Debromination, zinc dust

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. ... 

Debromination. wc-Dibromides are converted into alkenes by reaction with zinc dust and a catalytic amount of TiCl4 in THF at 0°. Isolated yields are generally in the range 80-90%. 

On the other hand, when 39 was heated with hydrogen bromide in acetic acid, 1,2-di-<9-acetyl-( 1,3/2,6)-3,4-dibromo-6-(bromomethyl)-l,2-cyclohexanediol (50) was obtained, which was converted into 1,2-di-0-acetyl-( 1,3/2)-3(bromomethyl)-5-cyclo-hexene-l,2-diol (51) by debromination with zinc dust in glacial acetic acid [21]. Hydro-xylation of 51 with osmium tetroxide, and successive acetylation yielded 1,2,3,4-tetra-C>-acetyl-6-bromo-6-deoxy-pseudo-a-DL-glucopyranose (52). Nucleophilic substitution reactions of 52 with sodium acetate gave pseudo-a-DL-glucopyranose pentaacetate (55), which gave pseudo-a-DL-glucopyranose (54) by usual hydrolysis [22]. Alternatively, the pentaacetate 55 was obtained as a minor component in a poor yield by nucleophilic substitutions of 2,3,4-tri-0-acetyl-l,6-dibromo-l,6-dideoxy-pseudo-... 

Zinc dust and hydrobromic acid in glacial acetic acid have been used to debrominate XXXV (It = Br) and its 21,22-dihydro derivative to deoxyisostrychnine (CXIV) and its dihydro derivative (56). 

The bromination of a double bond is an important and well-understood organic reaction. In the present experiment it is employed for the very practical purpose of purifying crude cholesterol through the process of bromination, crystallization, and then zinc dust debromination. 

The cholesterol dibromide that crystallizes from the reaction solution is collected, washed free of the impurities or their dehydrogenation products, and debrominated with zinc dust, with regeneration of cholesterol in pure form. Specific color tests can differentiate between pure cholesterol and tissue cholesterol purified by ordinary methods. 

Why might old zinc dust not react in the debromination reaction ... 

Debromination. Trimethyl phosphite debrominates cic-dibromides in which one or both halogens is adjacent to a carbonyl group." Use of sodium iodide leads to tars zinc dust reduces the double bond. 

Dehalogenation, dehalohydrination. See also yS-Chloroethyl ethyl ether. The debromination of cholesterol dibromide with zinc dust requires only a catalytic amount of acetic acid and once the reaction has started it proceeds with vigor, like... 

Debromination of 1,2-bisbromomethylcyclooctatetraene to give 7,8-dimethylene-A " -cyclooctatriene, the exocyclic double bonds of which constitute a system highly reactive to dienophiles, was effected in practically quantitative yield with zinc dust in DMF at room temperature. ... 

Dehalogenation, dehydrohalogenation. An early procedure for debromination of 5a,6/8-dibromocholestane-3-one without isomerization of the nonconjugated ketone initially formed involved reaction with zinc dust in hot ethanol, but the reaction proceeds better in ether containing a catalytic amount of acetic acid (see above ). 

Ordinary zinc dust is not suitable for debromination of the vinylic dibromide (1) to the cumulene (2), but can be activated by etching with sulfuric acid, drying, and treatment with a small amount of iodine. ... 

Various other reagents can be used for this reductive debromination. 1,2-Dipropylcyclopropane was prepared from 4,6-dibromononane using chromium(II) perchlorate in dimethylformami-de/water (yield 93%), lithium amalgam in tetrahydrofuran (75%), lithium biphenylide in te-trahydrofuran (78%), potassium-sodium alloy in tetrahydrofuran (68%), zinc dust and zinc(II) chloride in propan-2-ol/water (95%) and alkyllithiums in tetrahydrofuran (BuLi 16%, i-BuLi 18%, t-BuLi 47%). Ring closure of 1,3-dibromobutane to methylcyclopropane was achieved by treatment with zero-valent copper, which was obtained from reaction of lithium naphthalen-ide and copper(I) iodide/tributylphosphane in tetrahydrofuran (yield 91%) ... 

Spiro[2.n]alkanes 26 and 4-oxospiro[2. (n + 2)]alkanes 27 are readily obtained by a dehalogena-tion process. In most cases, 1,3-debromination with zinc dust is an appropriate method (for an example, see ref 54). 

Treatment of 3,3-dibromocamphor with bromine and chlorsulphonic acid (conditions under with 3,3,8-tribromocamphor is formed), followed by working-up the crude reaction mixture with zinc and hydrobromic acid to effect selective debromination, provided 8-bromocamphor (456) in a regiospecificprocess. Prolonged treatment of c(,/-camphor with excess BF3, OEt2 and ethyl diazoacetate in ether afforded a 2 1 mixture (63 % yield) of the camphene derivatives (457) and (458). Reaction of the mixture with zinc dust and glacial acetic acid at reflux gave (457) in essentially quantitative yield from (458). ... 

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