Dienes reactions with bromine

Bicyclo[2.2.1]hepta-2,5-diene, reaction with acetic acid to yield nortii-cyclyl acetate, 46, 74 Bicj clohexyl, 45, 61 2-Biphenylylmagnesium iodide, 46, 94 Bomyl chloride 46, 56 Bromination of y-butyrolactone, 46, 22 Bromine, reaction with i-butyrolactone in presence of red phosphorus, 46, 22... 

Propargyltrimethylsilanes are converted into 3-alkyl-3-halo-l,2-dienes by iodine or bromine. Reaction with acetyl chloride and AlClj results in a 3-acelyl-3-alkyl-l,2-dicnc, but in low yield. ... 

A"" -3-Ketones are more reactive than cross-conjugated A ""-3-ketones. A"" -3,3-CycIoethylenedioxy compounds can be easily prepared by acid-catalyzed reaction with ethylene glycol or by exchange dioxolanation. 3,3-Cycloethylenedioxy-A -dienes can be prepared from 3,3-cycloethy-lenedioxy-A -enes by allylic bromination and dehydrobromination. Acid hydrolysis yields A"" -3-ketosteroids. ... 

As in the case of the steroids, introduction of additional nuclear substituents yields morphine analogs of increased potency. The more important of these are derived from one of the minor alkaloids that occur in opium. Thebaine (14), present in crude opium in about one-tenth the amount of morphine, exhibits a reactive internal diene system that is well known to undergo various addition reactions in a 1,4 manner (e.g., bromination). Thus, reaction with hydrogen peroxide in acid may be visualized to afford first the 14-hydroxy-6-hemiketal (15). Hydrolysis yields the isolated unsaturated ketone (16). Catalytic reduction... 

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

The first step in this preparation, the epoxidation of 1,4,5,8-tetra-hydronaphthalene, exemplifies the well-known selectivity exerted by peracids in their reaction with alkenes possessing double bonds that differ in the degree of alkyl substitution.12 As regards the method of aromatization employed in the conversion of ll-oxatricyclo[4.4.1.01-6]-undeca-3,8-diene to l,6-oxido[10]annulene, the two-step bromination-dehydrobromination sequence is given preference to the one-step DDQ-dehydrogenation, which was advantageously applied in the synthesis of l,6-metliano[10]annulene,2,9 since it affords the product in higher yield and purity. 

Even simple dienes and polyenes are difficult to classify in comparison with alkenes. Whereas bromination, oxidation and reaction with tetranitromethane (TNM) can identify the number of double bonds and their location in the molecular structure, conjugated double bonds produce very complex mixtures. Furthermore, many of the tests based on 7r-complexation can also apply for aromatic moieties. An example is the TNM 7r-complex which is yellow with benzene and orange with naphthalene and the tests are therefore non-specific. 

After treatment of zirconacyclopentadienes with 2 equivalents of CuCl at room temperature, the addition of a halogenating agent such as NBS at —78 °C leads to the formation of octatetraenes 57a [7m]. This reaction involves slow bromination of the diene-dicopper compounds and coupling with the alkenyl bromide moiety (Eq. 2.40). 

Addition of the dicyanomethyl radical to propadiene (la) occurs exclusively at Q (not shown in Scheme 11.8) [60]. On the other hand, methyl-substituted allenes, e.g. Id, undergo /3-selective reactions with 2-bromomalodinitrile (15). The significant /3-selectivity has been associated with the steric demand of the incoming radical 16, which favors addition to the sterically least hindered site at the diene Id to provide allylic radical 17. However, it seems likely that a stabilization of an intermediate allylic radical, e.g. 17, by methyl substituents contributes significantly to the observed regioselectivity of product formation. Trapping of intermediate 17 with bromine atom donor 15 proceeds at the least substituted carbon to afford allylic bromide 18. 

Sulfolene, as a source of 1,3-buta-diene in situ, 50, 43 Sulfones, bromination, 50, 31 Sulfur, reaction with organo-lithium compounds, 50,105 Sulfuryl chloride, with 1,1-cyclo-butanedicarboxylic acid to give... 

