Stereochemistry dibromides

The stereochemistry of the dibromides does not appear to affect the usefulness for regenerating olefins with zinc or chromous ion however, significant differences in ease of reduction have been observed. Thus, the diequatorial 3j5,4a-dibromocholestane (121) is 1 % reacted under conditions which give a 91 % conversion of the diaxial 3a,4j5-isomer (122). 

Ambrettolide is obtained from hibiscus and has a musk-like odor. Its preparation from a compound A is outlined in the table that follows. Write structural formulas, ignoring stereochemistry, for compounds B through G in this synthesis. Hint Zinc, as used in step 4, converts vicinal dibromides to alkenes.)... 

Structure and stereochemistry of the dibromides 2a-h were elucidated by various NMR methods including ID proton-proton NOE difference spectra and 2D... 

Thus, as expected from earlier work 36,37), the dioxabicyclization proceeds with inversion of configuration at the 3-position of the m-2-frans-3-dibromide 30, the stereochemistry at the 2-position being unaffected. However, experiments with individual diastereoisomers unexpectedly showed that the franj-2-m-3-dibromide 31, also reacts with silver trifluoroacetate, albeit less efficiently, to give the same bicyclic peroxide. We feel that this probably proceeds via an isomerisation (Eq. 25). 

Table 7 Stereochemistry of dibromides in the bromination of /8-methylstyrenes in methylenechloride rate-stereoselectivity relationship."...

Table 8 Solvent-dependence of the stereochemistry of dibromides from rafi.s-/]-methylstyrene bromination. cis- and...

Polarographic half-wave potentials Table 4.8) for the reduction of 1,2-dibromides are sensitive to the relative stereochemistry of the halide substituents... 

Considering stereochemistry, there are two stereoisomers of the starting alkene (cis and trans) and three stereoisomers of the product dibromide (the d-, /-, and meso-isomers). The reaction of bromine with (Z)-2-butene is shown in Figure 11.2, and the reaction of... 

An analogous reaction of (Z)-4-octene produced the f/treo-tellurium dibromide, which formed 5-methoxy-3-octene. The stereochemistry of this compound was not determined3. 

Mechanism 7-1 Dehydrohalogenation by the E2 Mechanism 304 Mechanism 7-2 Stereochemistry of the E2 Reaction 306 Mechanism 7-3 E2 Debromination of a Vicinal Dibromide 310... 

This may be illustrated by re-examining a reaction that we have already studied. Earlier, we saw that when a 1,2-dibromide was treated with iodine, the reaction followed a normal anti-E2 pathway to eliminate a bromine molecule, which meant that the threo isomer yielded the trans isomer. If this elimination reaction is performed when either, or both, of the bromine atoms is, or are, attached to a primary carbon (instead of a secondary or tertiary carbon as was the case when we first looked at this reaction), then a different stereochemistry results. When this experiment is performed with deuteriated derivatives, it becomes apparent that the threo isomer gives the cis isomer. Suggest an explanation for this result. 

Q3. When bromine is added to an alkene in solution in CC14, a dibromide is produced. Explain why, when bromine is added to the alkene 1 in aqueous solution, the product is a halohydrin, i.e. HO and Br have been added to adjacent atoms. What is the nature and stereochemistry of the product ... 

The electrophile does not need to be a proton. Bromine adds to double bonds to give the 1,2-dibromo compound (reaction 5.21). The intermediate bromonium cation here is believed to have the cyclic structure 27 rather than the open structure 28 because of the stereochemistry of the products, trans-But-2-ene (29) reacts with bromine to give the optically inactive meso dibromide 31 as a single compound (reaction 5.22) [m-but-2-ene gives an equimolar mixture of the two enantiomers (R,R)- and (S,S)-2,3-dibromobutane, which is also optically inactive]. 

The irregular centrotriindane 16 was functionalized by bromination to the stereoisomeric dibromides 121 [52] (Scheme 25). Subsequent hydrolysis or oxidation to the diketone 122 followed by reduction gave triindanediols 123 or 124 respectively, with defined stereochemistry. Two-fold Friedel-Crafts reaction of 123 in benzene yielded a C2-symmetrical triindane, 125, whereas the dibromides afforded mainly the diphenyltriindene 126. This compound represents one of... 

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