Esterification nucleophilic displacement

In die -elimination reactions discussed next, the electron-withdrawing groups are first introduced chemically into the polysaccharide this may be done by esterification of a carboxylic acid group, by oxidation of an alcohol group to a carbonyl group, or by nucleophilic-displacement reactions. 

All four isomeric selenolopyridines which can be derived from benzoselenophene (423— 426 Scheme 123) have been described. Ethyl 3-hydroxyselenolo[2,3-fe]pyridine-2-carboxy-late (429) has been prepared as shown in Scheme 124 (73BSF704). Treatment of ethyl 2-chloropyridine-3-carboxylate with methaneselenol yields (427). Nucleophilic displacement of bromine in bromoacetic acid with subsequent loss of methyl bromide yields (428), which after esterification is cyclized under Dieckmann conditions to give (429). The parent compound (423 colorless oil with b.p. 92 °C/1 mmHg) is prepared either by cyclization of compound (430) and subsequent decarboxylation of the intermediate acid (equation 57) or by reduction of 2-nitroselenophene and subsequent condensation of the amino compound with malonaldehyde bis(diethyl acetal) in the presence of zinc chloride (equation 58) (76BSF883). Selenolo[3,2-6]pyridine (426 b.p. 127-129°C/10 mmHg m.p. 35.5-37.0°C) has been obtained in an analogous manner. 

Functionalisation of the polymer backbone through reaction of the hydroxyl groups (esterification and etherification), nucleophilic displacement reactions, oxidation and selective reactions at the terminal reducing ends... 

Sulfinic acids and sulfoxides are not particularly common, being readily oxidized to the sulfonic acids and sulfones, respectively. Sulfonic acids have high melting points and probably exist as zwitterions. They are amphoteric, but mainly display the characteristics of weak acids. The sulfonic acid group activates an adjacent halogen to nucleophilic displacement, and may itself be displaced, e.g. reaction of alkylamines with benzimidazole-2-sulfonic acid. Imidazolesulfonic acids resist esterification and acid chloride formation, and are only hydrolyzed by concentrated hydrochloric acid at 170 °C the 2-isomers are more resistant than the 4- or 5-isomers. Aqueous alkali converts the free acids into hydroxy derivatives. Sulfonyl chlorides are accessible via the thiols (Section 4.07.3.6.1) which react with ammonia to form sulfonamides, or are reduced by tin(II) chloride to thiols (77JHC889). 

As has already been detailed for the allyloxycarbonyl (Aloe) moiety, allyl esters also proved to be very versatile and useful carboxy-protecting groups. They can be easily constructed by azeotropic esterification or nucleophilic displacement on allylic halides. For the cleavage of these esters lithium di-methylcuprate can be used. However, a much milder method is found in the Rh -catalyzed isomerization of the allyl moiety to a propenyl ester which immediately hydrolyzes under the reaction conditions (Scheme 67). Even milder is the Pd°-catalyzed allyl transfer to morpholine as an accepting nucleophile. The removal of allyl ester protection has earlier been used in particular in -lactam anti-... 

The preparation of esters can be classified into two main categories (1) carboxylate activation with a good leaving group and (2) nucleophilic displacement of a carboxylate on an alkyl halide or sulfonate. For simple esters, acid-catalyzed esterification with azeotropic removal of water is also very effective, but limited to simple systems for the most part. The nucleophilic approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondary halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). The section on transesterification should also be consulted, since this technology can be quite useful for the preparation of esters from other esters. 

Inter and intramolecular nucleophilic displacement of alcohols with inversion by means of diethyl azodicarboxylate (DEAD)-triphenylphosphine and a nucleophile. Also dehydration, esterification of alcohols or alkylation of phenols and one step synthesis of nitriles from alcohols (see 1 st edition). 

It is well known that the nucleophilic displacement reactions at tosylated polysaccharides are limited or at least mainly directed towards the primary positions . Therefore, our interest was focused on 6-0-tosyl starch samples with DStos 1- One suitable synthesis path is the protection of 0-2 and the subsequent tosylation. A useful protecting group may be the acetyl ester function. It was recently found that in contrast to conventional esterification processes of starch with acetic anhydride, which leads to a statistic distribution of the ester groups, an acetylation of starch dissolved in DMSO with acetic acid vinyl ester in the presence of sodium chloride yields 2-0-acetyl starch of varying DSac from 0.1 to 1.0. The functionalisation patterns of these new starch products were unambiguously proved by means of various NMR measurements including two dimensional methods . 

