Bromine elimination

It was envisaged that the enones were produced following abstraction of H-1 (a process facilitated by the ability of sulfur atoms to stabilize radicals on bonded carbon centers), radical bromination, elimination of hydrogen bromide to give substituted glycals, allylic bromination at C-3, and loss of acetyl bromide. In the formation of compound 6, hydrogen abstraction from C-5 was deemed to compete with that from C-1, and to lead to substitution at the former site with the formation of a relatively stable product. 

A convenient synthesis of thiazolo-fused 2-pyridones 41 has been developed <07JOC4917>. Thus, A2-thiazolines 39 undergo acyl ketene imine cyclocondensation with Meldrum s acid derivatives 38 to give the dihydro thiazolo-fused 2-pyridones 40, which transform to 41 via bromination-elimination process. Pyridones 41 are converted to the 2-substituted analogs 43 and the saturated products 42 by Heck coupling and conjugate addition, respectively. 

The initial scission of cyclobutane ring of available pagodane derivative 37 is achieved as a result of homolytic bromination. The tribromide 38, thus formed, underwent facile bromine elimination-fragmentation (and hydrogenolysis of the C-Br bond at the a-methoxycarbonyl center) under the action of metals to form diene 39. Diimide reduction of 39 affected only one double bond and gave product 40, which served as a common precursor for both 35 and 36. The... 

Trapping is usually a strong indication for the intermediacy of cycloalkyne, but adducts may be formed without their intermediate occurrence. Thus, the reaction of 1,2-dibromocyclobutene (27)- and 1,2-bromoacenaphthylene (29) with magnesium yielded the corresponding cycloalkyne adducts, 28 and 30, in 8% and 4% yield, respectively. However, it was shown that the reactions proceeded via addition of 16 to 27 and 29 followed by bromine elimination by magnesium to give 28 and 30. 

The Ph2GaI is prepared from Ph3Ga by bromine elimination. The products are dimers in the solid and in benzene. Molten PhGaCl2 is conductive. 

Thiazoles can be readily prepared from thiazolines either by oxidation (e.g., 13 to 14) <04CEJ71> or by bromination-elimination protocol (e.g., 15 to 16) <04H(63)773>. The exclusive formation of thiazole-thiazoline 16 indicates that the C-4 acyl group of the thiazoline is essential for the formation of thiazoles under CBrCb/DBU conditions. In the case of the thiazoline Weinreb amide 17, the thiazole formation is carried out using CBrC /DBU (17 to 20) or through a novel base-induced transformation presumably via aziridone 18 (17 to 19) <04T12139>. 

The trialkylbismuthines are highly refractive liquids which fume in air and have a tendency to explode when heated. Concentrated sulphuric acid causes violent decomposition, concentrated hydrochloric acid acting with less vigour but with a similar result. Treatment with chlorine or bromine eliminates an alkyl radical forming compounds of the type RgBiX, and with trimethylbismuthme, methyl iodide at 200 C. yields methyldi-iodobismuthine. Mercuric chloride converts triethyl-bismuthme into ethyldichlorobismuthine, whilst hydrogen sulphide breaks it down to bismuth sulphide. 

Having proved this synthesis tractable for the formation of bent alkynes, a similar approach was applied to synthesize [6]CPPA and [8]CPPA (12 and 13, Fig. 7). The shotgun -style macrocyclization reaction of 4,4 -diformyl-Z-stilbene under McMurry conditions provided moderate yields of macrocycles 9-11 (Fig. 7). These macrocycles were then subjected as a mixture to the same bromination/ elimination sequence that provided [4JCMPA [15]. 

Use of quinones like chloranil and DDQ has also been employed for this dehydrogenation. Halogenation-elimination with bromine or NBS has been a chemical means to promote the dehydrogenation. However, with the bromination/elimination protocol, one must be careful as aromatic halogenation can occur. 

Alkene Synthesis, a-Alkylation of ketones with Mes SiCH2l, followed by a bromination/elimination sequence, gives rise to a-methylene ketones (eq 5). ... 

Copper(II) bromide has been used to a-brominate diketotetraquinanes and to introduce a double bond into a prostanoid nucleus in a one-pot bromination-elimination procedure (eq 3). 3,7-Dibromo-2//,6//-benzodithiophene-2,6-diones (eq 4) and 5-bromo-4-oxo-4,5,6,7-tetrahydroindoles (eq 5) are prepared by the selective a-bromination of their respective ketone starting materials without bromination of the aromatic or heterocyclic rings. 4-Carboxyoxazolines are converted to the corresponding oxazoles using a mixture of CuBra and l,8-diazahicyclo[5.4.0]undec-7-ene (eq 6). ... 

The first synthesis 104, 105) of racemic thienamycin by the Merck group made use of the azetidinone (113) derived from acetoxybutadiene and chlorosulphonyl isocyanate (CSI). Reduction, hydrolysis and condensation with acetone gave the 1,3-tetrahydrooxazine (114). Introduction of the hydroxyethyl side chain by way of an aldol condensation produced predominantly the thermodynamically more stable tra s-P-lactam (115) as a mixture of R)- and (5)-isomers in the side chain. On removal of the acetone residue a proportion of the unwanted (5)-isomer crystallised from the mixture. The synthesis was continued with the mixture by way of the aldehyde (116) and the thio-acetal (117). Bromination, elimination and introduction of the iV(l) malonate residue gave (119) ready for an intramolecular alkylation reaction. 

Bromine in dry chloroform added dropwise with stirring at room temp, during 5 hrs. to a soln. of benzylidene-p-nitraniline in the same solvent, and kept 6 hrs. at 4° dibromide (Y 71%) treated 6 hrs. at 35° with thiourea in 90%-ethanol benzylidene-p-nitraniline. Y 63%. Also bromine elimination by other nucleophiles such as iodide, thiosulfate, thiophenolate, and thiocyanate ions, or thio-cresol s. Nasir-ud-Din et al., Ghem. Ind. 1967, 1875. 

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