Esters isomerization

Thermal degradation occurs when heat causes compounds to undergo structural changes, leading to the formation of simpler species. For example, many organophosphorus esters isomerize when heated... 

Silyl Ketene Acetal to a-Silyl Ester Isomerization... 

Esters of other fatty acids are used to a lesser extent. In addition to benzyl esters and phenethyl esters, isomeric homologues with substituted side-chains are used in fairly large amounts in perfume compositions because of their special blossom odors. Not all have yet been found in nature. The esters are prepared from the corresponding alcohols via the customary routes. 

The maximum allylic ester isomerization possible can be measured when hexene is used in the reaction. Both hexene-l-yl acetates and hexen-2-yl acetates will give hexen-3-yl acetates upon allylic ester isomerization (Reactions 8a, 8b, and 8c). Therefore, the hexanol-3-acetate found after hydrogenating the hexenyl acetate product will be representative of the sum of both allylic attack on hexene during vinylation and allylic ester isomerization after vinylation and olefinic isomerization. If hexanol-3-acetate is found in only low levels in the reaction product, then both allylic attack during vinylation and allylic ester isomerization can be discounted in considering the major reaction pathways. 

Also, the low levels of hexanol-3-acetate found in all these experiments indicate that neither attack on the allylic position of hexene nor allylic ester isomerization is important under these reaction conditions. When the procedure of Kitching et al. (29) was repeated, a high level... 

The reactions and product distributions thus far reported have been exclusively concerned with hexene. It was of interest to see whether the high specificity of positional substitution could be maintained with the other hexene isomers. By positional substitution specificity is meant ester attachment on ether of the carbons involved in the original carbon-carbon double bond. Table VII shows the results of these studies. The internal olefins reacted more slowly than the a-olefin, and with both palladium chloride-cupric chloride and 7r-hexenylpalladium chloride-cupric chloride systems high substitutional specificity (> 95% ) was also maintained with 2-hexene (Table VII). However, with 3-hexene the specificity is considerably lower (80%). Whether this is caused by 3-hexene isomerization prior to vinylation or by allylic ester isomerization is not known. A surprisingly high ratio of 2-substitution to 3-substitution is found ( 7 1) in the products from 2-hexene. An effect this large... 

Several additional alkaloids were isolated from Tripterygium wilfordii.. The structures of wilforidine (70)[200], neowilforine (71) [201], and wilforzine (72) [190] were established by chemical and spectroscopic studies. The structure of isowilfordine (58) was determined using H and 3c NMR, and by treatment with NaOMe/MeOH to provide an ester isomeric with dimethyl hydroxywilfordate [202],... 

Carbonylation of a mixture of ethylene, ethanol, and diphenylacetylene in the presence of rhodium carbonyl gives the butenolide (10). 2 Another formation of a butenolide, that of compound (12), is by flash vacuum pyrolysis of diphenylmethyl propiolate, Ph2CH02C H. It has been suggested that the ester isomerizes to the methylene-carbene (11), which yields the product by intramolecular insertion. Ozonization of tetraphenyl-... 

EINECS 261-675-7 Hexanoic acid, 3,5,5-trimethyl-, isotridecyl ester Isomeric tetramethyl-1-nonyl 3,5,5-trimethyl hexanoate Isotridecyl isononanoate Isotridecyl 3,5,5-trimethylhexanoate Wiekeno 153. Silky emollienoe and solvent characteristics for skin and hair care products pigment wetter, moisturizer, CasChem. 

Meisenheimer et were the first to study the kinetics of allylic ester isomerizations. They followed the isomerization of a-phenylallyl p-nitroben-zoate and a-phenylallyl 3,4,5-tribromobenzoate to the corresponding cinnamyl benzoates by a tedious analytical procedure involving use of melting point-composition diagrams. They observed that isomerization of the molten esters at 137°C is autocatalytic due to partial slow decomposition of the esters to substituted benzoic acids and unidentified products, and demonstrated that the reaction is catalyzed by added benzoic acids. The rate law for these reactions is... 

Braude " later pointed out that mechanistic conclusions based on these early studies of allylic ester isomerization are of doubtful validity, since they do not account for the sensitivity of these reactions to adventitious acidic impurities. Braude et ti/, and Pocker " " " studied the kinetics of isomerization of a-phenylallyl esters [(3), R = = YCgH, R == H] in the weakly... 

In summary, the limited data available permit no clear-cut decision between the SnI mechanism with charge separation in the transition state, and the SnT mechanism with tight ion-pair intermediates. The fact that ally lie ester isomerizations almost certainly involve ionic intermediates under solvolytic conditions (see below) suggests that the reactions under non-solvolytic conditions probably also occur by an ionic mechanism,... 

In aqueous 90% acetone at 99.6°C, At for /r Ai.s-a,y-dimethylallyl p-nitro-benzoate is 2.6x10-6 ec" [(II), R = R = CH3 Y = p-O-.NQH.CO., SOH = HoO]. The rate is not affected by addition of small amounts of lithium p-nitrobenzoate or p-nitrobenzoic acid. In aqueous 60% acetone at 60 C, At = 1.5 X lO secBy using carbonyl- o-labeled ester, it was shown that 60 scrambling is about twice as fast as allylic rearrangement, which implies that the carboxyl group of the p-nitrobenzoate ion in the tight ion pair rotates relative to the allylic carbonium ion in some instances before the ions recombine to form enantiomeric esters . This and similar observations on other allylic esters (see below) rules out the concerted Sf i mechanism as the only mechanism for allylic ester isomerization under solvolytic conditions. 

Most of the anionotropic ester isomerizations whose kinetics have been studied involve simple allylic compounds. A few kinetic investigations of solvolysis and isomerization reactions of doubly unsaturated allylic esters and non-allylic unsaturated esters have been reported. Two examples of such reactions are mentioned here. Spirodienyl p-nitrobenzoate (V) undergoes simultaneous allylic isomerization to VI, viz. 

Regardless of the method chosen for the recovery of retinoids from biological samples, it is extremely important to remember that retinoids vary widely both in their solubility properties and in their stability. For these reasons, when an extraction method is being considered for the isolation of a given retinoid and/or its metabolites, two important points must be taken into account (1) whether the method chosen will result in a complete extraction of the retinoid of interest, and (2) whether it will contribute to the production of artifacts, either through hydrolysis of esters, isomerization, or production of nonphysiological compounds that may be mistaken for metabolites. 

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