Oxepin into (Z,Z

Alistair P. Henderson, Martine L. Barnes, Christine Bleasdale, Richard Cameron, William ... Dong Jiang , Xin Li , Esra Mutlu , Hannah J. Petty , Majid...
0 downloads 0 Views 160KB Size
Download PDF
1314

Chem. Res. Toxicol. 1997, 10, 1314-1318

Communications Dimethyldioxirane Converts Benzene Oxide/Oxepin into (Z,Z)-Muconaldehyde and sym-Oxepin Oxide: Modeling the Metabolism of Benzene and Its Photooxidative Degradation Christine Bleasdale, Richard Cameron, Christine Edwards, and Bernard T. Golding* Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K. Received July 16, 1997X

Oxidation of 7-oxabicyclo[4.1.0]hepta-2,4-diene (benzene oxide)/oxepin with dimethyldioxirane (DMDO) gave mainly (Z,Z)-muconaldehyde, with complete diastereoselectivity. Similarly, 2-methyl-7-oxabicyclo[4.1.0]hepta-2,4-diene (toluene 1,2-epoxide)/2-methyloxepin gave (Z,Z)1,6-dioxohepta-2,4-diene, while 2,6-dimethyl-7-oxabicyclo[4.1.0]hepta-2,4-diene (1,2-dimethylbenzene 1,2-epoxide)/2,7-dimethyloxepin gave (Z,Z)-2,7-dioxo-3,5-octadiene. By monitoring the DMDO oxidation of benzene oxide/oxepin by 1H NMR spectroscopy, a significant byproduct was identified as 4,8-dioxabicyclo[5.1.0]octa-2,5-diene (sym-oxepin oxide). This observation supports the hypothesis that the route to (Z,Z)-muconaldehyde proceeds from oxepin via 6,8dioxabicyclo[5.1.0]octa-2,4-diene (oxepin 2,3-oxide), with a minor pathway leading to sym-oxepin oxide. The DMDO oxidations described provide model systems for the cytochrome P450dependent metabolism of benzene and for the atmospheric photooxidation of benzenoid hydrocarbons.

Introduction (Z,Z)-Muconaldehyde is a putative product of both the atmospheric photooxidation of benzene and its metabolism. There is ample evidence to indicate that enals of the muconaldehyde type are potentially carcinogenic (1). Indeed, metabolically produced muconaldehydes are candidates for explaining the carcinogenicity of benzene (2, 3). Thus, attempts to elucidate the complex toxicology of benzene (4) are focused on the roles of the isomeric muconaldehydes 2a-c (2, 3), as well as the metabolites benzene oxide (1a) (5, 6) and 1,2- and 1,4-benzoquinone (7). We have shown that (Z,Z)-muconaldehyde (2a) and (E,Z)-muconaldehyde (2b) can form cyclic adducts with the DNA bases adenine and guanine (3). However, although (E,E)-muconaldehyde (2c) has been identified as a product of the metabolism of benzene in mouse liver microsomes (8, 9), the mechanism of its formation and that of the putative precursor (Z,Z)-muconaldehyde from benzene has not been elucidated. It was proposed (10, 11) that (Z,Z)-muconaldehyde could arise by a P450 monooxygenase-dependent oxidation of oxepin (1b) to 6,8dioxabicyclo[5.1.0]octa-2,4-diene (oxepin 2,3-oxide, 3a), which undergoes spontaneous cleavage to (Z,Z)-muconaldehyde (2a) (see Scheme 1). Oxepin (1b) is therefore trapped from its rapid equilibrium with the initial benzene metabolite benzene oxide (1a) (12). (Z,Z)Muconaldehyde is believed to isomerize by an electrocy* Corresponding author. Tel: (44 191) 222 6647. Fax: (44 191) 222 7070. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1997.

