Introduction

Pain is the most frequent cause of patient’s recourse for medical assistance. The pain syndrome is the main reason for a decrease in the quality-of-life of patients with the majority of diseases and dominates in the clinical picture, while being the major symptom of the disease. Traditional analgesic and anti-inflammatory agents, such as opiates, steroids, and non-steroidal anti-inflammatory drugs all have serious side effects, particularly at high doses taken by chronic users (Melnikova, 2010; Kinsey et al., 2011). The development of highly effective, low-toxic analgesics thus remains a challenge in pharmacology and medicinal chemistry, stimulating studies in this direction (Kankala et al., 2013; Deep et al., 2012; Nigade et al., 2012; Ingale et al., 2012; Tolstikova et al., 2012).

Recently, we showed that several chiral heterocyclic compounds that formed in reactions of verbenol epoxide 1 with aromatic aldehydes containing three methoxy groups at different positions of the aromatic ring had a considerable analgesic activity (Mikhalchenko et al., 2013). Importantly, compounds of different structural types showed activity, due to which it seems promising to perform systematic studies of biological properties for each type of framework.

Earlier, it was shown that epoxide 1 reacted with aldehydes 2a–d containing various substituents in the para-position of the aromatic ring in the presence of montmorillonite clay K10, giving compounds 3a–d with a benzodioxin framework (Scheme 1) (Il’ina et al., 2010). These compounds that formed as a single stereoisomer are promising for studies of their biological activity including the analgesic activity.

Scheme 1
scheme 1

The interaction of verbenol epoxide 1 with aldehydes 2a–d

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The goal of this study was to synthesize heterocyclic compounds of Type 3 with a benzodioxin framework with various substituents in the aromatic ring and study their analgesic activity.

Chemistry

To synthesized new compounds with a benzodioxin framework, we performed the reaction of verbenol epoxide 1 with aromatic aldehydes 2e–h in the presence of clay K10 (Scheme 2). The choice of substituents (halogen atoms and CF3 group) in aldehydes was dictated by the necessity of studying the structure–activity relationship and based on our knowledge of the effect of the nature of substituent in the aromatic ring on the structure of the intermolecular products formed in reactions with verbenol epoxide 1. For example, it was shown that the introduction of oxygen-containing substituents in the aromatic ring of aldehyde led to the formation of compounds with a hexahydro-2H-chromene but not benzodioxin type of framework (Mikhalchenko et al., 2013; Il’ina et al., 2007, 2011).

Scheme 2
scheme 2

The interaction of verbenol epoxide 1 with aldehydes 2e–h

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The reactions were performed in dichloromethane at room temperature. The individual products were isolated by column chromatography on silica gel. The yields of intermolecular products are listed in Table 1. In addition to the products of the reactions of 1 with aldehydes, compounds 4–6, which are the products of the isomerization of epoxide 1, formed in all instances.

Table 1 Yields of products in the reaction of (–)-cis-verbenol epoxide 1 with aldehydes 2e–h
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Earlier, it was shown that the main intermolecular products formed in the reaction of verbenol epoxide 1 with aromatic aldehydes 2a–c containing a halogen atom in the para-position in the presence of clay K10 were products 3a–c with a benzodioxin framework (Il’ina et al., 2010). The displacement of Cl and F atoms from para- to meta-position led to compounds with a hexahydro-2H-chromene framework as the main reaction products (7e, g). Compounds 3e, g were obtained in minor quantities. When dichlorobenzaldehyde 2f and 4-(trifluoromethyl)benzaldehyde 2h were used, compounds 3 and 7 formed in a comparable ratio. The highest yield of intermolecular products was observed in the reaction with dichlorobenzaldehyde 2f (59 %). This is a good yield for reactions with such a labile compound as verbenol epoxide 1.

Compounds 7 formed as a mixture of diastereomers at the C-5 carbon atom (Scheme 2) in comparable amounts.

Using aldehydes 2e and 2g containing a chlorine and fluorine atom, respectively, at the meta-position led to the formation of small amounts of compounds 8e and 8g, which are the products of the addition of two aldehyde molecules to monoterpenoid 1, and to compound 9 evidently formed as a result of dehydration in 7g (Fig. 1).

Fig. 1
figure 1

Structures of compounds 8e, g, and 9

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Thus we obtained the desired set of chiral heterocyclic compounds 3a–h with a benzodioxin framework, which allowed us to pass to studies of their biological activity.

Biology

The analgesic activity of the compounds in a dose of 10.0 mg/kg (oral administration) was studied in the standard experimental pain models, namely the acetic acid-induced writhing (0.75 % acetic acid, 0.1 ml for one animal, intraperitoneally) and hot plate (thermal stimulation, T = 54 ± 0.5 °C) tests (Koster et al., 1959; Eddy and Leimbach, 1953). Sodium diclofenac in the same dose was used as a reference drug.

According to the data of Table 2, among para-halogen-substituted compounds 3a–c, compound 3b containing a chlorine atom showed the highest analgesic activity in a dose of 10 mg/kg in the acetic acid-induced writhing test. In this test, compound 3b was almost equal to the sodium diclofenac reference. A displacement of the chlorine atom to the meta-position on passing to compound 3e led to considerable decrease in the analgesic effect, and dichloro derivative 3f did not show any reliable degree of analgesic activity at all. At the same time, the position of the fluorine atom (3a and 3g) did not affect the analgesic activity of these compounds which are both effective. Using bromo derivative 3c did not reveal any reliable analgesic effect. According to (Tolstikova et al., 2011), LD50 exceeds 1,000 mg/kg for 3a and is 350 mg/kg for sodium diclofenac (Syubaev et al., 1986).

Table 2 Analgesic activity of compounds 3a–h, 7e and sodium diclofenac in the acetic acid-induced writhing test (10 mg/kg dose)
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Compound 3d containing a nitro group showed a pronounced analgesic effect, while its analog 3h with a CF3 group was slightly less effective.

