Allyl bromide, addition

The most representative results of these additive systems are (1) allyl alcohol seems to inhibit the Si-face attack of the hydride, and (2) allyl bromide inhibits the. Re-face attack. In this system, if it is assumed that CF3 is sterically less demanding than the CH2COCH3 substituent, Prelog s rule holds for the yeast-allyl bromide additive reduction system, whereas it is not followed when the additive is allyl alcohol [26]. 

Cyclic enediynes have been obtained also by elimination of a suitable propargylic alcohol on a preformed cyclic diyne.5 An example is shown in Scheme 19.7, where the cyclization was accomplished through a Nozaki-Hiyama intramolecular allyl bromide addition to an aldehyde.5 This method proved successful for the synthesis of a particular enediyne that, because of conformational biases, could not be obtained by the strategies described above. 

The allyl bromide addition required 30 min in the hands of the checkers. 

If reaction does not occur when a little allyl bromide is first introduced, further addition must be discontinued until the reaction has commenced. Remove 2-3 ml. of the Grignard solution with a dropper pipette, add about 0-5 ml. of allyl bromide and warm gently to start the reaction after this has reacted well, add the solution to the main portion of the Grignard reagent. 

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

A 20% excess of ethylmagnesium bromide was prepared from magnesium (6.5 g) in ether (80 ml) by adding ethyl bromide (30 g) in ether (30 ml). Indole (25.8 g) in benzene (50 ml) was then added slowly with stirring and stirring was continued for 20 min after addition was complete. A solution of allyl bromide (29.2 g) in benzene (20 ml) was then added slowly. The mixture was stirred overnight and then diluted with ether and the product isolated and purified by distillation (22.7 g, 70% yield). 

Kharasch s earliest studies in this area were carried out in collab oration with graduate student Frank R Mayo Mayo performed over 400 experi ments in which allyl bromide (3 bromo 1 propene) was treated with hydrogen bromide under a variety of conditions and determined the distribution of the normal and abnormal products formed during the reaction What two prod ucts were formed Which is the product of addition in accordance with Markovmkovs rule Which one corresponds to addition opposite to the rule ... 

When however the ionic addition of hydrogen bromide to 1 3 butadiene is car ried out at room temperature the ratio of isomeric allylic bromides observed is differ ent from that which is formed at — 80°C At room temperature the 1 4 addition product predominates... 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

A mixture of 3.18 g (10 mmoles) of 17 -hydroxy-2-hydroxymethylene-5a-androstan-3-one, 20 ml dry dimethyl formamide and 0.3 g (13 mmoles) of sodium hydride is stirred for 0.5 hr at room temperature under nitrogen. A total of 1.51 g (12.5 mmoles) of redistilled allyl bromide is added and the mixture is stirred for 1 hr on the steam bath. Aqueous potassium hydroxide (2 g in 5 ml of water) is added and stirring is continued for 1 hr on the steam bath. The reaction mixture is diluted with 50 ml of methylene dichloride followed by careful addition of 300 ml of water. The organic phase is separated and the aqueous phase is again extracted with 50 ml of methylene dichloride. The combined extracts are washed with water, dried over sodium sulfate, filtered and chromatographed on 200 g of silica gel. Elution with pentane-ether (4 1) provides 2a-allyl-17j -hydroxy-5a-androstan-3-one 0.85 g (26%) mp 118-119° [aj 14° (CHCI3), after crystallization from ether-hexane. 

The application of this strategy to the synthesis of chiral cyclohexadienes has been demonstrated by Kiindig. Addition of MeLi to the Cr(CO)3-complexed chiral phenyl oxazoline 43 followed by reaction with allyl bromide produced cyclohexadiene 44 in 69% yield and >98% de. ... 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

One gram of 6,7-dihydro-5H-dibenz[c,e] azepine hydrochloride was dissolved in water, made alkaline with concentrated ammonia, and the resultant base extracted twice with benzene. The benzene layers were combined, dried with anhydrous potassium carbonate, and mixed with 0.261 g of allyl bromide at 25°-30°C. The reaction solution became turbid within a few minutes and showed a considerable crystalline deposit after standing 3 A days. The mixture was warmed VA hours on the steam bath in a loosely-stoppered flask, then cooled and filtered. The filtrate was washed twice with water and the benzene layer evaporated at diminished pressure. The liquid residue was dissolved in alcohol, shaken with charcoal and filtered. Addition to the filtrate of 0.3 gram of 85% phosphoric acid in alcohol gave a clear solution which, when seeded and rubbed, yielded 6-allyl-6,7-dihydro-5H-dlbenz[c,e] azepine phosphate, MP about 211°-215°C with decomposition. 