S0893-228x(97)00122-7 CCC: $14.00

clic process to the E,Z-isomer, which is converted into the E,E-isomer by either acidic or basic catalysis (13). Benzene and several of its methyl derivatives are significant environmental pollutants. These volatile organic compounds suffer atmospheric photooxidation to a variety of carbonyl products which include hexa-2,4diene-1,6-dials, for example, muconaldehyde from benzene (14, 15). The mechanism of formation of these dialdehydes has been studied by modeling atmospheric conditions in ‘smog chambers’ (16). These studies have indicated that hydroxy radical addition to 7-oxabicyclo[4.1.0]hepta-2,4-diene (benzene oxide)/oxepin (1a/1b), © 1997 American Chemical Society

Communications

Chem. Res. Toxicol., Vol. 10, No. 12, 1997 1315

Scheme 1. Hypothetical Metabolic Route from Benzene to (Z,Z)-Muconaldehyde 2a via Oxepin 2,3-Oxide 3aa

a

This conversion has been modeled using dioxirane oxidants, which also show the formation of sym-oxepin oxide (3b) (see text).

themselves derived by attack of hydroxyl radicals on benzene, may lead to (Z,Z)-muconaldehyde (2a), which is converted into its E,Z- and E,E-isomers (2b,c, respectively) under photochemical conditions. Oxepin 2,3-oxide is a possible intermediate in the conversion of oxepin to muconaldehydes. The formation of dialdehydes by atmospheric degradation of benzene needs to be considered in the context of the risk assessment of current levels of this VOC1 in urban environments (typically 1 ppb). In this Communication, we show that benzene oxide/ oxepin can be efficiently oxidized to (Z,Z)-muconaldehyde by dimethyldioxirane in acetone. Attempts to observe oxepin 2,3-oxide (3a) as an intermediate in the oxidation leading to (Z,Z)-muconaldehyde (2a) are described. Although 3a was not observed, a significant byproduct from the oxidation of 1a/1b was its isomer 4,8-dioxabicyclo[5.1.0]octa-2,5-diene (sym-oxepin oxide, 3b). We also show that methyl-substituted homologues (1c/1d and 1e/ 1f) of benzene oxide/oxepin are converted by DMDO into the corresponding methyl-substituted (Z,Z)-muconaldehydes 2d,e, respectively. This constitutes a useful synthetic method for accessing these valuable reference compounds. The significance of the results presented for understanding the metabolism and photooxidation of benzene is discussed.

Materials and Methods Hazardous Materials: Metabolites of benzene (potential carcinogens) and bromine must be handled in a well-ventilated hood; users should wear appropriate protective clothing. Instruments. NMR spectra were measured with Bruker instruments operating at frequencies given below with the data for each compound. 13C NMR spectra were measured with broad-band proton decoupling. The spectra were determined for solutions in the solvents cited below. Residual H-substituted solvent was used for referencing spectra. Mass spectra (EI mode) were recorded with a Kratos MS80 RF instrument. The pH values of solutions were measured with a Jenway 3020 meter. Chemicals. Chemicals and solvents were either AnalaR grade, which were used directly, or laboratory reagent grade purified further where appropriate. Preparation of Benzene Oxide/Oxepin (1a/1b) and Methyl-Substituted Derivatives 1c/1d and 1e/1f. Each mixture was obtained from the corresponding 1,4-dihydrobenzene by a 1Abbreviations: DMDO, dimethyldioxirane; MS (EI), mass spectrometry (electron impact mode); VOC, volatile organic compound.