For 3b, which showed the highest analgesic activity in the acetic acid-induced writhing test, ED50 (the ‘‘median effective dose’’ is a dose that produces 50 % protection) was determined to be 4.5 mg/kg. For comparison, ED50 was 5 mg/kg for sodium diclofenac and 155 mg/kg for aspirin in this test (Syubaev et al., 1986). Note that, an appreciable analgesic activity in compounds of this structural type was found for the first time.

In the hot plate test, reliable analgesic activity was found in compounds 3d and 3e in a dose of 10 mg/kg (Table 3). Compound 3d containing a nitro group exhibited more pronounced analgesic activity than the sodium diclofenac reference. It had a considerable analgesic activity in both the acetic acid-induced writhing and hot plate tests. Compound 3e showed less pronounced activity.

Table 3 Analgesic activity of compounds 3a–h and sodium diclofenac in the hot plate test (10 mg/kg dose)
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We also studied the analgesic activity of the compound 7e with a hexahydro-2H-chromene framework, which formed in the highest yield among compounds of this structural type. In the acetic acid-induced writhing test, compound 7e was equal to 3e having a benzodioxin framework in efficiency (Table 2), but proved inactive in the hot plate test (in contrast to 3e) (Table 3).

Conclusion

The reactions of verbenol epoxide 1 with aromatic aldehydes containing a halogen atom or a CF3 group in the presence of clay K10 gave a set of new chiral heterocyclic compounds 3, 7–9 with frameworks of different types. The structure and position of substituents in the aromatic ring produced a considerable effect on the yield and distribution of heterocyclic products.

For compounds 3e–h with a benzodioxin framework and previously prepared compounds 3a–d, the analgesic activity was studied on models in the acetic acid-induced writhing test and hot plate test.

The majority of the compounds under study exhibited pronounced analgesic activity in the acetic acid-induced writhing test in a dose of 10 mg/kg. Compound 3b containing a chlorine atom in the para-position of the aromatic ring showed the highest pain inhibition characteristics. The ED50 of this compound was 4.5 mg/kg.

In the hot plate test, 3d and 3e showed the analgesic activity; compound 3d was more effective that the sodium diclofenac taken in the same dose. Only compound 3d showed considerable analgesic activity in both tests.

Thus we have found analgesics 3b and 3d of a new structural type, which are very promising for further studies.

Experimental

Chemistry

General

All the chemicals reagents were of commercial grade. As catalyst, we used K10 clay (Fluka). The clay was calcinated at 110 °C for 3 h immediately before use. CH2Cl2 was passed through calcined alumina. (–)-cis-Verbenol epoxide (1) (\([\alpha ]_{580}^{20}\) = −60 (c = 0.41, CHCl3)) was synthesized according to (Il’ina et al., 2007) from (–)-verbenone (Aldrich), the content of the main substance was not less than 98.0 %. All product yields are given for pure compounds isolated by column chromatography. Column chromatography: silica gel (SiO2; 60–200 μ; Macherey–Nagel), 15 g per 1 g of reaction mixture; eluent hexane/EtOAc 100/0 → 0/100, acetone. GC/MS (purity control and products analysis): Agilent 7890A gas chromatograph equipped with a quadrupole mass spectrometer Agilent 5975C as a detector; quartz column HP-5MS (copolymer 5 %–diphenyl–95 %–dimethylsiloxane) of length 30 m, internal diameter 0.25 mm and stationary phase film thickness 0.25 μm was used for the analysis. Optical rotation: polAAr 3005 spectrometer, CHCl3 or MeOH soln., conc. g/100 ml. HR-MS: DFS-Thermo-Scientific spectrometer in a full scan mode (15–500 m/z, 70 eV electron-impact ionization, direct sample introduction). 1H- and 13C-NMR: Bruker DRX-500 apparatus at 500.13 MHz (1H) and 125.76 MHz (13C) in CDCl3 or CDCl3 + CD3OD (10:1, v/v); chemical shifts in ppm relative to residual CHCl3 ((H) 7.24, (C) 76.90 ppm), J in Hz; structure determinations by analyzing the 1H NMR spectra, including 1H–1H double resonance spectra and 1H–1H 2D homonuclear correlation, J-modulated 13C-NMR spectra (JMOD), and 13C–1H 2D heteronuclear correlation with one-bond and long-range spin–spin coupling constants (C–H COSY, 1 J(C,H) = 160 Hz, COLOC, 2,3 J(C,H) = 10 Hz).

General procedure for synthesis of (2S,4aR,8R,8aR)-2-(3-Aryl)-4,4,7-trimethyl-4a,5,8,8a-tetrahydro-4H-benzo[d][1,3]dioxin-8-ols

An appropriate aldehyde (4.2 μmol) was added to a suspension of clay K10 (4 g) in CH2Cl2 (25 ml). A solution of epoxide 1 (0.700 g, 4.2 μmol) in CH2Cl2 (10 ml) was added dropwise with stirring and the reaction mixture was stirred for 1.5 h at r.t. (20–25 °C). Then mixture of ethyl acetate (20 ml) and acetone (20 ml) was added. The catalyst was filtered off, the solvent was distilled off, and the residue was separated on a SiO2 column. The yields of the intermolecular products were calculated based on the consumed aldehyde.

With 3-chlorobenzaldehyde 2e

The following compounds were isolated: unreacted aldehyde 2e (0.270 g, conversion 54 %); intermolecular reaction products 3e (0.073 g, 11 %), 7e (S:R = 60:40) (0.253 g, 37 %) and 8e (0.260 g, 15 %); and isomerization products (1R,2R,6S)-3-methyl-6-(prop-1-en-2-yl)cyclohex-3-ene-1,2-diol 4 (0.129 g, 18 %), (S)-2-hydroxy-1-(2,2,3-trimethylcyclopent-3-enyl)ethanone 5 (0.066 g, 9 %), and (R)-2-((R)-2,2-dimethylcyclopent-3-enyl)-2-hydroxypropanal 6 (0.010 g, 1 %). The spectral data of 4, 5, and 6 coincided with those reported in the literature (Il’ina et al., 2007, 2012; Ardashov et al., 2007).