Kibayashi and coworkers reported that total synthesis of (+)-azimine (9) and (+)-carpaine (10) using intramolecular hetero-Diels—Alder reaction [38], The synthesis began with (5)-l,2,4-butanetriol (119) as a single source of chirality. Protection of 1,2-diol with PhCHO of triol 119 and subsequent oxidation of the primary alcohol and Wittig reaction followed by protection of the primary alcohol as a MOM ether and DIBALH reduction afforded 120. The resulting alcohol 120 was converted into tosylate and photoisomerized followed by conversion to hydroxamic acid via nucleophilic displacement of the tosylate by cyanide ion, alkaline hydrolysis, esterification with diazomethane, and... 

Cellulose, Polysaccharide, Activation, Cellulose solvents. Esterification, Etherification, Ionic liquids, NMR spectroscopy. Nucleophile displacement. Protecting groups, Regioselective functionalization... 

Polymeric herbicides were prepared with 2,4-D and CMPA covalently or ionically bound to oligoethylenoxylated polystyrene resins at different degrees of crosslinking. These adducts were formed by ion exchange, by nucleophilic displacement on chlo-romethyl groups, and by esterification of hydroxyl groups (Scheme 3.3) [127]. Herbicide release from polymer beads loaded with herbicide was monitored in aqueous solutions buffered at pH 4,7, and 9. For covalently bound herbicides, a release of... 

But we should wait a moment. Mechanisms other than the one shown in Figure 17.24 for Fischer esterification can be imagined, and it is not fair to dismiss them without evidence. For example, why not use the protonated alcohol as the electrophile and the carbonyl oxygen of the carboxylic acid as the nudeophile rather than the protonated carboxylic acid as the electrophile and the alcohol as the nucleophile Displacement of water using the acid as nucleophile could give the ester (Fig. 17.26). 

The next schemes 248 to 251 list a set of typical reactions of this kind of nucleophilic displacement esterification ... 

Nucleophilic aromatic substitution reactions should not be looked upon as a precise, rigidly bounded entity. These reactions might perhaps be considered as one aspect of the more general area of nucleophilic displacements on unsaturated carbon atoms. As such they are related to the many displacements occurring at carbonyl carbon atoms (the hydrolysis of esters, acid chlorides, anhydrides, and amides the aminolysis of esters, anhydrides, and acid chlorides the esterification of acids, acid chlorides, and anhydrides, etc.) and to nucleophilic displacements on double-bonded carbon in non-aromatic systems. Undoubtedly there are significant differences among these classes of reactions, but there are equally significant similarities. 

The next step is not immediately obvious. The generation of an ethyl ester from a lactone can be accommodated by transesterification (we might alternatively consider esterification of the free hydroxyacid). The incorporation of chlorine where we effectively had the alcohol part of the lactone leads us to nucleophilic substitution. That it can be SnI is a consequence of the tertiary site. Cyclopropane ring formation from an Sn2 reaction in which an enolate anion displaces a halide should be deducible from the structural relationships and basic conditions. 

Similarly, when the selective O-pivaloylation of trehalose was investigated by Richardson, Hough, and Cortes-Garcia, a number of useful derivatives were also obtained in quantities adequate to be of preparative value. Thus when 12 equivalents of pivaloyl chloride were employed for this esterification at — 20 °C, and the reactants were thereafter stored at room temperature for three days, a 61% yield of the 2,2 3,3, 4, 6,6 -heptapivalate was obtained, and following invertive chlorination with sulfuryl chloride, a displacement with different nucleophiles gave access to C-4-modified trehaloses, one example of which is shown in Scheme 17. 

The synthesis of one of the agents begins with nucleophilic aromatic displacement of bromine by cyanide in the highly fluorinated compound 78. Acid hydrolysis of the nitrile (79), followed by esterification of the newly formed acid, affords ester 80. Base-catalyzed condensation of the intermediate with diethyl malonate leads to the tricarbonyl derivative 81. 

In the esterification of an acid, an alcohol acts as a nucleophilic reagent in hydrolysis of an ester, an alcohol is displaced by a nucleophilic reagent. Knowing this, we are not surprised to find that one alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester is called transesterification. 

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