIONS The present study shows that development of alternative epoxy materials is a delicate balance between allergenic activity and polymerization properties. Measures to prevent exposure by protective clothing and education are not enough to reduce the high incidence of occupational ACD caused by ERMs. In the future, skin sensitization should always be of concern in the development of new commercial ERMs. Based on our research we propose that tuning of structural properties together with thorough investigation of polymerization conditions combined with studies on skin sensitization potential should be used in industrial research and development. ERM 1 in the present study could be used as a lead compound for further studies of aliphatic ERMs for specific applications while ERM C might be an alternative candidate to DGEBA.


23


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CONFLICT OF INTEREST DISCLOSURE The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Funding Financial support for this project was obtained from The Swedish Research Council for Health, Working Life and Welfare (Forte) and Sweden’s Innovation Agency Vinnova. The work was performed within the Centre for Skin Research at the University of Gothenburg.

ACKNOWLEDGEMENTS We thank Krister Holmberg for valuable discussions and Susanne Exing, Gabriella Wendt and Anders Eliasson for assistance with the LLNA experiments. We also thank Dr. David Bliman for assistance with spectroscopic analyses.

SUPPORTING INFORMATION AVAILABLE Experimental details on the polymerization procedure and thermogravimetric analysis. Complete LLNA information, fragment assignments from the MS-analysis after peptide depletion experiments and an MS spectrum of the peptide adduct with ketone 3. Nature of the transition states of model epoxide ring-opening reactions on DGEBA and C, showing the need for a water molecule bridge, and transition states for the reactions described in Figure 7. NMR spectra of compounds 1a, 1b, 1c and 1. This material is available free of charge via the Internet at http://pubs.acs.org.

ABBREVIATIONS ACD, allergic contact dermatitis; m-CPBA, 3-chloroperbenzoic acid; DFT, density functional theory; DGEBA, diglycidyl ether of bisphenol A; DGEBF, diglycidyl ether of bisphenol F; DMF, ACS Paragon Plus Environment

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dimethylformamide; DMSO, dimethyl sulfoxide; ERM, epoxy resin monomer; ERS, epoxy resin systems; LLNA, local lymph node assay; SI, stimulation index; TCA, trichloroacetic acid; TETA, triethylenetetramine; TGA, thermogravimetric analysis.

REFERENCES

(1)

(2) (3)

(4) (5) (6) (7) (8)

(9) (10) (11)

(12) (13)

Bangsgaard, N., Thyssen, J. P., Menne, T., Andersen, K. E., Mortz, C. G., Paulsen, E., Sommerlund, M., Veien, N. K., Laurberg, G., Kaaber, K., Thormann, J., Andersen, B. L., Danielsen, A., Avnstorp, C., Kristensen, B., Kristensen, O., Vissing, S., Nielsen, N. H., and Johansen, J. D. (2012) Contact allergy to epoxy resin: risk occupations and consequences. Contact dermatitis 67, 73-77. Anveden Berglind, I., Lind, M. L., and Liden, C. (2012) Epoxy pipe relining - an emerging contact allergy risk for workers. Contact dermatitis 67, 59-65. Geier, J., Krautheim, A., Uter, W., Lessmann, H., and Schnuch, A. (2011) Occupational contact allergy in the building trade in Germany: influence of preventive measures and changing exposure. International archives of occupational and environmental health 84, 403-411. Higgins, C., Cahill, J., Jolanki, R., and R., N. (2018) Epoxy Resins, In Kanerva's Occupational Dermatology (John, S., Johansen, J., Rustemeyer, T., Elsner, P., and Maibach, H., Eds.), Springer, Cham. Kanerva, L., Tarvainen, K., Pinola, A., Leino, T., Granlund, H., Estlander, T., Jolanki, R., and Forstrom, L. (1994) A single accidental exposure may result in a chemical burn, primary sensitization and allergic contact dermatitis. Contact dermatitis 31, 229-235. Henricks-Eckerman, M.-L., and Laijoki, T. (1986) Glycidyl ethers in epoxy resin products. Työterveyslaitoksen tutkimuksia 4, 41-46. Aalto-Korte, K., Kuuliala, O., Henriks-Eckerman, M. L., and Suuronen, K. (2015) Contact allergy to reactive diluents and related aliphatic epoxy resins. Contact dermatitis 72, 387397. Delaine, T., Niklasson, I. B., Emter, R., Luthman, K., Karlberg, A. T., and Natsch, A. (2011) Structure--activity relationship between the in vivo skin sensitizing potency of analogues of phenyl glycidyl ether and the induction of Nrf2-dependent luciferase activity in the KeratinoSens in vitro assay. Chemical research in toxicology 24, 1312-1318. Ponten, A., Zimerson, E., Sorensen, O., and Bruze, M. (2002) Sensitizing capacity and crossreaction pattern of the isomers of diglycidyl ether of bisphenol F in the guinea pig. Contact dermatitis 47, 293-298. Thorgeirsson, A., Fregert, S., and Ramnas, O. (1978) Sensitization capacity of epoxy resin oligomers in the guinea pig. Acta dermato-venereologica 58, 17-21. Basketter, D. A., Andersen, K. E., Liden, C., Van Loveren, H., Boman, A., Kimber, I., Alanko, K., and Berggren, E. (2005) Evaluation of the skin sensitizing potency of chemicals by using the existing methods and considerations of relevance for elicitation. Contact dermatitis 52, 39-43. Niklasson, I. B., Broo, K., Jonsson, C., Luthman, K., and Karlberg, A. T. (2009) Reduced sensitizing capacity of epoxy resin systems: a structure-activity relationship study. Chemical research in toxicology 22, 1787-1794. Niklasson, I. B., Delaine, T., Luthman, K., and Karlberg, A. T. (2011) Impact of a heteroatom in a structure-activity relationship study on analogues of phenyl glycidyl ether (PGE) from epoxy resin systems. Chemical research in toxicology 24, 542-548.