sequence of monoepoxidation with 3-chloroperoxybenzoic acid in dichloromethane, bromination of the resulting cyclohexa-1,4diene 1,2-epoxide with bromine in dichloromethane, and finally double dehydrobromination of the 4,5-dibromocyclohexene 1,2epoxide by potassium tert-butoxide in tetrahydrofuran (12, 17, 18). The products were purified by Kugelrohr distillation at water pump pressure. A typical procedure is given for the preparation of toluene 1,2-epoxide/2-methyloxepin (1c/1d). 1-Methyl-1,4-cyclohexadiene 1,2-Epoxide. A solution of 3-chloroperoxybenzoic acid (89% pure by titrimetric analysis, 4.14 g, 21.2 mmol) in dichloromethane (70 mL) was added dropwise to 1-methyl-1,4-cyclohexadiene (2.0 g, 21.2 mmol) with vigorous stirring. The mixture was heated at reflux for 3 h and then stirred at room temperature overnight. After the reaction mixture cooled in an ice bath, the white precipitate was removed by filtration. The filtrate was washed sequentially with 20% sodium bisulfite solution (3 × 40 mL) and brine (40 mL). After drying the organic layer over anhydrous magnesium sulfate, the solvent was removed. The residual product was purified by Kugelrohr distillation under water pump pressure and isolated as a colorless oil (1.31 g, 11.9 mmol, 56%): 1H NMR (200 MHz, CDCl3) δ 5.42 (2H, br s, 4-H, 5-H), 3.05 (1H, br s, 2-H), 2.612.18 (4H, m, 2 × 3-H, 2 × 6-H), 1.35 (3H, s, CH3); 13C NMR (50 MHz, CDCl3) δ 122.5, 121.4, 58.3, 56.6, 30.3, 26.2, 23.3; MS (EI) 110 (M+, 24), 95 ([M - CH3]+, 35), 91 (16), 81 (48), 67 (53), 53 (48), 50 (17), 43 (100), 32 (21). 1-Methyl-4,5-dibromocyclohexene 1,2-Epoxide. A solution of 1-methyl-1,4-cyclohexadiene 1,2-epoxide (1.3 g, 11.8 mmol) in dichloromethane (20 mL) was cooled to -70 °C. Bromine (0.55 mL, 1.71 g, 10.7 mmol) in dichloromethane (10 mL) was added dropwise. The reaction mixture was stirred at -70 °C for 2 h. Removal of the solvent and excess of bromine gave crude product that was purified by flash chromatography on silica gel using 20% ether in petroleum ether (40-60 °C) as eluent. The title compound was obtained as a colorless oil (1.82 g, 6.7 mmol, 57%): 1H NMR (200 MHz, CDCl3) δ 4.37-4.05 (2H, m, 4-H, 5-H), 3.04 (1H, dd, J ) 3.7, 8.1 Hz, 2-H), 2.94-2.23 (4H, m, 2 × 3-H, 2 × 6-H), 1.34 (3H, s, CH3); 13C NMR (50 MHz, CDCl3) δ 57.7, 57.5, 56.4, 50.0, 49.0, 48.3, 47.7, 39.1, 36.9, 36.9, 33.8, 33.8, 23.3, 23.2 (the compound is a mixture of diastereoisomers); MS (EI) 191 ([M - Br]+, 94), 121 (26), 110 ([M - Br2]+, 83), 95 (38), 91 (33), 81 (100), 67 (80), 53 (82). Toluene 1,2-Epoxide/2-Methyloxepin (1c/1d) (12). To a solution of 1-methyl-4,5-dibromocyclohexene 1,2-epoxide (500 mg, 1.85 mmol) in dry tetrahydrofuran (12 mL) was added potassium tert-butoxide (415 mg, 3.70 mmol) under nitrogen in three portions. The mixture became yellow, and a precipitate of potassium bromide was formed. After stirring for 15 min, the mixture was filtered to remove the precipitated potassium bromide, which was washed with diethyl ether. The solvent was removed in vacuo without heating to give crude product which

