Isomerization dehydrohalogenation

Azetidine, 7V-bromo-, 7, 240 Azetidine, AT-r-butyl- N NMR, 7, 11 Azetidine, AT-t-butyl-3-chloro-transannular nucleophilic attack, 7, 25 Azetidine, 3-chloro-isomerization, 7, 42 Azetidine, AT-chloro-, 7, 240 dehydrohalogenation, 7, 275 Azetidine, 7V-chloro-2-methyl-inversion, 7, 7 Azetidine, 3-halo-synthesis, 7, 246 Azetidine, AT-halo-synthesis, 7, 246 Azetidine, AT-hydroxy-synthesis, 7, 271 Azetidine, 2-imino-stability, 7, 256 Azetidine, 2-methoxy-synthesis, 7, 246 Azetidine, 2-methyl-circular dichroism, 7, 239 optical rotatory dispersion, 7, 239 Azetidine, AT-nitroso-deoxygenation, 7, 241 oxidation, 7, 240 synthesis, 7, 246 Azetidine, thioacyl-ring expansion, 7, 241 Azetidine-4-carboxylic acid, 2-oxo-oxidative decarboxylation, 7, 251 Azetidine-2-carboxylic acids absolute configuration, 7, 239 azetidin-2-ones from, 7, 263 synthesis, 7, 246... 

As reported by Shani and Sondheimer,1 the dehydrohalogenation of the tetrabromide by means of potassium hydroxide in ethanol at 50-55° affords a mixture, which is readily separated by chromatography on alumina, of l,6-oxido[10]annulene and the isomeric 1-benzo-xepin. The latter compound is also formed during chromatography of l,6-oxido[10]annulene on silica gel.7... 

Formal isomerization of the double bond of testosterone to the 1-position and methylation at the 2-position provides yet another anabolic/androgenic agent. Mannich condensation of the fully saturated androstane derivative 93 with formaldehyde and di-methylamine gives aminoketone 94. A/B-trans steroids normally enolize preferentially toward the 2-position, explaining the regiospecificity of this reaction. Catalytic reduction at elevated temperature affords the 2a-methyl isomer 95. It is not at all unlikely that the reaction proceeds via the 2-methylene intermediate. The observed stereochemistry is no doubt attributable to the fact that the product represents the more stable equatorial isomer. The initial product would be expected to be the p-isomer but this would experience a severe 1,3-diaxial non-bonded interaction and epimerize via the enol. Bromination of the ketone proceeds largely at the tertiary carbon adjacent to the carbonyl (96). Dehydrohalogenation... 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

By far the most thoroughly investigated azocines are the 2-methoxy derivatives prepared and studied by Paquette and coworkers (7lAG(E)ll). The synthesis involves addition of chlorosulfonyl isocyanate to a cyclohexadiene, conversion to the imidate and introduction of another double bond by allylic bromination and dehydrohalogenation. Valence isomerization then ensues and 2-methoxyazocine (98) or alkyl homologs are isolated from this sequence in multigram quantities as stable yellow oils. NMR data (see 98) clearly indicate the monocyclic azocine structure the spectra are invariant from -70 to 180 °C and indicate less than 2% of the bicyclic valence isomers. 

Olefinic azo compounds, in which the double bond is conjugated with the azo group, have been prepared from a-halocarbonyl compounds by reaction with a substituted hydrazine to form a hydrazone which, on treatment with a base, is dehydrohalogenated and isomerized to an olefinic azo compound. The reaction may be represented by Eqs. (19) and (20). 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

The second approach forms the triple bond by a double dehydrohalogenation of a dihalide. This reaction does not enlarge the carbon skeleton. Isomerization of the triple bond may occur (see Section 9-8), so dehydrohalogenation is useful only when the desired product has the triple bond in a thermodynamically favored position. 

A triple bond may be formed by dehydrohalogenation of dihalides and olefinic halides of the general types RCX= CHj, RCH= =CHX, RCH= =CXR, RCHXCHjX, RCHXCHXR, RCHjCHX, and RCXjCHjR. The choice of a base depends somewhat on the position desired for the triple bond in the product. Sodium amide tends to rearrange the triple bond toward the end of the chain," and potassium hydroxide favors reverse isomerization toward the center of the chain,Although neither rearrangement is dependable from a synthetic standpoint, it is best to choose the base favoring the desired product. 

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