(2S,4aR,8R,8aR)-2-(3-Chlorophenyl)-4,4,7-trimethyl-4a,5,8,8a-tetrahydro-4H-benzo[d][1,3]dioxin-8-ol 3e

1H NMR (CDCl3): 1.26 (s, 3H, H-18); 1.51 (s, 3H, H-17); 1.53 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.0 Hz, J(6a,1e) = 2.0 Hz, 1H, Ha-6); 1.80 (ddd, J(19,7a) = 2.5 Hz, J(19,7e) = 1.5 Hz, J(19,8) = 1.5 Hz, 3H, H-19); 2.07 (dddq, J(7e,7a) = 17.8 Hz, J(7e,6a) = 6.0 Hz, J(7e,8) = 5.3 Hz, J(7e,19) = 1.5 Hz, 1H, He-7); 2.42 (dddqd, J(7a,7e) = 17.8 Hz, J(7a,6a) = 10.8 Hz, J(7a,8) = 2.5 Hz, J(7a,19) = 2.5 Hz, J(7a,10e) = 1.2 Hz, 1H, Ha-7); 3.88 (br.s, 1H, He-10); 4.34 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.0 Hz, 1H, He-1); 5.65 (dm, J(8,7e) = 5.3 Hz, another J ≤ 2.5 Hz, 1H, H-8); 5.75 (s, 1H, H-3); 7.23–7.28 (m, 2H, H-14, H-15); 7.31 (ddd, J(16,15) = 6.7 Hz, J(16,14) = 2.1 Hz, J(16,12) = 1.8 Hz, 1H, H-16); 7.45 (dd, J(12,14) = 2.1 Hz, J(12,16) = 1.8 Hz, 1H, H-12). 13C-NMR (CDCl3): 75.16 (d, C-1); 95.06 (d, C-3); 74.84 (s, C-5); 34.00 (d, C-6); 22.95 (t, C-7); 125.40 (d, C-8); 130.71 (s, C-9); 70.43 (d, C-10); 140.74 (s, C-11); 126.58 (d, C-12); 134.08 (s, C-13); 128.80 (d, C-14); 129.44 (d, C-15); 124.60 (d, C-16); 22.66 (q, C-17); 27.14 (q, C-18); 20.49 (q, C-19). \([\alpha ]_{D}^{25}\) = −115.0 (c = 0.67, MeOH); HR-MS: 308.1169 (M+, C17H21ClO3 +; calc. 308.1174).

(2S,4S,4aR,8R,8aR)-2-(3-Chlorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((S)-7e)

1H NMR (CDCl3): 1.51 (s, 3H, H-17); 1.67 (ddd, J(4e,4a) = 13.4 Hz, J(4e,3a) = 2.7 Hz, J(4e,6) = 1.1 Hz; 1H, He-4); 1.77–1.85 (m, 1H, Ha-6); 1.81 (m, all J ≤ 2.5 Hz, 3H, H-18); 1.83 (dd, J(4a,4e) = 13.4 Hz, J(4a,3a) = 12.0 Hz, 1H, Ha-4); 2.12–2.18 (m, 2H, H-7); 3.79 (dd, J(1e,10e) = 2.4 Hz, J(1e,6a) = 2.2 Hz, 1H, He-1); 3.90 (br.s, 1H, He-10); 4.39 (dd, J(3a,4a) = 12.0 Hz, J(3a,4e) = 2.7 Hz, 1H, Ha-3); 5.63-5.66 (m, 1H, H-8); 7.15 (ddd, J(16,15) = 6.7 Hz, J(16,14) = 2.2 Hz, J(16,12) = 1.8 Hz, 1H, H-16); 7.17–7.22 (m, 2H, H-14, H-15); 7.27–7.30 (m, 1H, H-12). 13C-NMR (CDCl3): 77.67 (d, C-1); 76.80 (d, C-3); 43.08 (t, C-4); 70.97 (s, C-5); 38.25 (d, C-6); 22.61 (t, C-7); 124.55 (d, C-8); 131.34 (s, C-9); 70.45 (d, C-10); 143.90 (s, C-11); 125.91 (d, C-12); 134.14 (s, C-13); 127.57 (d, C-14); 129.53 (d, C-15); 123.83 (d, C-16); 26.96 (q, C-17); 20.67 (q, C-18). HR-MS: 308.1171 (M+, C17H21ClO3 +; calc. 308.1174).

(2S,4R,4aR,8R,8aR)-2-(3-Chlorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((R)-7e)

1H NMR (CDCl3): 1.22 (s, 3H, H-17); 1.63 (ddd, J(4e,4a) = 14.2 Hz, J(4e,3a) = 3.5 Hz, J(4e,6) = 1.3 Hz, 1H, He-4); 1.66–1.72 (m, 2H, Ha-4, Ha-6); 1.81 (m, all J ≤ 2.5 Hz, 3H, H-18); 1.95–2.02 (m, 2H, H-7); 3.92 (br.s, 1H, He-10); 4.23 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 4.75 (dd, J(3a,4a) = 11.0 Hz, J((3a,4e) = 3.5 Hz, 1H, Ha-3); 5.56–5.60 (m, 1H, H-8); 7.15 (ddd, J(16,15) = 6.7 Hz, J(16,14) = 2.2 Hz, J(16,12) = 1.8 Hz, 1H, H-16); 7.17–7.24 (m, 2H, H-14, H-15); 7.27–7.30 (m, 1H, H-12). 13C-NMR (CDCl3): 75.16 (d, C-1); 75.09 (d, C-3); 42.00 (t, C-4); 70.71 (s, C-5); 37.99 (d, C-6); 24.48 (t, C-7); 123.94 (d, C-8); 131.80 (s, C-9); 70.45 (d, C-10); 144.65 (s, C-11); 125.85 (d, C-12); 134.09 (s, C-13); 127.31 (d, C-14); 129.45 (d, C-15); 123.88 (d, C-16); 28.27 (q, C-17); 20.77 (q, C-18).

