1.4- Dioxin ring

Synthetic routes to 1,4-dithiins have been explored more thoroughly <66HC(21-2)i 112> than those to the 1,4-dioxin ring system. The parent compound is obtained in 47-60%... 

Fortnagel, P., Harms, H., Wittich, R.-M., Francke, W., Krohn, S. and Meyer, H. (1989). Cleavage of dibenzofuran and dibenzo-p-dioxin ring systems by a Pseudomonas bacterium, Naturwissenschaften, 76, 222-223. 

Benzil undergoes a cycloaddition reaction with stilbene and 1,1-diphenyl-ethylene to form adducts containing the 2,3-dihydro-[l,4]-dioxin ring system and with visnagin to form an oxetane. These reactions and the cycloaddition reactions of the analogous o-quinones are reviewed by Schonberg.87... 

Co(II)-Co(III) Catalysis Reductive 1,2-Dioxine Ring Opening and Tandem... 

The same technique was employed to determine the kinetics of the inversion of the dioxine ring in the radical cations of 29-34 [100]. 

Few structures of the benzo derivatives of these compounds have been reported, but a typical example of those which have is of 1,3-benzodioxin, in which the dioxin ring adopts an envelope conformation <88Mi 608-0i>. 

There was a 50% loss of relative to H. This result is consistent with the loss of the methyl group from the 6-methoxy group during the formation of the dioxin ring ). 

Chlorinated dibenzo ip-dioxins are contaminants of phenol-based pesticides and may enter the environment where they are subject to the action of sunlight. Rate measurements showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is more rapidly photolyzed in methanol than octachlorodi-benzo-p-dioxin. Initially TCDD yields 2,3,7-trichlorodiben-zo-p-dioxin, and subsequent reductive dechlorination is accompanied by ring fission. Pure dibenzo-p-dioxin gave polymeric material and some 2,2 -dihydroxybiphenyl on irradiation. Riboflavin-sensitized photolysis of the potential precursors of dioxins, 2,4-dichlorophenol and 2,4,5-trichloro-phenol, in water gave no detectable dioxins. The products identified were chlorinated phenoxyphenols and dihydroxy-biphenyls. In contrast, aqueous alkaline solutions of purified pentachlorophenol gave traces of octachlorodibenzo-p-dioxin on irradiation. 

More recently, another class of organic chlorine compounds has emerged as an environmental hazard. These are the dioxins, which, like DDT, contain ring compounds with chlorine substituents. A relatively simple example is 2,3,7,8-tetrachlorodibenzo-p-dioxin ... 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

Chlorinated dioxins occur in atmospheric deposition (Koester and Hites 1992), and will thereby enter the terrestrial environment and watercourses. The degradation of tetrachloro- through octa-chlorodibenzo[l,4]dioxins has been examined in low-nitrogen medium by Phanerochaete sor-dida YK-624 (Takada et al. 1996). All the compounds were extensively degraded, and the ring fission of 2,3,7,8-tetra- and octachlorodibenzo[l,4]dioxin produced 4,5-di- and tetrachlorocatechol. These results established important evidence for the biodegradability of even highly chlorinated dibenzodioxins. 

A series of dibenzo[l,4]dioxins bearing nitro and trifluoromethyl substituents in one ring and chlorine in the other was synthesized for comparison with impurities in commercial samples of the lampricide 3-trifluoromethyl-4-nitrophenol, and to assess their ability to induce MFO activity (Hewitt et al. 1998). Although they appeared to be relatively weak inducers, there were other unidentified components that were highly active. 

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