25


(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)

(27) (28) (29) (30) (31)

Page 26 of 39

O'Boyle, N. M., Delaine, T., Luthman, K., Natsch, A., and Karlberg, A. T. (2012) Analogues of the epoxy resin monomer diglycidyl ether of bisphenol F: effects on contact allergenic potency and cytotoxicity. Chemical research in toxicology 25, 2469-2478. O'Boyle, N. M., Niklasson, I. B., Tehrani-Bagha, A. R., Delaine, T., Holmberg, K., Luthman, K., and Karlberg, A. T. (2014) Epoxy resin monomers with reduced skin sensitizing potency. Chemical research in toxicology 27, 1002-1010. Gerberick, G. F., Ryan, C. A., Dearman, R. J., and Kimber, I. (2007) Local lymph node assay (LLNA) for detection of sensitization capacity of chemicals. Methods 41, 54-60. Hagvall, L., Niklasson, I. B., Rudback, J., O'Boyle, N. M., Niklasson, E., Luthman, K., and Karlberg, A. T. (2016) Assessment of cross-reactivity of new less sensitizing epoxy resin monomers in epoxy resin-allergic individuals. Contact dermatitis 75, 144-150. Delaine, T., Hagvall, L., Rudback, J., Luthman, K., and Karlberg, A. T. (2013) Skin sensitization of epoxyaldehydes: importance of conjugation. Chemical research in toxicology 26, 674-684. Ditchfield, R., Hehre, W. J., and Pople, J. A. (1971) Self-Consistent Molecular-Orbital Methods .9. Extended Gaussian-Type Basis for Molecular-Orbital Studies of Organic Molecules. J Chem Phys 54, 724-+. Becke, A. D. (1988) Density-Functional Exchange-Energy Approximation with Correct Asymptotic-Behavior. Phys Rev A 38, 3098-3100. Becke, A. D. (1993) Density-Functional Thermochemistry .3. The Role of Exact Exchange. J Chem Phys 98, 5648-5652. Stephens, P. J., Devlin, F. J., Chabalowski, C. F., and Frisch, M. J. (1994) Ab-Initio Calculation of Vibrational Absorption and Circular-Dichroism Spectra Using DensityFunctional Force-Fields. J Phys Chem-Us 98, 11623-11627. Grimme, S., Antony, J., Ehrlich, S., and Krieg, H. (2010) A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements HPu. J Chem Phys 132. Someya, H., Ohmiya, H., Yorimitsu, H., and Oshima, K. (2008) Silver-catalyzed benzylation and allylation reactions of tertiary and secondary alkyl halides with Grignard reagents. Organic Letters 10, 969-971. Wu, C. S., Liu, Y. L., Chiu, Y. C., and Chiu, Y. S. (2002) Thermal stability of epoxy resins containing flame retardant components: an evaluation with thermogravimetric analysis. Polym Degrad Stabil 78, 41-48. Coman, G., Zinsmeister, C., and Norris, P. (2015) Occupational Contact Dermatitis: Workers' Compensation Patch Test Results of Portland, Oregon, 2005-2014. Dermatitis : contact, atopic, occupational, drug : official journal of the American Contact Dermatitis Society, North American Contact Dermatitis Group 26, 276-283. Ponten, A., Carstensen, O., Rasmussen, K., Gruvberger, B., Isaksson, M., and Bruze, M. (2004) Epoxy-based production of wind turbine rotor blades: occupational dermatoses. Contact dermatitis 50, 329-338. Creytens, K., Gilissen, L., Huygens, S., and Goossens, A. (2017) A new application for epoxy resins resulting in occupational allergic contact dermatitis: the three-dimensional printing industry. Contact dermatitis 77, 349-351. Jackh, C., Fabian, E., van Ravenzwaay, B., and Landsiedel, R. (2012) Relevance of xenobiotic enzymes in human skin in vitro models to activate pro-sensitizers. J Immunotoxicol 9, 426-438. English, J. S., Foulds, I., White, I. R., and Rycroft, R. J. (1986) Allergic contact sensitization to the glycidyl ester of hexahydrophthalic acid in a cutting oil. Contact dermatitis 15, 66-68. Kanerva, L., Jolanki, R., and Estlander, T. (1991) Allergic contact dermatitis from nondiglycidyl-ether-of-bisphenol-A epoxy resins. Contact dermatitis 24, 293-300.


