Demethylation boron tribromide

DEMETHYLATION Boron tribromide. Chromic anhydride. Dimethylformamide. Methylmagnesium iodide. 

Xn. Demethylation of Aryl Methyl Ethers by Boron Tribromide... 

Pyrrolobenzotriazine 43 was obtained by reaction of 42 with nitrous acid. Its demethylation with boron tribromide gave 44, whose oxidation... 

Demethylation reactions proceed equally well using dry M-pentane or dry methylene chloride as the solvent for both the ether and the boron tribromide methylene chloride, having by far the more powerful solvent action, is to be preferred. 

Chemoselective demethylation of 1247 by treatment with boron tribromide led to a 4-hydroxy compound, which, after reduction with diborane, was reacted with triflic anhydride (Tf20) to afford the benzyl derivative 1248. Palladium(0)-catalyzed... 

Oxindole 89 was cleanly demethylated upon treatment with boron tribromide. The resulting oxindole 90 was subjected to the prenylation conditions, and the desired alkylated product 91 was obtained in 52% yield. The epoxidation/Lewis acid-mediated cyclization proved to be successful on this substrate. The epoxide product was directly treated with SnCl4 in THF to provided the desired 92. When oxindole 92 was treated with NaBHt (1.6 equivjand BF3 OEt2 (3.5 equiv) in THF, the desired 93 was obtained. The indole 93 was treated with TBDMSC1 and imidazole in DMF, to provide the required O-silylated indole, which was easily converted to the gramine 94 through the well known Mannich procedure. 

Since arucadiol and miltirone both have an aromatic "B" ring, enone 64 served as a common intermediate for both of these quinone pigments. The aromatization of 64 was readily achieved using 2,3-dicyano-5,6-dichloro-l,4-quinone (DDQ) (Equation 5.2). With substrate 65 in hand, only demethylation of the ethers was required to complete a synthesis of arucadiol (58). This transformation was accomplished in nearly quantitative yield using boron tribromide. Our synthetic arcudiol was spectrally identical with the natural material. 

Heterogeneous reactions lend themselves to continuous flow reactors, which are desirable as they minimise the reacting volume. This reduces operation risks, and allows smaller, more efficient plants to be built. Flow reactors designed for fluorous reactions with both liquid and gaseous substrates have been demonstrated to be effective, at least on a bench scale [49]. Fluorous solvents have also recently found applications as liquid membranes to control the rate of addition of reagents and so control exothermic reactions such as alkene bromination (Fig. 6), and demethylation of anisoles by reaction with boron tribromide [50], This has potential as a clean route as the kinetic control gives improved selectivity. 

In order to introduce a second hydroxyl group al C-l 1 of the ester (99) its sodium salt was treated with p-nitrobenzene diazonium chloride in methanol. The resulting product on methylation and then reduction with potassium dithionate afforded the amino ester (100). Diazotization and methanolysis produced ethyl ( )-camosate dimethyl ether (101), which on saponification produced camosic acid dimethyl ether (102). Wenkert [22] accomplished demethylation of dextrorotatory camosic dimethyl ether with boron tribromide. Thus the work of Meyer lacks only the... 

A new method for the demethylation of codeine to morphine, previously a capricious reaction, has been reported, the product being obtained in good yield. Demethylation by boron tribromide in chloroform gives 90—91%150 and by potassium t-butoxide in propanethiol gives 80% morphine.151 A patent describes an improved method for the preparation of codeinone from thebaine, by adding the alkaloid to anhydrous hydrogen bromide in solution in methylene chloride and dibutyl ether at -20 °C, in the presence of small quantities of iodine, followed by hydrolysis with aqueous sodium bicarbonate. The claimed yields of codeinone are 95% crude and 90% after purification.152 Codeinohe is an intermediate in the conversion of thebaine into codeine. An overall yield of 85% of codeine from thebaine, without purification of any of the intermediates, has been claimed for an... 

FAO (131) was synthesized from the 7a-amino-6,14-encfo-ethenomorphi-nan (140) which is available from thebaine in three steps as described by Bentley and his coworkers [201], Selective O-demethylation of the phenolic ether function of (140) with boron tribromide afforded the phenol (141) which was treated with methylfumaroyl chloride to yield FAO (131) [96],... 

As an intermediate of their synthesis of [2.2]metaparacyclophane-quinones, Tashiro et al. obtained 70, that could not be completely demethylated with boron tribromide [37], Using the tert-butyl group as a positional protecting group, these authors also tried the synthesis of [2.2]metacyclophanes 72a/b containing in-traannular halide-substituents [38], The preparation of the difluoro compound... 

Deethylation. Boron tribromide cleaves the ethyl ethers of phenols at 20° in yields consistently greater than 90%. The yields of phenols on demethylation of the corresponding methyl ethers with BBr, are erratic (15-90%). ... 

Three successive [2+4] cycloadditions were used in the synthesis of the pentacyclic methyl ether of G-2N by Kraus and Zhao [92] and later, by a slightly modified procedure, also of the natural product G-2N (118) [93] (Scheme 31). Thermal reaction of the cyclobutanol 112 with acrylic ester gave the dihydronaphthalene 113 which was demethylated by treatment with boron tribromide and converted into the exocyclic ketene acetal 114. This unstable diene was reacted in a second cycloaddition with 2,6-dichlorobenzoquinone (115) to afford the tetracyclic chloroquinone 116. In a last Diels-Alder reaction, ring E was anella-ted by treatment of 116 with l-methoxy-l,3-bis[(trimethylsilyl)oxy]-l,3-buta-diene (117) to yield the pentacyclic natural product G-2N (118) [93]. 

Quinazolinones 1 bearing a methoxy substituent in the benzene ring, readily obtained by cyclization of the appropriate precursors (cf. Section 6.3.1.1.1.), are transformed into the corresponding hydroxyquinazolinones 2 on treatment with hydrobromic acid, - hydriodic acid, aluminum trichloride,or boron tribromide. Similarly, quinazoline-2,4-diamines bearing a methoxy group at position 6 or 8 are demethylated with boron tribro-mide in dichloromethane, sodium methanethiolate in dimethylacetamide or by pyridini-um hydrochloride fusion at 200... 

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