Dehydrohalogenations, based-induced

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],... 

The isolation of compounds containing simple C=P double bonds parallels the triple-bond work. The first stable acyclic phosphaalkene was synthesized over fifteen years ago.17 Again, base-induced dehydrohalogenation and stabilization by bulky groups is important ... 

The double sequence of dehydrohalogenation of a 7,7-dihalobicyclo[4.1.0]heptane derivative to give a bicyclo[4.1.0]hept-l(7)-ene, followed by base-induced prototropic shifts to move the double bond into the six-membered ring132-141, finds continuing applications in routes to cycloproparenes100,142 ... 

Sec. 8.2) from dihalides by base-induced double dehydrohalogenation... 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

The intermediacy of system (23), the aza analogue of the corresponding thiirene dioxide, has been established in the base-induced successive dehydrohalogenations of the a,a-dibromobenzyl-sulfonamide (24) to provide (26) which afforded benzonitrile (27) on losing its S02 moiety (Scheme... 

Base-induced 1,3-dehydrohalogenation of a-halo ketones 1 leads to cyclopropanones 2, but these reactive species almost always react further with the nucleophiles present to give addition products at the carbonyl group. The resulting adducts may be isolated, but in most cases react further under the influence of base to ring open to carboxylic acid derivatives 3a, b. 

Nitrile oxides could also be prepared from nitroalkanes by reaction with (Boc)20 in the presence of catalytic amounts of DMAP at room temperature under N2 [276]. Alternatively they could also be prepared via base-induced dehydrohalogenation from hydroximinoyl chlorides, which can be either purchased or generated hy chlorination of the corresponding aldoximes with N-chlorosucdnimide. 

The formation of arynes has often been achieved by base-induced dehydrohalogenation, but for the formation of 3,4-didehydrothiophene, a fluoride-induced process can be used, following ipso electrophilic displacement of one of the silicons from 3,4-bis(trimethylsilyl)thiophene to generate the appropriate precursor. ... 

The mechanisms of base-induced beta-eliminations leading to alkynes are highly dependent on the type of base employed, and for this reason the dehydrohalogenations induced by metal alkoxides are considered separately from those initiated by metal amides or metal alkyls. 

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