1316 Chem. Res. Toxicol., Vol. 10, No. 12, 1997 was distilled (Kugelrohr) at water pump pressure (∼80 °C oven temperature). The title compound was obtained as a pale yellow liquid (180 mg, 1.67 mmol, 90%): 1H NMR (200 MHz, CDCl3) δ 6.10 (2H, m, 4-H, 5-H), 5.74 (1H, m, 6-H), 5.62 (1H, m, 3-H), 5.41 (1H, d, J ) 4.9 Hz, 7-H), 1.86 (3H, s, CH3); MS (EI) 108 (M+, 100), 91 (31), 79 (78), 72 (28), 65 (38), 59 (90), 52 (28), 43 (96). Benzene Oxide/Oxepin (1a/1b). The 1a/1b mixture was obtained (cf. ref 18) as a pale yellow oil: 1H NMR (200 MHz, CDCl3) δ 6.25 (2H, m, 4-H, 5-H), 5.90 (2H, m, 3-H, 6-H), 5.31 (2H, d, J ) 4.9 Hz, 2-H, 7-H) (oxepin numbering used); MS (EI) 95 (M+, 30), 88 (10), 77 (12), 67 (100), 57 (28), 46 (15), 41 (36), 32 (19). 1,2-Dimethylbenzene 1,2-Epoxide/2,7-Dimethyloxepin (1e/1f). This compound was prepared essentially as described (17) from 1,2-dimethylcyclohexa-1,4-diene: yellow liquid; υmax (film) 3027, 2979, 2942, 2912, 2875, 2728, 2424, 1695, 1658, 1594 cm-1; 1H NMR (200 MHz, CDCl3) δ 5.95 (2H, pseudo-t, J ) 3.4 Hz, 4-H, 5-H), 5.39 (2H, br s, 3-H, 6-H), 1.85 (6H, s, 2 × CH3); MS (EI) 123 ([M + 1]+, 54), 95 (5), 91 (9), 77 (32), 65 (7), 59 (18), 43 (100), 32 (16). Conversion of Oxepins 1b,d,f into the Corresponding Muconaldehydes 2a,d,e. Each benzene oxide/oxepin mixture (0.5 M solution in acetone) was treated with 1 mol equiv of DMDO (0.07 M in acetone) (19, 20) at 0 °C. After 15-30 min the solvent was removed in vacuo to give the corresponding muconaldehyde. (Z,Z)-Muconaldehyde (2a). This was obtained as a yellow solid (80 mg, 64%), which was purified by dissolution in the minimum amount of diethyl ether followed by the addition of cold petroleum ether: υmax (in KBr) 1663 cm-1; 1H NMR (200 MHz, CDCl3) δ 10.40 (2H, d, J ) 7.0 Hz, 1-H, 6-H), 8.16 (2H, dd, J ) 2.6, 7.6 Hz, 3-H, 4-H), 6.25 (2H, m, 2-H, 5-H); MS (EI) 110 (M+, 70), 88 (13), 81 ([M - CHO]+, 100), 57 (32), 53 (72), 44 (16), 32 (27) [data identical with that from an authentic sample (13)]. (Z,Z)-1,6-Dioxohepta-2,4-diene (2d). The crude product was taken up in diethyl ether (10 mL) and washed with water (5 mL) and brine (5 mL). The ether layer was dried over anhydrous sodium sulfate. Removal of the solvent afforded the title compound as a yellow oil (92 mg, 60%): υmax (film) 1674 cm-1; 1H NMR (200 MHz, acetone-d6) δ 10.37 (1H, d, J ) 7.6 Hz, 1-H), 8.05 (1H, t, J ) 11.2 Hz, 4-H), 7.60 (1H, t, J ) 11.2 Hz, 3-H), 6.62 (1H, d, J ) 11.4 Hz, H-5), 6.13 (1H, m, 2-H), 2.33 (3H, s, CH3); MS (EI) 124 (M+, 100), 95 (56), 81 (32), 69 (68), 53 (19), 43 (28), 32 (31). (Z,Z)-2,7-Dioxo-3,5-octadiene (2e). This was obtained as a yellow oil which partially crystallized at -20 °C (79 mg, 67%): υmax (film) 1674, 1690 cm-1; 1H NMR (200 MHz, acetoned6) δ 7.53 (2H, dd, J ) 2.4, 8.1 Hz, 4-H, 5-H), 7.46 (2H, dd, J ) 2.4, 8.1 Hz, 3-H, 6-H), 2.28 (6H, s, 2 × CH3); MS (EI) 138 (M+, 50), 123 (4), 95 (16), 85 (2), 77 (1), 64 (46), 46 (100), 32 (54). sym-Oxepin Oxide (3b). This was prepared from 1a/1b via its copper(I) complex 4 according to the procedure of Rastetter (21). Thus, demetalation of the copper(I) complex 4 was performed with 20% aqueous ammonia in the temperature range -20 to -30 °C, to liberate the azo diepoxide 5. This underwent decomposition in CDCl3 at room temperature, with extrusion of nitrogen, to give sym-oxepin oxide 3b (see Scheme 2): 1H NMR (200 MHz, CDCl3) δ 6.34 (2H, d), 5.14 (2H, ddd), 3.33 (2H, dd) (see ref 22 for analysis of this spin system). The solvent was removed (-42 °C at 0.02 mmHg) and the residual sym-oxepin oxide (3b) was taken up in deuterated acetone: 1H NMR (200 MHz, acetone-d6) δ 6.54 (2H, d, J ) 7.5 Hz, 2-H, 7-H), 5.33 (2H, ddd, J ) 2.1, 3.5, 7.5 Hz, 3-H, 6-H), 3.43 (2H, dd, J ) 2.1, 3.5 Hz, 4-H, 5-H). Reaction of Benzene Oxide/Oxepin with DMDO Monitored by NMR. A freshly prepared solution of DMDO in deuterated acetone (0.7 mL, 55 mM) at -78 °C was added to benzene oxide/oxepin (1a/1b) (97% pure by 1H NMR analysis, 12.2 mg, 0.13 mmol) in an NMR tube. The reaction was monitored by 500-MHz 1H NMR spectroscopy (see Figure 1). The formation of a 10:3 mixture of (Z,Z)-muconaldehyde (2a)

