Pyridinium bromide perbromide in the

Treatment of 2,7-di-/ert-butylthiepin (1) either directly with bromine at — 78 °C, or with pyridinium bromide perbromide at room temperature, gives the thiophene compound 2. In contrast, bromination with bromine-1,4-dioxane complex or pyridinium bromide perbromide in the presence of acetic acid leads to the thiopyran derivative 3.87 To account for these results a homothiopyrylium ion has been proposed as a common intermediate, formed by electrophilic bromination at C4 in the first step. 

Reagents and Equipment. Weigh and place 500 mg (3.37 mmol) of trans-cinnamic acid in a 25-mL round-bottom flask containing a magnetic stir bar. HOOD Now weigh and place (in the hood) 1.2 g of pyridinium bromide perbromide in the same flask. 

Acetoxyandrost-5-en-17-one (59) is converted into the ethylene ketal (60) by treatment with ethylene glycol, triethylorthoformate and p-toluenesulfonic acid. The ketal is brominated with pyridinium bromide perbromide in THF and then treated with sodium iodide to remove bromine from the 5 and 6 positions. This gives the 16a-bromo compound (61) which is hydrolyzed in methanol to the free alcohol (62). Dehydrobromination is effected with potassium Fbutoxide in DMSO to give the -compound (63). Acid catalyzed hydrolysis of the ketal in aqueous acetone gives the title compound (64). ... 

The transformation is also possible with hexamethyldisilane and pyridinium bromide perbromide as the source of bromine. This reaction, however, is slower in the case of primary and secondary alcohols. Benzylic, allylic, and tertiary alcohols react rapidly. Hence selective reactions are possible. In the case of secondary alcohols, the conversion occurs with 88% inversion. 

Treatment of the dihydropyridine diester (105) with pyridinium bromide perbromide in chloroform give the lactone (107), via the unstable bromi-nated intermediate (106) (Scheme 21) (84S617 90TL1479). 

Remove the condenser from the flask, and to the warm solution, in the hood, add 1.2 g (3.7 mmol) of pyridinium bromide perbromide in one portion. Wash down any perbromide adhering to the sides of the flask with an additional 6 mL of acetic add using a Pasteur pipet. Reattach the air condenser. 

Trimethylsilyl iodide 17, which can be generated in situ by reaction of trimethyl-silyl chloride (TCS) 14 with Nal in acetonitrile [1], converts alcohols 11, in high yields at room temperature, into their iodides 773a, HI, and hexamethyldisiloxane (HMDSO) 7 [1-8, 12]. Likewise esters such as benzyl benzoate are cleaved by Me3SiCl 14/NaI in acetonitrile under reflux [Ij. Reactions of alcohols 11 with trimethylsilyl bromide 16 in chloroform or, for in situ synthesis of 16 from liBr and TCS 14 in acetonitrile and with HMDS 2 and pyridinium bromide perbromide, proceed only on heating in acetonitrile or chloroform to give the bromides 773 b in nearly quantitative yield [3, 8, 12] (Scheme 6.1). 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

DetkioketaKzation. Various thioacetals and thioketals are readily hydrolyzed by pyridinium bromide perbromide (1 equivalent) under phase-transfer conditions. Tetrabutylammonium bromide is used as catalyst and aqueous methylene chloride as solvent. The reaction is more efficient in the presence of pyridine as buffer. Yields are generally 75-90%.1... 

Pyridinium bromide perbromide efficiently brominates pyrroles already substituted by electron-withdrawing substituents and also gives a high yield of 3-bromoindole in its reaction with indole. In conjunction with sulfuryl chloride, it has been used in the synthesis of 3-bromo-2-chloro-, 2-bromo-3-chloro- and 2,3-dibromo-indole (81SC253). 3-Methylindole reacts with A-bromosuccinimide in acetic acid to give the 2-bromo derivative which reacts further with an excess of A-bromosuccinimide to yield 2,6-dibromo-3-methylindole (B-70MI30500, 72HC(25-2)127) whilst in aqueous or alcoholic media, 3-bromo-3-methylox-indole is produced (cf. Scheme 15). All of these reactions proceed via the 3-bromo-3A-indolium cation, but the course of the reaction depends not only upon the orientation or... 

The enol acetate 77 of 3,4-dihydro-7-methoxy-5-methyl-l-(2l/)-naphthalenone was converted to the acid 78 by ozonolysis and hydrolysis and this by a Wittig reaction with a-methoxyethyltriphenyl-phosphonium chloride gave 79. Compound 79 was converted into 80 by a series of reactions, five in number, which in turn was converted into 81 by reaction with potassium in -butanol. The methyl ester of compound 81, one isomer of which was recognized as that having the correct stereo structure, was converted to 82 by heating with acetic anhydride and 10-camphorsulfonic acid. Subsequent steps involved ozonization, reaction with V,iV -carbonyldiimidazole, lactam formation, reaction with pyridinium bromide perbromide, reaction with sodium hydride, and a further series in which (+ )-oxodendrobine (83) was ultimately obtained. Reduction of the latter to ( )-dendrobine... 

In Fig. (12) keto ester (94) was selected as starting material. It was converted to the formyl derivative (95) which yielded a,P-unsaturated aldehyde (96) by treatment with DDQ. Michael addition of the sodium enolate of tert-butyl- isovalerylacetate to aldehyde (96) afforded the adduct (97) as a mixture of C-ll diastereomers. By fractional crystallization one of the adducts could be separated but for the synthetic purpose the mixture was not separated. Treatment of the adduct (97) with p-toluenesulfonic acid in glacial acetic acid caused t-butyl ester cleavage, decarboxylation and cyclodehydration leading the formation of tricyclic enedione (98) in 80% yield. This approach was previously utilized by Meyer in the synthesis of nimbiol [29], Treatment of (98) with pyridinium bromide perbromide, followed by hydrogenolysis with palladium and carbon caused aromatization of (98) leading the formation of the phenolic ester (99). 

Other acids can be used to promote chlorination or bromination. N-Bromosuccinimide and HBF4 can be used to brominate phenols with high pnra-selectivity, as can pyridinium bromide perbromide, and NBS in acetic acid with ultrasound is effective. The use of NBS with a catalytic amount of HCl has also been reported. Both NCS and NBS with aqueous BF3 gave the respective chloride or bromide. Note that NBS in an ionic liquid gave the brominated aromatic. Bromine on silica gel gave good yields of the brominated aromatic compound. HBr with hydrogen peroxide... 

Bromination of an olefin and double dehydrobromination of the resulting 1,2-dibromide is a classical method for the generation of 1,3-dienes (Table 1). Bromination of a double bond can be done with molecular bromine or, more conveniently, with pyridinium bromide perbromide . A variety of bases has been employed for dehydrobromination. While potassium hydroxide and sodium methoxide have been used for a long time, lithium carbonate-lithium chloride in DMF or hexamethylphosphoric triamide (HMPA) works well in many cases . Double dehydrobromination with hindered bases such as potassium r-butoxide or diazabicyclononene (DBN) and diazabicycloundecene (DBU) give good results. 

The compound is a brominating agent similar to pyridinium bromide perbromide but more stable. It is very soluble in tetrahydrofurane and is employed in this solvent for the a-bromination of ketones and cyclic ketals, for example ... 

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