(2R,4S,4aR,6S,7R,8aR,9S)-2,9-Bis(3-Chlorophenyl)-4,7-dimethylhexahydro-2H-4,6-(epoxymethano)chromen-8(5H)-one 8e

1H NMR (CDCl3): 1.12 (d, J(18,9) = 7.5 Hz, 3H, H-18); 1.48 (s, 3H, H-17); 1.95 (dd, J(4e,4a) = 14.9 Hz, J(4e,3a) = 2.6 Hz, 1H, He-4); 1.94–1.97 (m, all J ≤ 3.5 Hz, 1H, He-8); 2.19 (dd, J(4a,4e) = 14.9 Hz, J(4a,3a) = 13.2 Hz, 1H, Ha-4); 2.39–2.42 (m; 2H, H-7); 2.43 (br.q, J(9e,18) = 7.5 Hz, 1H, He-9); 2.57–2.61 (m, all J ≤ 5.2 Hz, 1H, He-6); 4.54 (d, J(1a,6e) = 5.2 Hz, 1H, Ha-1); 4.75 (dd, J(3a,4a) = 13.2 Hz, J(3a,4e) = 2.6 Hz, 1H, Ha-3); 4.79 (br.s, 1H, H-19); 7.15 (br.d, J(25,24) = 7.6 Hz, 1H, H-25); 7.18–7.28 (m, 4H, H-14, H-21, H-23, H-24); 7.29 (t, J(15,14(16)) = 7.6 Hz, 1H, H-15); 7.52 (ddd, J(16,15) = 7.6 Hz, J(16,14) ≈ 1.5 Hz, J(16,12) ≈ 1.5 Hz, 1H, H-16); 7.58 (dd, J(12,14) = 2.0 Hz, J(12,16) ≈ 1.5 Hz, 1H, H-12). 13C-NMR (CDCl3): 76.32 (d, C-1); 71.63 (d, C-3); 48.13 (t, C-4); 73.13 (s, C-5); 39.01 (d, C-6); 24.40 (t, C-7); 43.30 (d, C-8); 42.60 (d, C-9); 211.43 (s, C-10); 144.01 (s, C-11); 126.72 (d, C-12); 134.13 (s, C-13); 127.88 (d, C-14); 129.67 (d, C-15); 124.75 (d, C-16); 23.16 (q, C-17); 17.70 (q, C-18); 74.97 (d, C-19); 142.25 (s, C-20); 125.81 (d, C-21); 134.07 (s, C-22); 127.28 (d, C-23); 129.50 (d, C-24); 123.93 (d, C-25). \([\alpha ]_{D}^{25}\) = −56.7 (c = 0.53, MeOH); HR-MS: 430.1091 (M+, C24H24Cl2O3 +; calc. 430.1097).

With 3,4-dichlorobenzaldehyde 2f

The following compounds were isolated: unreacted aldehyde 2f (0.433 g, conversion 41 %); intermolecular reaction products 3f (0.169 g, 29 %) and 7f (S:R = 55:45) (0.180 g, 30 %); and isomerization products 4 (0.079 g, 11 %), 5 (0.124 g, 18 %), and 6 (0.049 g, 7 %).

(2S,4aR,8R,8aR)-2-(3,4-Dichlorophenyl)-4,4,7-trimethyl-4a,5,8,8a-tetrahydro-4H-benzo[d][1,3]dioxin-8-ol 3f

1H NMR (CDCl3): 1.25 (s, 3H, H-18); 1.50 (s, 3H, H-17); 1.52 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.2 Hz, J(6a,1e) = 2.1 Hz, 1H, Ha-6); 1.80 (br.s, 3H, H-19); 2.06 (dddq, J(7e,7a) = 17.7 Hz, J(7e,6a) = 6.2 Hz, J(7e,8) = 5.3 Hz, J(7e,19) = 1.5 Hz, 1H, He-7); 2.38 (dddqd, J(7a,7e) = 17.7 Hz, J(7a,6a) = 10.8 Hz, J(7a,8) = 2.5 Hz, J(7a,19) = 2.5 Hz, J(7a,10e) = 1.3 Hz, 1H, Ha-7); 3.86 (br.s, 1H, He-10); 4.32 (dd, J(1e,10e) = 2.4 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 5.63 (ddq, J(8,7e) = 5.3 Hz, J(8,7a) = 2.5 Hz, J(8,19) = 1.5 Hz, 1H, H-8); 5.72 (s, 1H, H-3); 7.26 (dd, J(16,15) = 8.2 Hz, J(16,12) = 1.8 Hz, 1H, H-16); 7.38 (d, J(15,16) = 8.2 Hz, 1H, H-15); 7.53 (d, J(12,16) = 1.8 Hz, 1H, H-12). 13C-NMR (CDCl3): 75.15 (d, C-1); 94.33 (d, C-3); 74.96 (s, C-5); 33.94 (d, C-6); 22.90 (t, C-7); 125.28 (d, C-8); 130.69 (s, C-9); 70.34 (d, C-10), 139.00 (s, C-11); 128.45 (d, C-12); 132.23 and 132.56 (2 s, C-13, C-14); 130.07 (d, C-15); 125.74 (d, C-16); 22.62 (q, C-17); 27.07 (q, C-18); 20.46 (q, C-19). \([\alpha ]_{D}^{25}\) = −101.5 (c = 0.40, MeOH); HR-MS: 342.0782 (M+, C17H20Cl2O3 +; calc. 342.0784).