Communications Scheme 2. Synthesis of sym-Oxepin Oxide (3b)a

a

cf. ref 21.

and sym-oxepin oxide (3b) began at -60 °C. The major conversion of 1a/1b into these products occurred between -40 and -30 °C. After 30 min at -30 °C (total reaction time 2.5 h), the oxidations were adjudged to have reached completion and the contents of the tube were slowly warmed to 10 °C. At this temperature the rearrangement of sym-oxepin oxide (3b) into 4H-pyran-4-carboxaldehyde (6) was observed (cf. ref 21), with complete conversion being achieved over ca. 45 min. Isomerization of (Z,Z)-muconaldehyde occurred at room temperature over several hours. Preparation of 4H-Pyran-4-carboxaldehyde (6). symOxepin oxide (3b) in deuterated acetone, generated as described, quantitatively rearranged at -20 °C over 3 days to 4H-pyran4-carboxaldehyde (6): 1H NMR (200 MHz, acetone-d6) δ 9.60 (1H, d, J ) 1.2 Hz, CHO), 6.72-6.67 (2H, m, 2-H, 6-H), 5.145.09 (2H, m, 3-H, 5-H), 3.81-3.76 (1H, m, 4-H).

Results and Discussion Preparation of Arene Oxides/Oxepins. These were prepared essentially as described (12, 17, 18). Thus, benzene oxide/oxepin (1a/1b) was prepared from 1,4cyclohexadiene by epoxidation followed by bromination and dehydrobromination of the intermediate 4,5-dibromocyclohexene 1,2-epoxide using potassium tert-butoxide. Toluene 1,2-oxide/2-methyloxepin (1c/1d) and 2,7-dimethyloxepin (1f) [this is the almost exclusive component in equilibrium with 1e (12)] were synthesized in similar ways from 1-methyl- and 1,2-dimethyl-1,4-cyclohexadiene, respectively. Oxidation of Oxepins Using DMDO. Benzene was reported to be oxidized to a mixture of products containing (Z,Z)- and (E,E)-muconaldehyde using methyl(trifluoromethyl)dioxirane at 0 °C (23). This conversion may be presumed to occur via benzene oxide/oxepin (1a/1b), which Whitham and Davies had already shown to be converted into muconaldehydes by oxidation with peroxy acids (10). At 80 °C a mixture of the E,Z- and E,E-isomers was obtained, while at 0 °C a low yield of (Z,Z)muconaldehyde was obtained (10). These workers also reported that oxidation of 2,7-dimethyloxepin using peroxybenzoic acid afforded a mixture of isomers of 2,7-dioxo3,5-octadiene. In these studies the propensity of (Z,Z)muconaldehyde to rearrange to its E,Z-isomer by an uncatalyzed electrocyclic process was not realized. DMDO (24) is regarded as a good model oxidant for P450-dependent monooxygenases and has been previously used for the conversion of enol ethers, including hexamethyloxepin, into labile alkoxyoxiranes (20, 2530). In a manner similar to that described for furan (28, 29), we employed this oxidant in an attempt to generate oxepin 2,3-oxide from benzene oxide/oxepin (1a/1b). We have found that when benzene oxide/oxepin (1a/1b) is

