Dibromide erythro

The bromination products, dibromide in methylene chloride and methoxybromide in methanol, are a mixture of erythro- and threo-diastereoisomers, obtained in a ratio, Erythro/Threo = 70 / 30, which does not depend on the substituents or on the solvent. As expected, the reaction in the protic solvent is fiilly regioselective, i.e. methanol only traps the intermediate... 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

The method of Horton and coworkers,123 just mentioned, for bro-mination in methanol in the presence of silver acetate, has also been applied to the 2,3-dideoxy-6-0-trityl-o -D-erythro-hex-2-enopyrano-glycan derived from a 2,3-di-0-p-tolylsulfonyl-6-0-trityl derivative of amylose a 2,3-dibromide having a degree of substitution of approximately 2.0 was obtained.124... 

Addition to alkenes1 and alkynes.2 The reagent converts (E)-alkenes into meso- or erythro-dibromides in CHC13 in 80-95% yield. It is also useful for selective a-bromination of ketals (75-80% yield). 

To account for the production of bromo chlorides and bromo nitrates, it is proposed that the reaction of anions (Br, Cl , NO ) in step 5 is very fast. Independent evidence for the existence ot a metastable, cyclic bro-monium ion comes from the observation that the bromine atoms are usually in a threo (trans) orientation in the final product and that in the reverse reaction of acetolysis of dibromides, there is retention of configuration. This would be consistent with the Br-assisted SnI ionization of the dibromide. The assisting Br atom would attack the neighboring C atom from the back side to the leaving Br ion in a typical Walden inversion. Thus from irans-butene-2 we obtain erythro-2,3-dihromo butane ... 

Aryl-substituted vic-dibromides undergo debromination to produce the corresponding E-alkenes when treated with indium metal in MeOH. Since debromination occurs by the usual trans-elimination, meso/erythro- and d,Z-/f/zreo-vic-dibromides would give trans- or cis-alkenes, respectively, as shown in Scheme 4.6. It is thus suggested that in this case the reaction occurs via a common relatively stable radical or anion intermediate, which directly collapses to -alkene. 

Upon ionic addition of bromine, cii-l-phenyl-1-propene gives a mixture of 17% erythro dibromide and ST/o- three /ra/i5-l-phenyI-l-propene gives 88% erythro, 12% threo and /m/75-l-(p-methoxyphenyl)propene gives 63% erythro, 37% threo. 

First, Winstein and Lucas found (Fig. 28.3) that (racemic) erythro bromo-hydrin yields only the meso dibromide, and (racemic) threo bromohydrin yields... 

They carried out the same reaction again but this time used opticaily active starting materials (Fig. 28.4). From optically active erythro bromohydrin they obtained, of course, optically inactive product the meso dibromide. But optically active erythro bromohydrin also yielded optically inactive product the racemic dibromide. 

Debromination of vic-dihromides. rioDibromides are debrominated, usually in high yield, by treatment With excess sodium thiosulfate in DMSO (60°, 8 hr.). For example, erythro-stilbene dibromide is converted into trans-stilbene (99 % yield). The reaction is apparently irons stereospecific. 

Tike zinc, the new reagent effects trans-elimination of dibromides however, erythro- and threo-a-hydroxy bromides give the same mixture of cis- and transolefins.5 Likewise reduction of either cis- or fran.v-4-t-butyl-1 -chloro-1 -methyl-cyclohexane (1 or 2) affords predominantly (62 6%) cis-4-t-butyTTmethyl-cyclohexane (3).6 If butanethiol is added to the reduction (method of Barton and... 

In agreement with earlier studies on the chalcone dibromides , the dehydro-chlorinations of 4,4 -dichlorochalcone dichlorides are not stereospecific . The threo isomer yields the trans olefin chiefly (see (30) below), as expected for elimination of HX from an " anti conformation (see Section 2.3), but the erythro isomer gives a mixture of olefins in which the trans form predominates over the cis by a factor of 2 1 (31). 

The stereospecificity of halogen addition to the acid was studied for the gas-solid state with or without cyclodextrin and in carbon tetrachloride solution without cyclodextrin, and the typical result was shown in Table II. Bromination yielded eryt/2ro-dibromide stereospecifically in 100 % in three different conditions, but chlorination gave an excess of f/jreo-dichloride in 72-87 % yields to the erythro-lsomer. The composition of the reaction mixture showed no variation with extent of coTver-sion in three reaction states at various temperatures and the remaining acid had retained nearly 100 % isomeric purity. Since chlorinated products were found to be stable under the conditions, these lower stereospecificities in the chlorination might be caused by the different addition mechanism from that of bromination which proceeded stereospecifically. Similar result was observed for chlorine addition to... 

The acid in the narrower cavity of a-cyclodextrin did not react with bromine at all after 24 h, but reacted after exposure for a long time (see Table I). After 100 h of bromination, it converted in 61 % and yielded 40 % of a 2 % e.e, of erythro-dibromide with the opposite configuration to that of product obtained from the reaction in 3-cyclodextrin matrix, and many peaks in the X-ray diffraction pattern disappeared as depicted in Figure 3. a-Cyclodextrin alone kept highly crystalline in spite of exposure to bromine under the same condition except a little change in intensities of the diffraction peaks (Figure 4). 

The effect of racemization on this reaction was examined by using the optical active eryt/iro-dibromide ([a]g -27.2°, 40 % e.e. calculated from the value [a] -68.3° in literature22), as shown in Table IV. When the optical active dibromide included in 3-cyclodextrin was exposed to bromine at 25°C for 3 h, the e.e. of recovered erythro-dihro-mide, identified by NMR, decreased remarkably, its decrease corresponding to 78 % of racemization, and thoi the dibromide racemized perfectly after 10 h. In contrast, the optically active dibromide alone did not racemize even after 20 h of exposure to bromine. This result shows that the racemization was catalyzed by 3-cyclodextrin. 

Though only (-)-e2yt/3ro-dibromide was obtained on the bromination of trans-cinnamic acid, the chlorination gave + )-erythro- and threo-dichlo-rides the e.e. of the trans-adduct reached 37 % (calculated from the JJ IQO value [a]p +67.3°in literature " ) and the... 

Fig. 1. A change of X-ray diffraction diagrams of the inclusion complex of ethyl trans-cinnamate with a-cyclodextrin on the gas-solid bromination at 25oc, and a diagram of a mixture of a-cyclodextrin and erythro-dibromide of the ester...

No chlorination was observed in the gas-solid chlorination of ethyl trans-cinnamate included in the cavity of a-cyclodextrin at temperatures from -5 to 50 C for 20-45 hr, as shown in Table II. The chlorination of the ester gave mixtures of erythro- and t/ireo-dichlorides having optical activities, while (+)-erz/t/zro-dibromide was the only product on the bromination of the ester inclusion complex with 3-cyclodextrin (Table I). The [a] value in D line or 589 nm at 25 C cannot be found for the pure... 

In the present case, conjugate addition has the potential to result in two diastereomers. As in the case of the starting dibromide, these stereoisomers can be either erythro or threo. The distribution between these two products is important because it ultimately determines the ratio of products in the next step (third) of the reaction. 

Bromination of 4,4-dimethylchalcone with bromine (gas-solid reaction) gave optically active erythro-dibromide (6% ee) along with minor amount of a product (Scheme 4). 

In Mechanism 1 the initial Sn2 attack by the Te atom at a C-Br bond of the dibromide, leads to inversion of the configuration at the carbon. In consequence, i/zreo-dibromides will produce erythro-sa is 7 and e //zro-dibromides will produce i/zreo-salts 8 exclusively (Scheme 29.5). 

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