(2S,4S,4aR,8R,8aR)-2-(3,4-Dichlorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((S)-7f)

1H NMR (CDCl3 + CD3OD): 1.51 (s, 3H, H-17); 1.63–1.68 (m, 1H, He-4); 1.82 (br.s, 3H, H-18); 1.80–1.89 (m, 2H, Ha-4, Ha-6); 2.11–2.18 (m, 2H, H-7); 3.80 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 3.85 (br.s, 1H, He-10); 4.43 (dd, J(3a,4a) = 12.0 Hz, J(3a,4e) = 2.7 Hz, 1H, Ha-3); 5.63–5.67 (m, 1H, H-8); 7.16 (dd, J(16,15) = 8.2 Hz, J(16,12) = 2.0 Hz, 1H, H-16); 7.39 (d, J(15,16) = 8.2 Hz, 1H, H-15); 7.42 (d, J(12,16) = 2.0 Hz, 1H, H-12). 13C-NMR (CDCl3 + CD3OD): 77.71 (d, C-1); 75.72 (d, C-3); 41.95 (t, C-4); 69.84 (s, C-5); 37.77 (d, C-6); 22.21 (t, C-7); 123.80 (d, C-8); 130.78 (s, C-9); 69.57 (d, C-10); 142.25 (s, C-11); 127.33 (d, C-12); 130.35 and 131.73 (2 s, C-13, C-14); 129.70 (d, C-15); 124.77 (d, C-16); 25.87 (q, C-17); 19.91 (q, C-18). HR-MS: 342.0788 (M+, C17H20Cl2O3 +; calc. 342.0784).

(2S,4R,4aR,8R,8aR)-2-(3,4-Dichlorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((R)-7f)

1H NMR (CDCl3): 1.23 (s, 3H, H-17); 1.59–1.67 (m, 2H, H-4), 1.70 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.2 Hz, J(6a,1e) = 2.1 Hz, 1H, Ha-6); 1.82 (br.s, 3H, H-18); 1.88–1.98 (m, 2H, H-7); 3.91 (br.s, 1H, He-10); 4.23 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 4.74 (dd, J(3a,4a) = 9.4 Hz, J(3a,4e) = 5.0 Hz, 1H, Ha-3); 5.57–5.60 (m, 1H, H-8); 7.10 (dd, J(16,15) = 8.2 Hz, J(16,12) = 2.0 Hz, 1H, H-16); 7.34 (d, J(15,16) = 8.2 Hz, 1H, H-15); 7.38 (d, J(12,16) = 2.0 Hz, 1H, H-12). 13C-NMR (CDCl3): 75.21 (d, C-1); 74.51 (d, C-3); 41.89 (t, C-4); 70.70 (s, C-5); 38.02 (d, C-6); 24.49 (t, C-7); 123.91 (d, C-8); 131.85 (s, C-9); 70.43 (d, C-10); 142.89 (s, C-11); 127.74 (d, C-12); 130.99 and 132.27 (2 s, C-13, C-14); 130.12 (d, C-15); 125.10 (d, C-16); 28.30 (q, C-17); 20.77 (q, C-18).

With 3-fluorobenzaldehyde 2g

The following compounds were isolated: unreacted aldehyde 2g (0.145 g, conversion 72 %); intermolecular reaction products 3g (0.085 g, 10 %), 7g (S:R = 60:40) (0.247 g, 28 %), 8g (0.007 g, 0.5 %), and 9 (0.012 g, 1.5 %); and isomerization products 4 (0.062 g, 9 %), 5 (0.069 g, 10 %), and 6 (0.012 g, 2 %).

2S,4aR,8R,8aR)-2-(3-Fluorophenyl)-4,4,7-trimethyl-4a,5,8,8a-tetrahydro-4H-benzo[d][1,3]dioxin-8-ol 3g

(1H NMR (CDCl3): 1.25 (s, 3H, H-18); 1.51 (s, 3H, H-17); 1.53 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.1 Hz, J(6a,1e) = 2.0 Hz, 1H, Ha-6); 1.79 (m, all J ≤ 2.5 Hz, 3H, H-19); 2.06 (dddq, J(7e,7a) = 17.7 Hz, J(7e,6a) = 6.1 Hz, J(7e,8) = 5.3 Hz, J(7e,19) = 1.5 Hz, 1H, He-7); 2.38–2.47 (m, 1H, Ha-7); 3.87 (br.s, 1H, He-10); 4.34 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.0 Hz, 1H, He-1); 5.64 (dm, J(8,7e) = 5.3 Hz, 1H, H-8); 5.77 (s, 1H, H-3); 6.97 (dddd, J(14,15) = 8.2 Hz, J(14,F) = 8.2 Hz, J(14,16) = 2.5 Hz, J(14,12) = 1.6 Hz, 1H, H-14); 7.17 (ddd, J(12,F) = 9.6 Hz, J(12,14) = 2.5 Hz, J(12,16) = 1.8 Hz, 1H, H-12); 7.21 (br.d, J(16,15) = 7.7 Hz, 1H, H-16); 7.25–7.30 (m, 1H, H-15). 13C-NMR (CDCl3): 75.12 (d, C-1); 95.00 (4 J(C,F) = 2.2 Hz, C-3); 74.77 (s, C-5); 33.98 (d, C-6); 22.93 (t, C-7); 125.31 (d, C-8); 130.71 (s, C-9); 70.39 (d, C-10); 141.28 (3 J(C,F) = 7.1 Hz, C-11); 113.37 (2 J(C,F) = 22.1 Hz, C-12); 162.66 (1 J(C,F) = 245.5 Hz, C-13); 115.52 (2 J(C,F) = 21.2 Hz, C-14); 129.66 (3 J(C,F) = 8.4 Hz, C-15); 122.01 (4 J(C,F) = 3.1 Hz, C-16); 22.64 (q, C-17); 27.12 (q, C-18); 20.47 (q, C-19). \([\alpha ]_{D}^{24}\) = −90.8 (c = 0.83, MeOH); HR-MS: 292.1462 (M+, C17H21FO3 +; calc. 292.1469).