Communications

Chem. Res. Toxicol., Vol. 10, No. 12, 1997 1317

appearance of 2a over time and also the formation of a small amount of another product, assigned as sym-oxepin oxide (3b). The spectrum of 3b (acetone-d6) was comparable with that reported for authentic sym-oxepin oxide (in CDCl3) (cf. Figure 1). The identity of this byproduct was proved by synthesis of a reference sample (21) and direct comparison with the product of the oxidation of benzene oxide/oxepin (1a/1b) with DMDO. On warming the reaction mixture to 10 °C the sym-oxepin oxide (3b) was converted into 4H-pyran-4-carboxaldehyde (6), which was identical with an authentic sample (21). There was no apparent decomposition of (Z,Z)-muconaldehyde (2a) into its isomers during the time scale and temperature range of this experiment. However, in another experiment, after keeping the reaction mixture for 3 days at room temperature, partial isomerization of 2a to (E,Z)and (E,E)-muconaldehyde (2b,c) was observed. The ratio of 2a/3b from the oxidation of benzene oxide/ oxepin (1a/1b) by DMDO was ca. 10:3. If the selectivity of oxidation of the triene system of oxepin were determined solely by statistical factors, then the ratio of 2a/ 3b would be 2:1. The larger ratio must be a consequence of the greater nucleophilicity of the double bonds nearer to the ring oxygen atom toward DMDO.

Conclusions

Figure 1. 1H NMR spectra of the reaction between benzene oxide/oxepin and dimethyldioxirane at -60 °C (spectrum 2), -40 °C (spectrum 3), -30 °C (spectrum 4), -10 °C (spectrum 5), 10 °C (spectrum 6), and 10 °C (spectrum 7, taken 20 min after spectrum 6). Spectrum 1 shows benzene oxide/oxepin for reference, and spectrum 8 is a vertical expansion of spectrum 6 [b, (Z,Z)-muconaldehyde; 9, 4H-pyran-4-carboxaldehyde; (, symoxepin oxide; 2, benzene oxide/oxepin].

treated with a solution of DMDO in acetone, (Z,Z)muconaldehyde (2b) is formed stereospecifically in ca. 80% yield. Similar oxidations of 2-methyloxepin (1d) and 2,7-dimethyloxepin (1f) gave the corresponding methylsubstituted muconaldehydes, (Z,Z)-1,6-dioxohepta-2,4diene (2d) and (Z,Z)-2,7-dioxo-3,5-octadiene (2e), respectively. We propose that the muconaldehydes arise by trapping of the more nucleophilic tautomer 1b by epoxidation to give the intermediate oxepin oxide 3a, which is converted into a muconaldehyde, probably via a zwitterionic intermediate (cf. Scheme 1). The overall process is stereospecific. Oxidation of Benzene Oxide/Oxepin (1a/1b) Monitored by 1H NMR. Monitoring the oxidation of benzene oxide/oxepin (1a/1b) by DMDO in acetone-d6 by 1H NMR in the temperature range -60 to -10 °C enabled the formation of (Z,Z)-muconaldehyde and sym-oxepin oxide to be observed directly but failed to detect oxepin 2,3oxide. Representative spectra are shown in Figure 1. Thus, the rate of formation of oxepin 2,3-oxide is much slower than its conversion into (Z,Z)-muconaldehyde. The onset of product formation occurred around -60 °C in acetone-d6 and became relatively rapid in the range -30 to -40 °C. This relatively polar solvent may facilitate the conversion of oxepin 2,3-oxide into muconaldehyde via a zwitterionic intermediate, as shown in Scheme 1. The spectra showed the disappearance of 1a/1b and the

We have shown that benzene oxide/oxepin (1a/1b) is stereospecifically oxidized to (Z,Z)-muconaldehyde by DMDO in acetone. This model system supports the postulated route of metabolism of benzene involving oxidation of benzene oxide/oxepin (1a/1b) by P450dependent monooxygenase [isozyme 2E1 (31)]. The intermediacy of oxepin 2,3-oxide in the DMDO-induced conversion could not be confirmed by 1H NMR monitoring of the reaction, presumably because the rate of ring cleavage of this intermediate exceeded its rate of formation in acetone solvent. However, its isomer sym-oxepin oxide was observed as a byproduct of the oxidation showing that the regioselectivity of DMDO for the double bonds of oxepin is 10:3 in favor of the 2,3-double bond (or 6,7) over the 4,5-double bond. It is interesting to consider the selectivity toward the double bonds of oxepin by cytochrome P450 monooxygenase. This raises the question as to whether sym-oxepin oxide (3b) is a bona fide metabolite of benzene, and if so, what are the toxicological consequences (see ref 32 for a discussion of this issue). The results described also provide a model for the conversion of benzene into muconaldehydes under conditions of photooxidation. The possible formation of sym-oxepin oxide (3b) under these conditions (′atmospheric metabolism′) needs to be explored.

