Bromine-Silver acetate

ALLYLIC ACETOXYLATION Bromine-Silver acetate. Palladiunli(II) trifluoroacetate. 

Partial oxidation of 158 by bromine yields bromine-free 159 together with mono-, di-, and tribromo derivatives. Other useful reagents are nitrous acid (54JCS286) or the one-electron oxidants silver acetate (in substoichio-metric quantity) and cerium(IV) ammonium nitrate (only for the methyl ether of 158 86BCJ511). 

Alternatively, l-oxo-l,2,3,4-tetrahydroquinolizinium ion (111 Scheme 73) and its congeners may be converted to derivatives of 1-hydroxyquinolizinium ion (63JCS2203). The bicyclic ketone (111) can be brominated to yield the 2-bromo (112) or the 2,2-dibromo derivative (113). Action of boiling acetic anhydride converts the monobromo ketone (112) to 2-bromoquinolizinium bromide (Table 2). If the same bromo ketone (112) is heated with ion-exchange resin (Amberlite IRA-400) loaded with hydroxyl ions or, alternatively, heated with silver acetate in water, the 1,2-dihydroxyquinolizinium ion is formed and may be recovered as the picrate (Scheme 74). 

In another study,125 methyl 5-0-benzoyl-2,3-dideoxy-/8 D-g(t/cero-pent-2-enofuranoside (81) was treated with bromine in methanol in the presence of silver acetate and barium carbonate the two monobenzoates (shown as 84) of methyl 2-bromo-2-deoxy-j8-D-xylofurano-side were obtained. A rationalization for this reaction involves formation of an intermediate bromonium ion (82), produced by attack of Br on the less-hindered side of the double bond, which then undergoes trans-attack by the benzoate group at C-5 to give the benzoxonium intermediate (83) the monobenzoates (84) are obtained on processing of the reaction mixture with water. 

Lemieux and Fraser-Reid174 have reported that the reaction of methyl 3,4,6-tri-0-acetyl-2-deoxy-2-iodo-j3-D-glucopyranoside (137) with bromine and silver acetate in acetic acid containing potassium acetate gives an almost quantitative yield of 1,3,4,6-tetra-O-acetyl-2,5-anhydro-D-mannose methyl hemiacetal (139), obtained as an... 

Displacement of the bromine atom in 4-bromo-4-deoxy-DL-threonic acid (7) with silver acetate gave15 DL-threonic acid (10). 

When acetobromoglucose was treated with moist silver oxide, a mixture of bromine-free acetates was formed from which was obtained, besides glucose tetraacetate, a small proportion of a crystalline disaccharide octa-... 

Ag2Se (c, II). Fabre2 measured the heats of reaction of silver selenide with bromine water and of aqueous silver acetate with gaseous hydrogen selenide. See also Pelabon1 and Tubandt and Reingold.1... 

Treatment of the steroid derivative 1 with bromine and silver acetate in a mixture of chloroform and pyridine at low temperature followed by quenching of the reaction mixture with aqueous acid gave a mixture of compounds, one of which was shown to be 2. 

The bromine atom on carbon 1 of the dibromide is very reactive. This reactivity corresponds to that of the bromine of tetraacetyl-D-glucopyranosyl bromide further increased by the proximity of the second bromine atom. By treatment with silver acetate the very labile bromine atom can be exchanged with an acetoxy group to form 1,3,4,6-tetraacetyl-2-desoxy-2-bromo-D-hexose (IX).8 The 3,4,6-triacetyl-2-desoxy-2-bromo-... 

Chlorination by sulfuryl chloride occurs at position 3 for l-oxa-6,6aA4-dithiapentalenes (71AHC(13)161, p. 181). Bromination of l,6-dioxa-6aA4-thiapentalene occurs readily and leads to the 3,4-dibromo derivative. With the same compound, iodine and silver acetate give the 3-iodo and the 3,4-diiodo derivatives (72CC1283). 

Pyridine /V-oxide is unreactive toward iron-catalyzed bromination at 110°C (55JA2902), but silver sulfate-catalyzed bromination in sulfuric acid at 200°C gives a 10% yield of 2- and 4-bromination in the ratio 1 2 (6ITL32). With bromine in oleum the main product is 3-bromopyridine /V-oxide (60%) together with the 2,5-dibromo (—35%), and 2,3- and 3,4-dibromo compounds (—5%) (62T227). Presumably the N-oxide function is here complexed with sulfur trioxide, which causes deactivation and 3-orientation. Bromination in acetic anhydride also gives 3-substitution (35%) an addition-elimination mechanism has been proposed (65JPJ62). 

AUylic acetoxylation. Some years ago Petrow reported that the reaction of dehy-droisoandrosterone (1) with bromine and silver acetate resulted in acetoxylation at C4 to give 2. The reaction proceeds via the 5a,6p-dibromide (a). The same reaction is observed... 

Alkoxy radicals produced by homolysis of hypohalites can attack non-activated C-H bonds, and lead to tetrahydro-furanoid structures after basic treatment of the intermediate halo-alcohols. Hypochlorites of [2y,2g] and C(20)-tertiary alcohols [28,2gJ gave 6 8,19- and 18,20-oxido derivatives respectively on irradiation and hydrolysis, but the more reactive hypobromites and hypoiodites are now preferred. Akhtar ef al. [30] distinguish between two modes of reaction of a 6jS-hypobromite (2), generated in situ by treating the alcohol with bromine and silver acetate. Experiments with... 

The conversion of 5a-cholestan-6/5-ol into the 6yS, 19-epoxide, by reaction with bromine and silver oxide, has been improved by the use of certain silver salts with bromine in pentane, in the dark. Silver acetate, carbonate, and... 

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