Dioxin isomers, structures

Structure— Function Relationships. Since PCBs and related HAHs are found in the environment as complex mixtures of isomers and congeners, any meaninghil risk and hazard assessment of these mixtures must consider the quaUtative and quantitative stmcture—function relationships. Several studies have investigated the stmcture—activity relationships for PCBs that exhibit 2,3,7,8-tetrachlorodibenzo-p-dioxin [1746-01-6] (1)... 

The oxidation behavior of 3-oxa-chromanols was mainly studied by means of the 2,4-dimethyl-substituted compound 2,4,5,7,8-pentamethylM /-benzo[ 1,3]dioxin-6-ol (59) applied as mixture of isomers 27a it showed an extreme dependence on the amount of coreacting water present. In aqueous media, 59 was oxidized by one oxidation equivalent to 2,5-dihydroxy-3,4,6-trimethyl-acetophenone (61) via 2-(l-hydroxyethyl)-3,5,6-trimethylbenzo-l,4-quinone (60) that could be isolated at low temperatures (Fig. 6.41). This detour explained why the seemingly quite inert benzyl ether position was oxidized while the labile hydroquinone structure remained intact. Two oxidation equivalents gave directly the corresponding para-quinone 62. Upon oxidation, C-2 of the 3-oxa-chroman system carrying the methyl substituent was always lost in the form of acetaldehyde. 

The phosphorescence spectra of chlorinated 1,4-dibenzodioxins, in hexane solutions at 77 and 4.2 K, have a well-resolved vibronic structure with distinctions in quasi-linear phosphorescence spectra even for closely related isomers of polychlorinated dioxins <1998MI129>. 

The large number of potential congeners and isomers of some semivolatile compounds is illustrated in Table 8.1 which shows the numbers of possible chlorinated biphenyl, chlorinated dibenzo-/ -dioxin, and chlorinated dibenzofuran congeners and isomers. The total number of chlorinated congeners of each parent compound is the sum of the possible isomers at each level of chlorination. Dibenzofuran has more possible chlorinated congeners and isomers than chlorinated dibenzo-/ -dioxin (Fig. 8.2) because it has just one ring oxygen and a less symmetrical structure. If F, Br, I, CH3 or any other uniform... 

Donnelly JR, Sovocool GW (1992), Chemosphere 25 1299-1304. Gas chromatographic elution order and elution shift modelling for isomer specific analysis of halogenated dioxins" Donnelly JR, Munslow WD, Grange AH Pettit TL, Simmons RD, Sovocool GW (1991a), J. Chromat. 540 293-310. Correlation of structure with linear retention index for bromo- and bromochlorodibenzo-p-dioxins and bromodibenzofurans"... 

Dioxins and furans are planar tricyclic compounds that have very similar chemical structures and properties. They may contain between 1 and 8 chlorine atoms dioxins have 75 possible positional isomers and furans have 135 positional isomers. They are generally highly insoluble in water, lipophilic and very persistent. Neither dioxins nor furans are... 

The most toxic PCDD is 2,3,7,8-tetrachloro-dibenzo-p-dioxin. Draw the structure of this compound. Also draw the structures of two other isomers containing four chlorine atoms. 

As noted above, closely related isomers of DDD and DDT had very different abilities to inhibit binding to the alligator ER. Two factors are involved. First is the conformation of the receptor, and the second is the three-dimensional structure of the xenobiotic and its resemblance to a natural ligand. As a model system to investigate the structure-activity relationships of molecules that react to specific sites in hormones, we will use the well-studied molecules 1,3,7,8 TCDD (dioxin) and various PCBs. 

The latter when subjected to a second Ullmann condensation with bromomethoxyphenylacetic acid produces the acid (83). Removal of the /v-butyry1 group and internal amide formation, followed by Bischler-Napieralski cyclisation, A -methy1ation and reduction gives a mixture of diastereoisomeric bases of structure (84), one of which has been reduced to the 4 -benzyl compound and completely 0-demethylated to the tetrahydric phenol (85). This is cyclised to the dioxin by hydrobromic acid at 140°, methylated with diazomethane and reductively debenzy-lated to trilobine (86). The other isomer of the base (84) has been similarly converted into obaberine, the C-1 epimer of trilobine (Y. Inubushi et al.. Tetrahedron Letters, 1976, 2857 Chem.Pharm.Bull., 1977,, 1636). 

Polybrominated dibenzo-p-dioxins (PB-DDs) are highly toxic however, less toxic than the chloro compounds. The effects and symptoms are comparable to those of chlorinated dioxins. In rats, 2,3,7,8-bromo isomer was 3 to 10 times less toxic than 2,3,7,8-TCDD (Loeser and Ivens 1988). Mason et al. (1987b) have investigated the biological and toxic effects and structure-activity relationships of brominated and chlorinated... 

Tandem MS monitoring selected fragment ions produced in the collision cell of a sector or multiquadrupole instrument or in an ion-trap chamber offers advantages over single-stage MS in terms of selectivity of analysis and specificity in molecular structure determination [104]. Tandem MS techniques have been widely used with good results for the analysis of polyhalogenated compounds, such as isomers of the polychlorinated dibenzo-y dioxines and dibenzofurans [105], PCBs [106], PCNs, and related compounds [107]. However, the applica tion of MS-MS to the analysis of PCTs and toxaphene has been very limited. 

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