Acknowledgment. We thank NERC and BBSRC for support of this research, Dr. I. Barnes (Bergische Universita¨t, Wuppertal, Germany) for advice on atmospheric chemistry, Dr. W. P. Watson for helpful discussions, and Prof. W. McFarlane and Drs. M. N. S. Hill and N. J. Rees (Newcastle) for NMR spectra.

References (1) Chung, F. L., Young, R., and Hecht, S. S. (1989) Detection of cyclic 1,N-propanodeoxyguanosine adducts in DNA of rats treated with N-nitrosopyrrolidine and mice treated with crotonaldehyde. Carcinogenesis 10, 1291-1297. (2) Bleasdale, C., Golding, B. T., Kennedy, G., MacGregor, J. O., and Watson, W. P. (1993) Reactions of muconaldehyde isomers with nucleophiles including tri-O-acetylguanosine: formation of 1,2disubstituted pyrroles from reactions of the (Z,Z)-isomer with primary amines. Chem. Res. Toxicol. 6, 407-412.

1318 Chem. Res. Toxicol., Vol. 10, No. 12, 1997 (3) Golding, B. T., Bleasdale, C., MacGregor, J. O., Nieschalk, J., Pearce, K., and Watson, W. P. (1996) Chemistry of muconaldehydes of possible relevance to the toxicology of benzene. Environ. Health Perspect. 104 (Suppl. 6), 1201-1209. (4) Snyder, R., Witz, G., and Goldstein, B. D. (1993) The toxicology of benzene. Environ. Health Perspect. 100, 293-306. (5) Lindstrom, A. B., Yeowell-O’Connell, K., Waidyanatha, S., Golding, B. T., Tornero-Velez, R., and Rappaport, S. M. (1997) Measurement of benzene oxide in the blood of rats following administration of benzene, Carcinogenesis 18, 1637-1641. (6) Lovern, M. R., Turner, M. J., Meyer, M., Kedderis, G. L., Bechtold, W. E., and Schlosser, P. M. (1997) Identification of benzene oxide as a product of benzene metabolism by mouse, rat, and human liver microsomes. Carcinogenesis 18, 1695-1700. (7) Snyder, R., and Hedli, C. C. (1996) An overview of benzene metabolism. Environ. Health Perspect. 104 (Suppl. 6), 1165-1171. (8) Latriano, L., Goldstein, B. D., and Witz, G. (1986) Formation of muconaldehyde, an open-ring metabolite of benzene, in mouse liver microsomes: an additional pathway for toxic metabolites. Proc. Natl. Acad. Sci. U.S.A. 83, 8356-8360. (9) Witz, G., Zhang, Z. H., and Goldstein, B. D. (1996) Reactive ringopened aldehyde metabolites in benzene hematoxicity. Environ. Health Perspect. 104 (Suppl. 6), 1195-1199. (10) Davies, S. G., and Whitham, G. H. (1977) Benzene oxide-oxepin. Oxidation to muconaldehyde. J. Chem. Soc., Perkin Trans. 1 1346-1347. (11) George, P., Bock, C. W., and Glusker, J. P. (1996) Fission of the epoxide ring in 6,8-dioxabicyclo[5.1.0]octa-2,4-diene (epoxyoxepin) and in 8-oxabicyclo[5.1.0]octa-2,4-diene - an ab initio molecular orbital study. J. Mol. Struct. 376, 425-435. (12) Vogel, E., and Gu¨nther, H. (1967) Benzene oxide-oxepin valence tautomerism. Angew. Chem., Int. Ed. Engl. 6, 385-401. (13) Golding, B. T., Kennedy, G., and Watson, W. P. (1988) Simple syntheses of isomers of muconaldehyde and 2-methylmuconaldehyde. Tetrahedron Lett. 29, 5991-5994. (14) Dumdei, B. E., and O’Brien, R. J. (1984) Toluene degradation products in simulated atmospheric conditions. Nature 311, 248250. (15) Yu, J. Z., Jeffries, H. E., and Sexton, K. G. (1997) Atmospheric photooxidation of alkylbenzenes. 1. Carbonyl product analyses. Atmos. Environ. 31, 2261-2280. (16) Klotz, B., Barnes, I., Becker, K. H., and Golding, B. T. (1997) atmospheric chemistry of benzene oxide/oxepin. J. Chem. Soc., Faraday Trans. 93, 1507-1516. (17) Paquette, L. A., and Barrett, J. H. (1969) 2,7-Dimethyloxepin. Org. Synth. 49, 62-65. (18) Gillard, J. R., Newlands, M. J., Bridson, J. N., and Burnell, D. J. (1991) The π-facial stereoselectivity in the Diels-Alder reactions of benzene oxides. Can. J. Chem. 69, 1337-1343.

