Bromo olefins

The cis-fagaramide (J) was synthesized as outlined below. The required acetylenic acid (c) was prepared from piperonal (a) by the Corey s procedure.Treatment of piperonal with carbon tetrabromide, triphenylphosphine and zinc gave the bromo olefin (b) as an oil in 71% yield. The bromo olefin (b) was treated with 2 equivalents of n-butyl lithium followed by quenching with dry ice to give acetylenic acid (c) in 54% yield. Treatment of (c) with excess thionyl chloride without solvent at 50 followed by addition of isobutyl amine in benzene gave the acetylenic amide (d) as a viscous oil in 96% yield. Partial reduction of (d) gave cis-fagarmide (7 ) in 89% yield. 

Julia and co-workers (87, 88) have developed a novel synthesis of homoallyl-ic bromides by rearrangement of cyclopropyl carbinols on treatment with hydrobromic acid. For instance, the secondary cyclopropyl carbinols 301 have been converted into trans-bromo olefins 302 with 90-95% stereoselectivity. 

The Newman projections 304 and 305 illustrate the required transition state geometries for a concerted process leading to the trans and cis-bromo olefins 302 and 303 respectively. Models show that severe non-bonded steric interactions between the C- and C3 methylene groups and the R group strongly destabilize arrangement 305 relative to 304 so that the predominant formation of the trans olefin 302 (from 304) is readily explained. 

Pancralistatin, [29], The first asymmetric total synthesis of (+)-pancratistatin (94) was reported by Hudlicky 130,26], Thus the bromo olefin 114 (Scheme 15), obtained by a-addition of copper nitrenoid generated from (N-tosyiimino) phenyliodinane to 45, was debrominated to the olefinic aziridine 117. The latter underwent Irons 1,2-ring opening with diarylcyancuprate... 

The element efFect is usually studied for the easily available chloro-and bromo-olefins, but larger differences are expected on comparing them to the fluoro-olefin. Only two systems have been investigated in this respect. In the reaction of piperidine with PhCO.CMe=CHX (X = F, Cl) at 30° in ethanol and in dimethylformamide the kF/ka values are 204 and 263, respectively (Beltrame et al., 1968). In the reaction of 1,1-diphenylvinyl halide with ethoxide ion in ethanol,... 

Fig. 4.23. <7wt -Selectivity of an E2 elimination as the reason for the chemoselective formation of a 1,3-diene instead of a bromo-olefin.

Finally treatment of ot,a-dibromomethyl alkohols with zinc and glacial acetic acid causes P-elimination of HOBr yielding E- and Z-bromo-olefins, as shown by D. R. Williams et al. for a variety of compounds 23) ... 

Several conjugated diolefins have been made by heating bromo olefins with solid potassium hydroxide or excess quinoline. In the latter case, the bromo olefins were made available by allylic bromination of olefins with N-bromosuccinimide. /S-phenylbutadiene is obtained in 46% yield by the action of pyridine on the corresponding secondary chloride. Chlorination of n-butyl chloride gives an isomeric mixture of dichlorides from which low yields (18-30%) of butadiene are obtained by passing the vapors over soda lime at about 700°. ... 

Kumaraswamy, S., and Swamy, K.C.K., A convenient route to aryl substituted chloro and bromo olefins. Tetrahedron Lett., 38, 2183, 1997. 

It involves treating the bromo-olefin (174) with butylcopper lithium to afford an organometallic derivative which adds to the carbonyl to give (175). 

An interesting transannular cyclization was uncovered by Helmlin-GER and Ourisson (772) who found that mere chromatography of bromo olefin (777) over silica gel (but not alumina) gives the cyclized product (172). Acetolysis of (171) gives both (172) and (173). Structure (172) is fully secured by an X-ray crystallographic analysis study (113). 

Scheme 6.57 Specific 7-exo-Mizoroki-Heck-cyclization of bromo olefin 198.

Jin and Weinreb [137, 138] reported enantioselective total syntheses of the 5,11-methanomorphanthridine Amaryllidaceae alkaloids (—)-pancracine and (-)-coccinine. They nsed a specific Mizoroki-Heck-cyclization to convert bromo olefin 197 into seven-membered exocyclic alkene 198 in good yield (Scheme 6.57). 

Bromo-Olefins from Verongia Sponges.—Full details have been published on the structural elucidation of the bromo-compounds Aeroplysinin-1, Aerothionin, and Homoaerothionin isolated from Verongia sponges. In addition, a new bromo-compound, designated Aeroplysinin-2, has been isolated from the same source, which has added further sustenance to biosynthesis commentary among this closely related group of compounds. 

Aromatic and aliphatic aldehydes are converted into terminal acetylenes with one extra carbon atom on treatment with bromomethyltriphenylphosphonium bromide and two molar equivalents of potassium t-butoxide. The first reaction step is Wittig olefination the second, dehydrobromination, is sometimes incomplete since the acetylenes are often contaminated with small quantities of 1-bromo-olefins e.g. Scheme 108). 

Z-Trisubstituted allylic alcohols are obtained from protected a-hydroxy-ketones in a lithium-free Wittig reaction and 1-bromo-olefins (and hence acetylenes) are produced from the reaction of Ph3P=CHBr with aldehydes. References to two syntheses of substituted-allenes have appeared Ph3P=C(R )C02R and R R C=C=0 (generated in situ from the acid chloride... 

Product distributions from the disproportionation of l,l,2,3-tetrachloro-3, 3-difluoropropene (2CFsCFCCl CClj- CFj Ca Cai+CFa,CCI CCIg 2CFCljCCl CCl4- CF,ClCa CCl, + CCl,Ca Ca,) at 50 C with aluminium halides, titanium tetrachloride, and antimony pentachloride have been determined. Formation of the bromo-olefin CF,-CCl CBrCl in the reaction with aluminium bromide was rationalized in trams of the HSAB concept, and related known reactions were interpreted similarly. Replacement of vinylic fluorine by iodine has been shown to occur udien tetrafluoro-ethylene or chlorotrifluoroethylene is treated with methyl iodide and aluminium chloride. ... 

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