(2S,4S,4aR,8R,8aR)-2-(3-Fluorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((S)-7g)

1H NMR (CDCl3): 1.52 (s, 3H, H-17); 1.69 (ddd, J(4e,4a) = 13.4 Hz, J(4e,3a) = 2.8 Hz, J(4e,6a) = 1.1 Hz, 1H, He-4); 1.79–1.88 (m, 2H, Ha-4, Ha-6); 1.82 (br.s, 3H, H-18); 2.12–2.19 (m, 2H, H-7); 3.81 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 3.92 (br.s, 1H, He-10); 4.42 (dd, J(3a,4a) = 12.0 Hz, J(3a,4e) = 2.8 Hz, 1H, Ha-3); 5.63-5.67 (m, 1H, H-8); 6.91 (dddd, J(14, 15) = 8.0 Hz, J(14,F) = 8.0 Hz, J(14,16) = 2.5 Hz, J(14,12) = 1.5 Hz, 1H, H-14); 7.00–7.06 (m, 2H, H-12, H-16); 7.25 (ddd, J(15,14) = 8.0 Hz, J(15,16) = 8.0 Hz, J(15,F) = 5.6 Hz, 1H, H-15). 13C-NMR (CDCl3): 77.52 (d, C-1); 76.75 (4 J(C,F) = 1.8 Hz, C-3); 43.06 (t, C-4); 71.01 (s, C-5); 38.19 (d, C-6); 22.60 (t, C-7); 124.56 (d, C-8); 131.31 (s, C-9); 70.42 (d, C-10); 144.42 (3 J(C,F) = 7.1 Hz, C-11); 112.69 (2 J(C,F) = 22.1 Hz, C-12); 162.73 (1 J(C,F) = 245.9 Hz, C-13); 114.28 (2 J(C,F) = 21.2 Hz, C-14); 129.76 (3 J(C,F) = 8.4 Hz, C-15); 121.21 (4 J(C,F) = 3.0 Hz, C-16); 26.96 (q, C-17); 20.70 (q, C-18). HR-MS: 292.1467 (M +, C17H21FO3 +; calc. 292.1469).

(2S,4R,4aR,8R,8aR)-2-(3-Fluorophenyl)-4,7-dimethyl-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((R)-7g)

1H NMR (CDCl3): 1.23 (s, 3H, H-17); 1.61-1.73 (m, 3H, 2H-4, Ha-6); 1.82 (br.s, 3H, H-18); 1.95–2.05 (m, 2H, H-7); 3.93 (br.s, 1H, He-10); 4.24 (dd, J(1e,10e) = 2.4 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 4.78 (dd, J(3a,4a) = 11.0 Hz, J(3a,4e) = 3.5 Hz, 1H, Ha-3); 5.57–5.60 (m, 1H, H-8); 6.89 (dddd, J(14,15) = 8.0 Hz, J(14,F) = 8.0 Hz, J(14,16) = 2.5 Hz, J(14,12) = 1.5 Hz, 1H, H-14); 7.00–7.06 (m, 2H, H-12, H-16); 7.25 (ddd, J(15,14) = 8.0 Hz, J(15,16) = 8.0 Hz, J(15,F) = 5.6 Hz, 1H, H-15). 13C-NMR (CDCl3): 75.02 (d, C-1); 75.05 (4 J(C,F) = 1.8 Hz, C-3); 42.00 (t, C-4); 70.75 (s, C-5); 37.96 (d, C-6); 24.49 (t, C-7); 123.95 (d, C-8); 131.76 (s, C-9); 70.42 (d, C-10); 145.19 (3 J(C,F) = 7.1 Hz, C-11); 112.63 (2 J(C,F) = 22.1 Hz, C-12); 162.74 (1 J(C,F) = 245.0 Hz, C-13); 114.00 (2 J(C,F) = 21.2 Hz, C-14); 129.67 (3 J(C,F) = 8.4 Hz, C-15); 121.22 (4 J(C,F) = 3.0 Hz, C-16); 28.30 (q, C-17); 20.80 (q, C-18).

(2R,4S,4aR,6S,7R,8aR,9S)-2,9-Bis(3-fluorophenyl)-4,7-dimethylhexahydro-2H-4,6-(epoxymethano)chromen-8(5H)-one 8g

1H NMR (CDCl3): 1.12 (d, J(18,9) = 7.6 Hz, 3H, H-18); 1.48 (s, 3H, H-17); 1.94–1.97 (m, all J ≤ 3.5 Hz, 1H, He-8); 1.97 (dd, J(4e,4a) = 14.9 Hz, J(4e,3a) = 2.6 Hz, 1H, He-4); 2.20 (dd, J(4a,4e) = 14.9 Hz, J(4a,3a) = 13.2 Hz, 1H, Ha-4); 2.40–2.47 (m, 3H, H-7, He-9); 2.59 (dm, J(6e,1a) = 5.2 Hz, another J ≤ 3.5 Hz, 1H, He-6); 4.55 (d, J(1a,6e) = 5.2 Hz, 1H, Ha-1); 4.77 (dd, J(3a,4a) = 13.2 Hz, J(3a,4e) = 2.6 Hz, 1H, Ha-3); 4.81 (br.s, 1H, H-19); 6.89–6.99 (m, 3H, H-14, H-23; H-12 or H-21); 7.02 (br.d, J(H,H) = 7.7 Hz, 1H, H-16 or H-25); 7.35 (br.d, J(H,H) = 7.7 Hz, 1H, H-25 or H-16); 7.25–7.33 (m, 2H, H-15, H-24); 7.37 (ddd, J(H,F) = 9.8 Hz, J(H,H) = 2.6 Hz, J(H,H) = 1.8 Hz, 1H, H-21 or H-12). 13C-NMR (CDCl3): 76.28 (d, C-1); 71,61 (4 J(C,F) = 2.0 Hz, C-3); 48.19 (d, C-4); 73.05 (s, C-5); 39.04 (d, C-6); 24.34 (t, C-7); 43.34 (d, C-8); 42.63 (d, C-9); 211.51 (s, C-10); 144.59 (3 J(C,F) = 7.1 Hz, C-11); 112.83 (2 J(C,F) = 22.1 Hz), 113.57 (2 J(C,F) = 22.1 Hz), 113.94 (2 J(C,F) = 21.2 Hz) and 114.55 (2 J(C,F) = 21.2 Hz)–C-12, C-14, C-21 and C-23; 162.74 (1 J(C,F) = 245.5 Hz) and 162.79 (1 J(C,F) = 245.5 Hz)–C-13 and C-22; 129.75 (3 J(C,F) = 8.4 Hz, C-15, C-24); 122.01 (4 J(C,F) = 3.1 Hz) and 121.16 (4 J(C,F) = 3.1 Hz)–C-16 and C-25; 23.15 (q, C-17); 17.68 (q, C-18), 74.92 (4 J(C,F) = 2.0 Hz, C-19); 142.83 (3 J(C,F) = 7.1 Hz, C-20). HR-MS: 398.1684 (M+, C24H24F2O3 +; calc. 398.1688).