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TABLES Table 1 A comparison of peptide reactivity data (% of AcPHCKRM remaining after 40 min), LLNA EC3 values (M), calculated reactivity (ΔG‡ and ΔrG, kJ mol-1, B3LYP-D3/6-31+G**, and reaction of truncated models with MeS- in the presence of one water molecule) Compound

% AcPHCKRM

EC3 /M

ΔG‡ /kJ mol-1

ΔrG /kJ mol-1

DGEBA

26

0.036a

46

-91

C

60

0.091b

55

-75

1

76

0.38

66

-78

2

36

0.027

51

-70

34c

-73c

61e

-71e

17f

-14f

43

-83

3

4 aFrom bEC 3

n.t.d

11

45

0.054

reference15

for C similar to EC3 for F15

cModelled

as PhC(=O)Me end unit (Figure 7)

dCompound eModelled

3 was not tested of ethical reasons as 2 can be metabolically oxidized to 3 in vivo.

as PhC(=O)C=C end unit (Figure 7)

fReaction

with MeNH2 to form imine on central ketone group; ΔG‡ for initial rate-determining step (Figure 7).


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Table 2 Thermal Stability and Degradation Data of the ERMs from Thermogravimetric Analysis under Nitrogen Atmosphere

aIDT=

Test compound

IDTa (°C)

Tmaxb (°C)

Rmaxc (% / °C)

Ead (kJ/mol)

DGEBAe

358

408

-2.17

168

Cf

376

404

-0.90

110

1

326

357/442g

-0.69/-0.73

60.4/81.2

2

307

371

-0.71

72.4

3

298

338

-0.78

96.3

4

340

372

-1.01

88.6

Initial decomposition temperature which was determined with the temperature of onset of the weight loss of the

sample bT

max= Temperature at maximum rate of weight loss which was taken as the peak value of the differential thermogravimetric thermograms cR

max=

dE

a=

Maximum weight loss rate or the slope of weight loss at Tmax

Activation energy for the decomposition of the cured epoxy resins

eData

calculated as a mean value of experiments performed in triplicate. The new data are in agreement with data from our previous investigation.15 fData gFor