Based on bis-silylated dienes another approach to quinoxaline derivatives such as 80 (Scheme 4.10) was found [97]. Fast [4+2] cycloaddition takes place by treatment of Cgo with 2,3-bis(trimethylsilyloxy)butadiene 98, yielding the acyloin-fused fullerene derivative 100 in good yields (Scheme 4.16). The silylated diene is formed in situ by treatment of 98 at 180 °C in o-dichlorobenzene. Controlled bromination of the intermediate 99 leads to the transient diketone 101, which reacts readily in a one-pot reaction with various o-diaminoarenes to yield the quinoxaline-fused fullerenes 102. 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

The syntheses described in this subsection rely upon the availability of o-diaroylbenzenes. A few methods seem to be of broader applicability. As Adams and co-workers have shown, the Diels-Alder reaction of dienes and diaroylethylenes gives 4,5-diaroylcyclohexenes, which on subsequent treatment with catalytic amounts of acid (sulfuric or phosphoric) form 4,7-dihydrobenzo[c]furans. Subjecting these compounds to the action of 2 moles of bromine in acetic acid in the presence of sodium acetate resulted in diaroylbenzenes (Eq. fi). - - ... 

Pyrrolidine readily forms an IV-nitroso derivative. This can be lithiated in the 2-position, and subsequent reaction with electrophiles and deprotection yields 2-substituted pyrrolidines (780S(58)l 13), as illustrated in Scheme 54. The transformation of tetrahydrothiophene 1,1-dioxide (238) into its 2-bromo derivative (239) is similar in principle. This involves deprotonation with ethylmagnesium bromide followed by electrophilic attack by bromine. Sodium ethoxide treatment of (239) gives buta-1,3-diene in 74% yield (80LA1540). 

Many other examples have been described that illustrate the possibility of carrying out selective reactions with PTT which would be impossible with bromine for example, 3/3-acetoxy-20-oxo-21-bromopregna-5,16-diene can be obtained from 3j8-acetoxy-20-ethylenedioxypregna-5,16-diene 6 anisyl cyclohexyl ketone gives the a-bromoketone in very good yield with the aromatic ring remaining unattacked 8 5,7-... 

The most intriguing, and apparently general reaction for the preparation of dehydrohomoadamantanes involves the (2+2) cycloaddition of 3,7-di-methylene-bicyclo[3.3.1]nonane. Photolysis of the parent diene gives 3,6-dehydrohomoadamantane which, upon reaction with bromine, gives 1-bromo-3-bromomethyladamantane (Eq. (44))136). Thermodynamic control un-... 

An improved route to 2a-hydroxycholesterol has been devised as part of the preparation of 2a-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = R3 = OH).123 Hydroxylation of the A bond of cholesta-l,5-dien-3/3-ol by means of 9-borabicyclo[3,3,l]nonane followed by reaction with alkaline hydrogen peroxide produced the 2-equatorial 2a,3a-diol in 70—80% yield. The conventional four-step sequence, acetylation, bromination, dehydrobromination, and hydrolysis, gave 2a -hydroxycholesta-5,7-dien-3/3-ol which was converted into 2a-hydroxy-vitamin D3. The isomeric 2/3-hydroxy-vitamin D3 has also been reported.124 Reaction of the 1/6,2/3-oxide obtained by peroxidation of the adduct (265) with lithium aluminium hydride results in a mixture of 2/3,3/3-dihydroxycholest-5,7-diene and its 1/3,3/3-dihydroxy-epimer in the ratio 8 1. Irradiation of the former 5,7-diene furnished the expected previtamin, which on equilibration gave 2/3-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = a-OH, R3 = OH). 

Dihydromuscimol, GABA agonist, synthesis, 60, 297 Diimidazo[ 1,2-h 2, 1 -/Ipyridazine, bromination, 59, 329 Diiron enneacarbonyl, reaction with 1-azabuta-1,3-diene, 57, 6 Diisoquino[3,2-i 2, 3 -y)[3,8J-... 

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