Isomerisation, /// . -ester

The method is generally applicable when other modes of esterification are either slow, inefficient, or likely to cause isomerisation it is, however, time-consuming and expensive. Small quantities of acid impurities are sometimes produced, hence it is advisable to wash the ester with saturated sodium bicarbonate solution. The silver salt can usually be prepared by dissolving the acid in the calculated quantity of standard ammonium hydroxide solution and... 

This stereospecific oxidation does not occur for all dioximes, probably due to isomerisation of the dioxime during the reaction or to different reaction mechanisms involved in the use of different oxidants. When the lipophilic-hydrophilic balance of the two furoxan isomers is appropriate, they are easily separated by chromatography or fractional crystallisation. For example, the synthesis of 4-hydroxymethyl-3-furoxancarboxamide (CAS 1609), one of the most promising furoxancarboxamide vasodilators (see later), passes through the intermediate formation of a mixture of the two isomeric methyl hydroxymethylfuroxancarboxylic esters, which can easily be separated by recrystallisation from isopropyl acetate [18]. 

In spite of this, the hydride catalyses the isomerisation of higher a-olefins to an equilibrium mixture of all the possible internal isomers and catalyses also the methoxycarbonylation of a-olefins and of internal olefins to the linear esters with 99% regioselectivity. 

Preliminary mechanistic studies on the methoxycarbonylation of 1-octene showed that two pathways to methyl nonanoate occur, one involving the direct carbonylation of 1-octene to the linear ester, the other the alkene isomerisation in competition with the first one. Subsequently, the linear product forms by tandem isomerisation of the internal alkenes, with the terminal alkyl intermediate being trapped by migration to CO at a higher rate than any branched alkyl species. This has been confirmed by the analysis of products... 

By isomerisation, a-pinene can be converted into camphene, and this can then be esterified to obtain an ester of isoborneate, which can be saponified to isoborneol. Isoborneol can be dehydrogenated to camphor, which can be reduced again to borneol, which is used in many fruit flavours. 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

Fang X, Jin F, Jin H, von Sonntag C (1998) Reaction of the superoxide radical with the A/-centered radical derived from N-acetyltryptophan methyl ester. J Chem Soc Perkin Trans 2 259-263 Ferreri C, Costantino C, Landi L, Mulazzani QG, Chatgilialoglu C (1999) The thiyl radical-mediated isomerisation of c/s-monounsaturated fatty acid residues in phospholipids a novel path of membrane damage Chem Commun 407-408... 

Reaction of cyclopropenes with bases such as alkoxide or amide ions often leads to a methylenecyclopropene by removal of an allylic hydrogen and reprotonation 6 9-71) though other reactions such as nucleophilic addition (see Section 5) or metallation at a vinylic position (see Section 2) may compete. Thus the ester (209) is isomerised by KOH to (210), and under more vigorous conditions to (211)144) ... 

C but isomerise to the more stable enolate on warming (Scheme 5.91). The use of two different ot-halo esters allows access to more complex samarium enolates before quenching with benzaldehyde.144... 

Animals. Within 48 hr 80% is eliminated via the feces, 14% via urine. Metabolism proceeds via hydrolysis of the methyl ester, isomerisation at the thiadiazole ring and hydroxylation of the tetrahydropyridazine moiety... 

H. G. Heller and M. Szewczyk, Synthesis, stereochemistry, and isomerisation of phenylethylidene succinic esters and related compounds, J. Chem. Soc., Perkin. Trans. 1, 1974, 1483—1487. 

Benzyl bromides 15 derived from either salicylaldehydes or 2-methylphenols by reaction with alkyl propynoates afford 277-1 -benzopyrans on treatment with tetrahydrothiophene and K2CO3. A sulfonium ylide is proposed which initiates a Michael addition - elimination -substitution sequence. When CS2CO3 is used as the base, 477-1-benzopyrans result almost exclusively, presumably through isomerisation of the 277-1-benzopyran. The protocol thus offers access to both chromene isomers through simple variation of the reaction conditions <06OL3853>. The Cu-catalysed intramolecular O-arylation of a-(2-bromobenzyl)-P-keto esters in refluxing THF offers a useful route to 2-substituted 477-l-benzopyran-3-carboxylates <06JOC6427>. 

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