Boron tribromide elimination

Dimethyl-4-bromo-27/-chromenes are accessible via a boron tribromide induced cyclization of phenol derivatives 31 (Scheme 9). The reaction is proposed to proceed via O-deprotection and subsequent elimination of hydroxide to form the intermediate zwitterionic species 32. Addition of bromide, pyran ring closure and protonation provides 4-bromo-2/7-chromenes in moderate yield (Scheme 9) < 1997H(45) 1131 >. 

Aluminum and boron halides are sometimes used to dealkylate alkyl aryl ethers to phenols. Boron tribromide cleaves aliphatic ethers to alcohols and alkyl halides, but the reaction has no preparative value in the aliphatic series. Aluminum halide and the ether first form a complex from which a molecule of alkyl halide is eliminated upon heating. 

Homochiral epoxides are versatile intermediates for the synthesis of a variety of natural products. The four-carbon bifiinctional chiron (i )-l- erNbutyldimethylsilyl-3,4-epoxybut-l-yne (228) is conveniently prepared from 141 as shown in Scheme 53. The conversion of 141 to chloride 225 followed by base-induced chloride elimination in liquid ammonia proceeds without any detectable epimerization (as determined by both hplc and nmr analysis of the corresponding Mosher ester) to provide the i -alcohol 226 in good yield. Subsequent silyl protection followed by treatment with boron tribromide results in a highly stereoselective bromination, together with simultaneous debenzylation to the bromohydrin 227, which under mild basic conditions is converted to epoxide 228. The optical purity of 228 (ee = 99%) demonstrates the high selectivity in this new bromination reaction [80,81]. 

The closing sequence comes from Kenner C. Rice and comprises seven stages. [103] Acetalisation of the keto-function and elimination of methanol with p-toluenesulfonic acid in chloroform is followed by addition of methyl hypobromite. Ehmination of HBr and hydrolysis of the ketal leads to the unsaturated ketone, which is reduced stereoselectively with sodium borohydride to produce (-)-codeine. This generates the fifth stereogenic centre. The methyl ether is finally cleaved with boron tribromide. 

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