taken from our previous investigation15

1 the curve shows two steps, therefore two values are given


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FIGURE LEGENDS

Figure 1. Structures of DGEBA, DGEBF and previously synthesized epoxy resin monomers (A-F) with decreased sensitizing potency but maintained technical polymerization properties.15 Figure 2. Structures of the synthesized analogues 1-4 Figure 3. a) Results from the LLNA for alternative ERMs compared to those from DGEBA and DGEBF obtained in our previous study.15 b) Enlarged portion showing concentrations from 0 to 0.02 M. Dose-response curves for DGEBA (), DGEBF (), 1 (▲), 2 (+), and 4 (●). SI=stimulation index. EC3 values; DGEBA: 0.036 M; DGEBF: 0.036 M; 1: 0.38 M; 2: 0.027 M; and 4: 0.054 M. LLNA of 3 was never performed due to ethical considerations. Figure 4. Depletion curves at 6 h of DGEBA (), C (), 1 (▲), 2 (+), 3 (♦), and 4 (●), with AcPHCKRM (10:1 excess of hapten, DMSO/phosphate buffer 1:1) Figure 5. Thermogravimetric thermograms showing % weight loss at increasing temperatures of epoxy resins based on different ERMs under N2 atmosphere. DGEBA (blue line), C (black line), 1 (orange line), 2 (red line), 3 (green line), and 4 (purple line). The curve for C was taken from O’Boyle et al.15 Figure 6. Schematic representations of the hydrogen-bonding networks discussed. a) Hydrogen bonding observed in DGEBA; b) Hydrogen bonding in DGEBA with more than one water molecule; c) Hydrogen bonding observed in C and 2-4. Figure 7. Reactivity parameters (∆Gǂ and ∆Gr, free energy changes of activation and reaction, respectively) for nucleophilic attack of a methanethiolate nucleophile on the epoxides of truncated models of the compounds under investigation. Reactivity parameters for the attack of methylamine on the carbonyl carbon of 3 to form the corresponding imine are also shown.


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Figure 1.


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Figure 2. OH O

O

O

O

1

2

O O O

O 3

O

O 4


31


Figure 3.

a) 70 60 50

DGEBA

SI

40

DGEBF

30

1

20

2

10

4

0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 Concentration (M)

b)

SI

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

40 35 30 25 20 15 10 5 0

DGEBA DGEBF 1 2 4

0

0.05

0.1

0.15

0.2

Concentration (M)


32

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Figure 4.


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Figure 5.


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Figure 6.

H a

H O

O

b

H O

O H

O Nu-

H O H O

c

H

O

H

O R

Nu-

Nu-


35


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Figure 7. O O

O

H2O, MeS-

O

G‡ = 46 rG = -91

O

H2O, MeS-

O

O

O

O

O

SMe OH O SMe

O O

G‡ = 34 rG = -73 H2O, MeS-

O

O

G‡ = 51 rG = -70

H2O, MeSO

SMe O

G‡ = 43 rG = -83 H2O, MeS-

OH

O

G‡ = 55 rG = -75 H2O, MeS-

O

SMe

SMe

O O

G‡ = 61 rG = -71 H2O, MeNH2

SMe

N

G‡ = 17 rG = -13

O

H2O, MeSG‡ = 66 rG = -78

O SMe


36

Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1. Synthesis of Compound 1a

i HO

OH

X

X 1a X = OTBDMS 1b X = Br ii

O

O 1

aReagents

iii 1c

and conditions: (i) a: tert-Butyldimethylsilyl chloride, imidazole, DMF, room temp.,

overnight; b: BBr3, CH2Cl2, 0 °C → room temp., 16 h; (ii) AgNO3, allylmagnesium bromide, Et2O, room temp., 5 h; (iii) m-CPBA, CH2Cl2, 0 °C → room temp., 16 h.


37


Page 38 of 39

Scheme 2. Synthesis of Compounds 2 and 3a O

O i

Br

Br

2a

ii

OH

OH

iii O

O 2

2b iv

O O

O 3

aReagents

and conditions: (i) Tetrakis(triphenylphosphine)palladium, allyltributylstannane, dry

DMF, 160 °C, microwave irradiation, 45 min; (ii) NaBH4, MeOH, 0 °C to room temp., 2h; (iii) mCPBA, CHCl3, room temp, overnight; (iv) MnO2, CH2Cl2, room temp, overnight.


38

Page 39 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 3. Synthesis of Compound 4a O HO

O

i OH

TfO

OTf

4a ii

O O

O 4

aReagents

iii

O

4b

and conditions: (i) Trifluoromethanesulfonic anhydride, pyridine, CH2Cl2, 0 °C to room

temp, 48h; (ii) tetrakis(triphenylphosphine)palladium, allyltributylstannane, dry DMF, 160 °C, microwave irradiation, 45 min; (iii) m-CPBA, CHCl3, room temp., 72h.


39

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