(2S,4aS,8R,8aR)-2-(3-Fluorophenyl)-7-methyl-4-methylene-3,4,4a,5,8,8a-hexahydro-2H-chromen-8-ol 9

1H NMR (CDCl3): 1.83 (br.s, 3H, H-18); 1.96 (dddq, J(7e,7a) = 17.8 Hz, J(7e,6a) = 6.5 Hz, J(7e,8) = 5.2 Hz, J(7e,18) = 1.5 Hz, 1H, He-7); 2.27–2.35 (m, 1H, Ha-7); 2.33 (dd, J(4e,4a) = 14.1 Hz, J(4e,3a) = 3.0 Hz, 1H, He-4); 2.44 (ddt, J(4a,4e) = 14.1 Hz, J(4a,3a) = 11.5 Hz, J(4a,17) = 2.0 Hz, 1H, Ha-4); 2.54 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.5 Hz, J(6a,1e) = 2.2 Hz, 1H, Ha-6); 3.73 dd, J(1e,10e) = 2.4 Hz, J(1e,6a) = 2.2 Hz, 1H, He-1); 3.92 (br.s, 1H, He-10); 4.38 (dd, J(3a,4a) = 11.5 Hz, J(3a,4e) = 3.0 Hz, 1H, Ha-3); 4.82 (dd, J(17,17′) = 2.3 Hz, J(17,4a) = 2.0 Hz, 1H, H-17); 4.92 (dd, J(17′,17) = 2.3 Hz, J(17′,4a) = 2.0 Hz, 1H, H-17′); 5.61–5.65 (m, 1H, H-8); 6.93 (dddd, J(14,15) = 8.2 Hz, J(14,F) = 8.2 Hz, J(14,16) = 2.6 Hz, J(14,12) = 1.5 Hz, 1H, H-14); 7.06 (ddd, J(12,F) = 9.5 Hz, J(12,14) = 2.5 Hz, J(12,16) = 1.8 Hz, 1H, H-12); 7.08 (br.d, J(16,15) = 7.5 Hz, 1H, H-16); 7.26 (ddd, J(15,14) = 8.2 Hz, J(15,16) = 7.5 Hz, J(15,F) = 5.6 Hz, 1H, H-15). 1C-NMR (CDCl3): 80.46 (d, C-1); 79.77 (4 J(C,F) = 2.0 Hz, C-3); 38.59 (t, C-4); 146.36 (s, C-5); 36.63 (d, C-6); 26.17 (t, C-7); 124.39 (d, C-8); 131.49 (s, C-9); 70.29 (d, C-10); 144.78 (3 J(C,F) = 7.1 Hz, C-11); 112.66 (2 J(C,F) = 22.1 Hz, C-12); 162.86 (1 J(C,F) = 245.5 Hz, C-13); 114.21 (2 J(C,F) = 21.2 Hz, C-14); 129.71 (3 J(C,F) = 8.4 Hz, C-15); 121.20 (4 J(C,F) = 2.9 Hz, C-16); 110.05 (t, C-17); 20.86 (q, C-18).

With 4-(trifluoromethyl)benzaldehyde 2h

The following compounds were isolated: unreacted aldehyde 2h (0.410 g, conversion 44 %); intermolecular reaction products 3h (0.131 g, 21 %) and 7h (S:R = 50:50) (0.114 g, 18 %); and isomerization products 4 (0.031 g, 4 %), 5 (0.093 g, 13 %), and 6 (0.066 g, 9 %).

(2S,4aR,8R,8aR)-4,4,7-Trimethyl-2-(4-(trifluoromethyl)phenyl)-4a,5,8,8a-tetrahydro-4H-benzo[d][1,3]dioxin-8-ol 3h

1H NMR (CDCl3): 1.26 (s, 3H, H-18); 1.53 (s, 3H, H-17); 1.55 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.1 Hz, J(6a,1e) = 2.0 Hz, 1H, Ha-6); 1.80 (ddd, J(19,7a) = 2.5 Hz, J(19,7e) = 1.5 Hz, J((19,8) = 1.5 Hz, 3H, H-19); 2.08 (dddq, J(7e,7a) = 17.8 Hz, J(7e,6a) = 6.1 Hz, J((7e,8) = 5.2 Hz, J(7e,19) = 1.5 Hz, 1H, He-7); 2.42 (dddqd, J(7a,7e) = 17.8 Hz, J(7a,6a) = 10.8 Hz, J(7a,8) = 2.5 Hz, J(7a,19) = 2.5 Hz, J(7a,10e) = 1.2 Hz, 1H, Ha-7); 3.89 (br.s, 1H, He-10); 4.37 (dd, J(1e,10e) = 2.3 Hz, J(1e,6a) = 2.0 Hz, 1H, He-1); 5.65 (dm, J(8,7e) = 5.2 Hz, 1H, H-8); 5.83 (s, 1H, H-3); 7.54–7.63 (m, 4H, H-12, H-13, H-15, H-16). 13C-NMR (CDCl3): 75.16 (d, C-1); 94.95 (d, C-3); 74.89 (s, C-5); 34.00 (d, C-6); 22.94 (t, C-7); 125.30 (d, C-8); 130.73 (s, C-9); 70.40 (d, C-10); 142.52 (s, C-11); 126.75 (d, C-12, C-16); 125.11 (3 J(C,F) = 4.0 Hz, C-13, C-15); 130.77 (2 J(C,F) = 32.3 Hz, C-14); 22.63 (q, C-17); 27.10 (q, C-18); 20.47 (q, C-19); 121.8 (1 J(C,F) = 272.5 Hz, C-20). \([\alpha ]_{D}^{25}\) = −80.3 (c = 0.43, MeOH); HR-MS: 342.1334 (M+, C18H21F3O3 +; calc. 342.1437).

