Debrominative transformations

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

Analogously, for preparation of racemic carba-a-glucopyranose 49 from 52, esterification of (—)-52 furnished the ester 95, which was transformed into compound 96 by debromination with zinc dust and acetic acid. Stereoselective hydroxylation of 96 with osmium tetraoxide and hydrogen peroxide, followed by acetylation, gave compound 97. Lithium aluminum hydride reduction of 97, and acetylation of the product, gave pentaacetate 98, which was converted into 99 by hydrolysis. ... 

Reaction of 47 with NBS in carbon tetrachloride afforded the tribromide (240, 100%). After replacement of the primary bromo group with benzoyl-oxyl, the product (241,47%) was debrominated with zinc dust in ethanol to give the diene (242,64%). Epoxidation of242 produced the isomeric compounds 243 and 244, which were transformed into the azides (245 and 246), convertible into valienamine isomers. ... 

Enantiomerically pure tetrahydro-l//-pyrrolo[2, l -acrylamides derived from proline (see Section 11.11.7.4), are versatile intermediates for the synthesis of natural products or drugs. Compound 86a was submitted to debromination with Bu3SnH followed by ring opening in KOH and further reduction with BHj to give diol 89 that was then easily transformed into (A)-4-(2,2,4-trimethyl-l,3-dioxolan-4-yl)-lT>utanol 90, a key intermediate for )-frontalin, <2002TA155>,... 

Hydrolytic cleavage plays an important role in the metabolic fate of both carbromal (4.210) and bromisoval (4.213). A product of hydrolysis of carbro-mal, 2-bromo-2-ethylbutyramide (4.211), was isolated from the urine of patients intoxicated with carbromal as well as from the urine of rats, mice, and dogs dosed with the drug [138], A suspected further metabolite is 2-bromo-2-ethylbutyric acid (4.212). Other metabolic transformations of carbromal involve hydroxylation and debromination. 

Two years later, the same group reported a formal synthesis of ellipticine (228) using 6-benzyl-6H-pyrido[4,3-f>]carbazole-5,ll-quinone (6-benzylellipticine quinone) (1241) as intermediate (716). The optimized conditions, reaction of 1.2 equivalents of 3-bromo-4-lithiopyridine (1238) with M-benzylindole-2,3-dicarboxylic anhydride (852) at —96°C, led regioselectively to the 2-acylindole-3-carboxylic acid 1233 in 42% yield. Compound 1233 was converted to the corresponding amide 1239 by treatment with oxalyl chloride, followed by diethylamine. The ketone 1239 was reduced to the corresponding alcohol 1240 by reaction with sodium borohydride. Reaction of the alcohol 1240 with f-butyllithium led to the desired 6-benzylellipticine quinone (1241), along with a debrominated alcohol 1242, in 40% and 19% yield, respectively. 6-Benzylellipticine quinone (1241) was transformed to 6-benzylellipticine (1243) in 38% yield by treatment with methyllithium, then hydroiodic acid, followed... 

Reaction of the chiral enolate of 6 with the chiral bromide 7 proceeds with excellent double asymmetric induction to provide the alkylated product (R,S)/(S,R)-866. However, the yield of this transformation is quite low due to decomposition via competitive debromination of ester 7. 

Polybrominated Diphenyl Ethers. A limited amount of data is available on the toxicokinetics of PBDEs. There are data gaps in a number of areas, particularly for octaBDE and pentaBDE mixtures and the tetia and hexa congeners that are most prevalent in the environment. Quantitative absorption studies could corroborate the conclusions on oral uptake in animals that are based on elimination and excretion data. Metabolism studies would help to characterize the enzymes involved as well as the transformation of some congeners to biologically active hydroxylated BDEs and the debromination of decaBDE to lower brominated BDEs. 

Tysklind M, Sellstom U, Soderstrom G, et al. 2001. Abiotic transformation of polybrominated diphenylethers (PBDEs) Photolytic debromination of decambro diphenyl ether. BFR 51-54. 

Isopropyl alcohol under UV irradiation converts bromobenzene to benzene in 72% yield (Table 4). Similar replacement of bromine by hydrogen is accomplished by treatment of aryl bromides dissolved in dichloromethane with a mixture of ethanethiol and anhydrous aluminum chloride. This hard acid-soft base combination reacts with polycyclic aromatic halides and halogenated phenols by an addition-elimination mechanism, leading to an aryl ethyl sulfide through a radical anion intermediate. This is converted by another molecule of ethanethiol to the debrominated arene and diethyl disulfide. 1-Bro-monaphthalene is thus transformed into naphthalene (equation 59), 2,4,6-tribromophenol into phenol (equation 60), and bromochlorophenols into chlorophenols in 61-91% yields. ... 

This assignment was confirmed by the work of Kaneko (9). Tuberostemonine was converted to the lactam XX which was transformed by reduction with lithium aluminum hydride followed by cleavage with cyanogen bromide into XXI. Debromination by hydrogen over palladium followed by reductive removal of the cyano group gave XXII. The... 

Diels-Alder reaction of pyran-2-ones. Diels-Alder reaction of 2-pyrones, if successful, can provide unusual cyclohexenecarboxylic acids, but thermally promoted cycloadditions with these electron-deficient dienes usually result in decarboxylation and aromatization of the adducts as a result of the required high temperatures (6,291-292). Successful Diels-Alder reactions of 3-bromo-2-pyrone (1) with the electron-rich dioxole 2 can be effected with a catalytic amount of ethyldiisopropylamine at 90° (4 days) to give the major adduct (endo-3) in 63% yield. The adduct is hydrolyzed by p-toluenesulfonic acid in methanol to 4 as the only diastereomer. The trisilyl ether of 4 was transformed to the a,/8-unsaturated ester 5 by radical debromination and DBU isomerization. ... 

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