Bromide intermediates, epoxide

Important related studies have been published by House, tJ- u and by Naqvi end co-workers,1218 on the subject of magnesium bromide-initiated epoxide isomerization. Their results, discussed in section IV.2.A, suggest that abnormal addition of Qrignard reagents to dicyelio a-haloketones does not proceed by way of an ephemeral poxide intermediate, in accordance with the view expressed previously by Gciesman and Akawie.4 4... 

The first poly-O-acetylaldosyl azide was synthesized by Bertho by treating tetra-O-acetyl-a-D-glucopyranosyl bromide with a metal azide. In addition to the aldosyl azide derivatives that are known (see Table IV), derivatives of 2-amino-2-deoxy-D-glucosyl azide have been prepared - (see Table V). If the trans hydroxyl group at C-2 is not acetylated, an intermediate epoxide of the type suggested for the aldosyl fluorides can form, and the azide obtained has the. same anomeric configuration as the original halide, as. shown by the formulas. The acetylated azides, like the... 

The release of ring strain in epoxides is probably responsible for the high reactivity of these special ethers. HI opens epoxides under mild conditions stereospecifically to iodohydrins (Scheme 26). The mechanism is similar to the reaction of bromide with epoxides (see Section 1.7.3.3). It should be noted, however, that reduction of epoxides to alkenes may occur if vicinal diiodides are intermediately formed, which can lose I2 under the reaction conditions. With the combination of acyl chloride and Nal unstable diiodides are avoided and 2-iodoethyl esters are formed from oxiranes (Scheme 27). ° ... 

In the enantioselective synthesis of (+)-pah.sadin B 46, (- -)-12-hydroxy-palisadin B 47, (+)-palisadin A 48, and (—)-aplysistain 49, the key bromide intermediate 45 was formed with this method. Treatment of epoxide 44 with LiBr and pyridinium p-toluenesulfonate (PPTS) in A-methyl-2-pyrrolidone (NMP) led to the corresponding bromohydrin 45 (Scheme 42.15). 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Chiral PTC has been used effectively for making intermediates for drugs. Dolling and coworkers have used 8-R, 9-5, N-(p-trifluoromethylbenzyl) cinchonium bromide to carry out an important asymmetric alkylation, giving 95% ee (Starks, 1987). Nucleophilic epoxidations of enones, Darzens reaction, Michael additions, etc. are some examples of reactions rendered asymmetric through chiral PTCs (Nelson, 1999). 

Campholenic Aldehyde Manufacture. Campholenic aldehyde is readily obtained by the Lewis-acid-catalyzed rearrangement of a-pinene oxide. It has become an important intermediate for the synthesis of a wide range of sandalwood fragrance compounds. Epoxidation of (+)- Ct-pinene (8) also gives the (+)-o -a-pinene epoxide [1686-14-2] (80) and rearrangement with zinc bromide is highly stereospecific and gives (-)-campholenic aldehyde... 

---