(2S,4S,4aR,8R,8aR)-4,7-Dimethyl-2-(4-(trifluoromethyl)phenyl)-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((S) 7h)

1H NMR (CDCl3 + CD3OD): 1.43 (s, 3H, H-17); 1.59 (dd, J(4e,4a) = 13.4 Hz, J(4e,3a) = 2.7 Hz, 1H, He-4); 1.73 (br.s, 3H, H-18); 1.72–1.80 (m, 2H, Ha-4, Ha-6); 2.03–2.12 (m, 2H, H-7); 3.72 (br.t, J(1e,10e) ≈ J(1e,6a) ≈ 2.2 Hz, 1H, He-1); 3.78 (br.s, 1H, He-10); 4.41 (dd, J(3a,4a) = 12.0 Hz, J(3a,4e) = 2.7 Hz, 1H, Ha-3); 5.55–5.59 (m, 1H, H-8); 7.34 (d, J(12,13) = J(16,15) = 8.2 Hz, 2H, H-12, H-16); 7.47 (d, J(13,12) = J(15,16) = 8.2 Hz, 2H, H-13, H-15). 13C-NMR (CDCl3 + CD3OD): 77.77 (d, C-1); 76.65 (d, C-3); 42.49 (t, C-4); 70.34 (s, C-5); 38.04 (d, C-6); 22.51 (t, C-7); 124.17 (d, C-8); 131.13 (s, C-9); 69.96 (d, C-10); 145.96 (s, C-11); 125.88 (d, C-12, C-16); 125.01 (3 J(C,F) = 4.0 Hz, C-13, C-15); 129.43 (2 J(C,F) = 32.3 Hz, C-14); 26.43 (q, C-17); 20.40 (q, C-18); 123.87 (1 J(C,F) = 272.5 Hz, C-19). HR-MS: 342.1442 (M+, C18H21F3O3 +; calc. 342.1437).

(2S,4R,4aR,8R,8aR)-4,7-Dimethyl-2-(4-(trifluoromethyl)phenyl)-3,4,4a,5,8,8a-hexahydro-2H-chromene-4,8-diol ((R)-7h)

1H NMR (CDCl3 + CD3OD): 1.15 (s, 3H, H-17); 1.60 (d, J(4,3) = 7.3 Hz, 2H, H-4); 1.65 (ddd, J(6a,7a) = 10.8 Hz, J(6a,7e) = 6.3 Hz; J(6a,1e) = 2.1 Hz, 1H, Ha-6); 1.75 (br.s, 3H, H-18); 1.88–2.00 (m, 2H, H-7); 3.82 (br.s, 1H, He-10); 4.16 (dd, J(1e,10e) = 2.4 Hz, J(1e,6a) = 2.1 Hz, 1H, He-1); 4.78 (t, J(3,4) = 7.3 Hz, 1H, Ha-3); 5.51–5.55 (m, 1H, H-8); 7.35 (d, J(12,13) = J(16,15) = 8.2 Hz, 2H, H-12, H-16); 7.47 (d, J(13,12) = J(15,16) = 8.2 Hz, 2H, H-13, H-15). 13C-NMR (CDCl3 + CD3OD): 75.17 (d, C-1); 75.09 (d, C-3); 41.74 (t, C-4); 70.08 (s, C-5); 37.54 (d, C-6); 24.36 (t, C-7); 123.57 (d, C-8); 131.72 (s, C-9); 70.00 (d, C-10); 146.71 (s, C-11); 125.84 (d, C-12, C-16); 124.98 (3 J(C,F) = 4.0 Hz, C-13, C-15); 129.20 (2 J(C,F) = 32.3 Hz, C-14); 27.70 (q, C-17); 20.54 (q, C-18); 124.02 (1 J(C,F) = 272.0 Hz, C-19).

Pharmacology

Animals

All studies were carried out on non-breeding albino mice (male) weighting 20–25 g, eight animals in each group (SPF-vivarium of the Institute of Cytology and Genetics of the Siberian Branch of the Russian Academy of Sciences). Mice were maintained at 22–25 °C on a 12 h light–dark cycle with food and water available ad libitum. All work with animals was performed in strict accordance with the legislation of the Russian Federation, the regulations of the European Convention for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes, and the requirements and recommendations of the Guide for the Care and Use of Laboratory Animals.

Analgesic tests

Agents were dissolved in saline-containing 0.5 % Tween 80 just before use and were administered per os, 1 h before testing. Saline was administered per os in blank mice (control group), 1 h before testing. Analgesic activity was assessed using acetic acid-induced writhing test and hot plate test.

In the acetic acid-induced writhing test, the pain reaction was determined by the number of abdominal convulsions, recorded from the 5th to 8th min following the acetic acid injection (0.75 %, 0.1 ml/mouse) (Koster et al., 1959). The percentage of pain reaction inhibition was calculated according to the following equation: % inhibition = 100 × (A−B)/A, where A is the mean number of writhes in the control group, and B is the mean number of writhes in the test group.

In the hot plate test, animals were placed individually on a metallic plate warmed to 54 ± 0.5 °C and the time until either licking of the hind paw or jumping occurred was recorded by a stopwatch (Eddy and Leimbach, 1953).

The results of pharmacological testing were partly presented by us in the patent (Tolstikova et al., 2011).

Statistical data processing was carried out using a Statistica 6.0 program.