Preparation of Diethyl Allyl (1-Methyl-2-Pentynyl) Malonate A solution of 12.1 g of sodium in 182 ml of absolute ethanol was prepared, and thereto were added 126.6 g of diethyl (1-methyl-2-pentynyl) malonate. Most of the ethanol was then distilled off under reduced pressure, and the residue was cooled and 63.5 g of allyl bromide were slowly added thereto. After completion of the addition, the mixture was refluxed for about 1 hour. 

Propyl-methyl-carbinyl allyl barbituric acid (also called allyl 1-methyl-butyl barbituric acid) may be prepared as follows 1 mol of propyl-methyl-carbinyl barbituric acid is dissolved in a suitable vessel In a 10 to 35% aqueous solution of 1 mol of potassium hydroxide. To this are added somewhat in excess of 1 mol of allyl bromide, and alcohol equal to about 10% of the total volume of the solution. The vessel Is agitated for 50 to 75 hours. At the end of this time, the solution, which may still exhibit two layers, is concentrated to about one-half its volume to remove the excess allyl bromide and the alcohol. On cooling, an oily layer, which is propyl-methyl-carbinyl allyl barbituric acid, separates out as a sticky viscous mass. It is dried, washed with petroleum ether, and dissolved in the minimum amount of benzene. Any unreacted propyl-methyl-carbinyl barbituric acid, which does not dissolve, is filtered off. The addition of petroleum ether to the clear filtrate causes the propyl-methyl-carbinyl allyl barbituric acid to precipitate as an oily mass. 

Allylic bromides can also serve as progenitors for nucleophilic organochromium reagents. An elegant example is found in Still and Mobilio s synthesis of the 14-membered cembranoid asperdiol (4) (see Scheme 2).7 In the key step, reduction of the carbon-bromine bond in 2 with chromium(n) chloride in THF is attended by intramolecular carbonyl addition, affording a 4 1 mixture of cembranoid diastereoisomers in favor of the desired isomer 3. Reductive cleav-... 

Chromium(II) Chloride Mediated Addition of Allyl Bromides to Aldehydes General Procedure12 ... 

An example for synthesis of the chiral p-keto ester 69 is illustrated in equation 64. It involves conjugate addition of the dipotassium / -keto ester 68 to vinyl sulfone 67 followed by in situ quenching with allyl bromide . The method provides a new procedure to sevenring annulation product 70 that is a potential precursor for (l)-(—)-cytochalasin C. 

Mediated by Tin. In 1983, Nokami et al. observed an acceleration of the reaction rate during the allylation of carbonyl compounds with diallyltin dibromide in ether through the addition of water to the reaction mixture.74 In one case, by the use of a 1 1 mixture of ether/water as solvent, benzaldehyde was allylated in 75% yield in 1.5 h, while the same reaction gave only less than 50% yield in a variety of other organic solvents such as ether, benzene, or ethyl acetate, even after a reaction time of 10 h. The reaction was equally successful with a combination of allyl bromide, tin metal, and a catalytic amount of hydrobromic acid. In the latter case, the addition of metallic aluminum powder or foil to the reaction mixture dramatically improved the yield of the product. The use of allyl chloride for such a reaction,... 

Normally, the addition of C-nucleophiles to chiral a-alkoxyaldehydes in organic solvents is opposite to Cram s rule (Scheme 8.15). The anti-Cram selectivity has been rationalized on the basis of chelation control.142 The same anti preference was observed in the reactions of a-alkoxyaldehydes with allyl bromide/indium in water.143 However, for the allylation of a-hydroxyaldehydes with allyl bromide/indium, the syn isomer is the major product. The syn selectivity can be as high as 10 1 syn anti) in the reaction of arabinose. It is argued that in this case, the allylindium intermediate coordinates with both the hydroxy and the carbonyl function leading to the syn adduct. 

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