Bromohydrin ester

In accordance with this interpretation, the dibromide rearrangement showed rather low susceptibility to changes in the polarity of the solvent, whereas the rates of reaction of a series of bromohydrin esters (3,4) were more sensitive to solvent character [5]. 

The stereospecific preparation of cis- and tra s-oxiranes can be achieved from the same diol via the acetal. The trans-oxirane is obtained via the glycol monotosyl ester and the c/s-oxirane via the bromohydrin ester (Eq. 53). ... 

In 2013, Braddock s group reported that enantiospedfic polyene cyclization was initialed by the formation of an enantiopure bromiranium ion (Scheme 9.10) [18]. Enantiopure bromohydrin esters were synthesized from the corresponding olefins in a three-step sequential reaction. The reaction gave... 

Bromohydrins can be prepared direcdy from polyhydric alcohols using hydrobromic acid and acetic acid catalyst, followed by distillation of water and acetic acid (21). Reaction conditions must be carehiUy controlled to avoid production of simple acetate esters (22). The raw product is usually a mixture of the mono-, di-and tribromohydrins. 

Conversion of bromohydrins to alkenes. The 14,15-epoxide (1) of arachidonic acid methyl ester has been converted into the 11,12-epoxide (5) in two steps. The first is conversion of 1 into a mixture of isomeric bromohydrins (2 and 3) with KBr. The mixture is then epoxidized under the conditions of Sharplcss (5, 75 76) to give, after chromatographic purification, 4, the epoxide of 2. Conversion of 4 into 5 presented a problem, but was eventually achieved by treatment of the w c-bromo Inflate with P[N(CHj)2]j as a Br acceptor. 

See also page 251, Section 2, for elimination via diols, epoxides, /1-dihalides, /J-halo esters and bromohydrins. Reviews ... 

Resolution of bromohydrifts.2 Diastereoisomeric esters of bromohydrins with MTPA are readily separated by fractional crystallization and characterized by NMR. The optically pure bromo MTPA esters are convertible by known methods into chiral alcohols, diols, and epoxides, including arene oxides. 

Most of the published syntheses of XIV have been previously summarized (2). One of the more practical large-scale syntheses of XIV in high purity is the one outlined in Figures 3 and 4, which is based on an earlier synthesis by Jacobson and co-workers (41 see also 20b). Thus the diol is converted to the bromohydrin VI by reaction with aqueous HBr in a two-phase system. The crude product is purified by conversion of the alcohol group to a non-volatile borate ester with triethyl borate and removal of the 1,8-dibromooctane by distillation. After hydrolysis of the borate any residual diol is removed by extraction with water. 

The synthesis of 6a-methyldigitoxigenin acetate (394) has been reported according to Scheme 19.198 Pregn-4-en-21-ol-3,20-dione was converted into its 6a-methyl derivative (387) using a previously described five-step reaction sequence biological hydroxylation furnished the 14a,12-diol (388) and reduction of the derived 21-acetate gave the 5/3-dihydro-steroid (389). Dehydration furnished the A14-olefin (390) which was converted into the 21-mesylate and thence into the lactone (391) by reaction with the monoethyl ester of malonic acid. The crude lactone was decarbox-ylated, reduced to the 3/3-alcohol (392), and converted into the bromohydrin (393) via its 3/3-acetate and thence by debromination into 6a-methyldigitoxigen 3-acetate... 

If desired, glycidic esters can be derived from a,3-dihydroxy esters, such as (110), by either of two methods. In one method, reaction of the diol with an arenesulfonyl chloride is regioselective, producing the a-arenesulfonate (111) in preference to the 3-sulfbnate. Treatment of (111) with an equivalent of base produces the erythro-glycidic ester (112) in good yield. In the second method, the diol is converted to a bromohydrin (114) via the acetoxy bromide (113). The brotmhydrin (114) affords the threo-g ycidic ester (115) on exposure to potassium caitxHiate in methanol. 

Cleavage of lactones and carbonates. Lactones and carbonates react with bromotrimethylsilane to afford bromocarboxylic acid derivatives (equation I) and bromohydrin trimethylsilyl ethers (equation II), respectively acyclic, aliphatic esters do not react with bromotrimethylsilane. lodotrimethylsilane reacts in an analogous fashion with lactones, but in reaction with ethylene carbonate the main product is 1,2-diiodoethane (equation III). The >-bromocarboxylate derivatives are converted into acid chlorides by reaction with SOCL (equation I). 

Products with low enantiomeric purity are obtained by direct application of this chemistry to unsubstituted acetate esters. However, aldol reactions of f-butyl bromoacetate mediated by (5) afford synthetically useful bromohydrins (6) with high selectivities (eq 8). h These may be reductively dehalogenated or converted to a variety of compounds by way of the derived epoxides. 

Transesterification can be used to cleave the acyl group from an ester to release the alcohol. The mildness of the reaction conditions enables chemoselective transformation. A siloxy group /S to a ketone group was not eliminated (Eq. 225) [524], and formation of an epoxide from the unprotected bromohydrin did not occur (Eq. 226) [525]. Similarly, in the synthesis of an avermectin derivative, delactonization was carried out by the titanium-based method as shown in Eq. (227) [526]. 

The plant bufadienolide scillarenin (500) has been synthesized. The starting material was 15a-hydroxycortexone (501), which was converted into the diketone ketal (502) by cupric acetate oxidation at C(21), followed by selective ketalization and tosylate elimination. Protection at C(3) as the dienol ether, oxiran formation at C(20) with dimethylsulphonium methylide, and regeneration of the C(3)- and C(21)-oxo-groups by acid hydrolysis then provided (503). Selective reaction at C(21) with the sodium salt of diethyl methoxycarbonyl-methylphosphonate, and boron trifluoride rearrangement of the epoxide ring to the aldehydo-unsaturated ester (504), was followed by enol lactonization to the bufadienolide (505). This was converted, in turn, to scillarenin (500) via the 14,15-bromohydrin, by standard reactions. Unsubstituted bufadienolides have also been prepared by the same method. 

The ring-closure mechanism of 2-chloroethanol has been studied on the basis of kinetic and equilibrium chlorine isotope effects. Epoxidation of the terminal double bond of farnesyl acetate has been achieved via the bromohydrin, obtained with NBS. A stereospecific method has been elaborated for the preparation of 1-alkynyloxiranes, starting from the monotosylate ester of acetylenic diols. 1-Alkynyloxiranes are also formed from a-hydroxy quaternary ammonium salts in alkaline medium (Eq. 57). ... 

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