Communications (19) Adam, W., Chan, Y.-Y., Cremer, D., Gauss, J., Scheutzow, D., and Schindler, M. (1987) Spectral and chemical properties of dimethyldioxirane as determined by experiment and ab initio calculations. J. Org. Chem. 52, 2800-2803. (20) Bleasdale, C., Golding, B. T., and Kitson, S. L. (1994) Preparation of ethenyl and oxiranol carbamate and their (2-2H2)-derivatives using selenium chemistry; thermolysis of oxiranol carbamate and reaction with adenosine. Bioorg. Med. Chem. Lett. 4, 291-296. (21) Rastetter, W. H. (1976) sym-Oxepin oxide. J. Am. Chem. Soc. 98, 6350-6353. (22) Haas, D. D., and Rastetter, W. H. (1976) Proton magnetic resonance studies of sym-oxepin oxide. J. Am. Chem. Soc. 98, 6353-6359. (23) Mello, R., Ciminale, F., Fusco, C., Prencipe, T., and Curci, R. (1990) Oxidations by methyl(trifluoromethyl)dioxirane. 4. Oxyfunctionalization of aromatic hydrocarbons. Tetrahedron Lett. 31, 6097-6100. (24) Murray, R. W., and Jeyaraman, R. (1985) Dioxiranes and reactions of methyldioxiranes. J. Org. Chem. 50, 2847-2853. (25) Baertschi, S. W., Raney, K. D., Stone, M. P., and Harris, T. M. (1988) Preparation of the 8,9-epoxide of the mycotoxin aflatoxinB1 - the ultimate carcinogenic species. J. Am. Chem. Soc. 110, 7929-7931. (26) Murray, R. W. (1989) Chemistry of dioxiranes. Chem. Rev. 89, 1187-1201. (27) Park, K.-K., Surh, Y.-J., Stewart, B. C., and Miller, J. A. (1990) Synthesis and properties of vinyl carbamate epoxide, a possible ultimate electrophilic and carcinogenic metabolite of vinyl carbamate and ethyl carbamate. Biochem. Biophys. Res. Commun. 169, 1094-1098. (28) Adger, B. M., Barrett, C., Brennan, J., McKervey, M. A., and Murray, R. W. (1991) Oxidation of furans with dimethyldioxirane. J. Chem. Soc., Chem Commun. 1553-1554. (29) Chen, L.-J., Hecht, S. S., and Peterson, L. (1995) Identification of cis-2-butene-1,4-dial as a microsomal metabolite of furan. Chem. Res. Toxicol. 8, 903-906. (30) Murray, R. W., Singh, M., and Rath, N. P. (1996) Reaction of hexamethylbenzene with dimethyldioxirane. J. Org. Chem. 61, 7660-7661. (31) Gut, I., Nedelcheva, V., Soucek, P., Stopka, P., Vodicka, P., Gelboin, H. V., and Ingelmann-Sundberg, M. (1996) The role of CYP2E1 and 2B1 in metabolic activation of benzene derivatives. Arch. Toxicol. 71, 45-56. (32) Stark, A. A., and Rastetter, W. H. (1996) Structure-activityrelationships in the mutagenicity and cytotoxicity of putative metabolites and related analogues of benzene derived the valence tautomers benzene oxide and oxepin. Environ. Mol. Mutagen. 28, 284-293.

TX970122D