Transesterification halides

Important examples of this type of compounds include pinacolborane, catecholborane, etc. These compounds are readily available by the reaction of pinacol 331 or catechol with either borane or boron halides. Transesterification also provides an attractive alternative for the synthesis of these compounds (Scheme 55). 

In our previous works[8,9] on the synthesis of various 5-membered cyclic carbonate, quaternary ammonium salts such as tetrabutylammonium halides showed excellent catalytic activities in relatively mild reaction conditions, under atmospheric pressure and below 140 U. hi this work, several kinds of quaternary ammonium salts have been used for the transesterification reactions of the ethylaie carbonate with methanol to DMC and ethylene glycol. 

The various tri-, di- and monoorganotin hydroxides and oxides that can be obtained by partial or complete hydrolysis of the corresponding halides or carboxylates often behave as catalysts for reactions such as esterification or transesterification, and it is partly with the aim of obtaining better and more selective catalysts that a lot of work has been carried out on these compounds in the past decade.347... 

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. 

The esters of salicylic acid account for an increasing fraction of the salicylic acid produced, about 15% in the 1990s. Typically, the esters are commercially produced by esterification of salicylic acid with the appropriate alcohol using a strong mineral acid such as sulfuric as a catalyst. To complete the esterification, the excess alcohol and water are distilled away and recovered. The cmde product is further purified, generally by distillation. For the manufacture of higher esters of salicylic acid, transesterification of methyl salicylate with the appropriate alcohol is the usual route of choice. However, another reaction method uses sodium salicylate and the corresponding alkyl halide to form the desired ester. 

Alcoholysis of nitriles, of ortho and thio ortho esters (transesterification), and of halides is the most common method of preparing the ortho ester functional groups (see Eqs. 2-7). A less practical method is the addition of alcohols to ketene acetals. The latter method is used only when the other methods are not found applicable to the synthesis of specially substituted mixed ortho esters. 

Alcoholysis of trihalides 0-6 Hydrolysis of ortho esters 0-20 Alcoholysis of acyl halides 0-21 Alcoholysis of anhydrides 0-22 Esterification of carboxylic acids 0-23 Transesterification 0-24 Alkylation of carboxylic acid salts 0-25 Cleavage of ethers with anhydrides 0-26 Alkylation of carboxylic acids with diazo compounds... 

Enol Ethers and Esters 0-15 O-Alkylation of carbonyl compounds with diazo alkanes 0-17 Transetherification 0-20 Reaction between acyl halides and active hydrogen compounds 0-23 Transesterification 0-24 Acylation of vinylic halides 0-94 Alkylation with ortho esters 0-107 O-Acylation of 1,3-dicarbonyl compounds... 

Because a substantial excess of the halide is used in the Koenigs-Knorr synthesis of cardenolides, it is usually difficult, if not impossible, to secure the acylated intermediate in crystalline form. Hence, Reichstein nd coworkers18 introduced a procedure whereby the whole of the products arising from the coupling reaction are saponified, employing a solution of potassium hydrogen carbonate in aqueous methanol. This modified transesterification procedure renders all extraneous carbohydrate materials water-soluble the methanol is removed by evaporation, leaving an aqueous solution from which the (deacylated) cardenolide may be obtained by... 

Isosorbide 2-acetate may also be prepared by transesterification of equimolar amounts of isosorbide 2,5-diacetate and isosorbide in the presence of acidic or basic catalysts (KOH or p-toluenesulfonic acid, for 1 hat 140°, and distillation104,107 of the 2-acetate at 100° at 27 kPa 89%). Subsequent esterification of the resulting isosorbide mononitrates with acyl and aroyl halides... 

Tin(iv) enolates are normally generated by transmetallation from lithium enolates with trialkyltin halides or transesterification between enol acetates with trialkyltin alkoxides.220 Other types of generation systems are described below. 

The 4-methoxybenzyl esters are prepared by transesterification of N-protected amino acid nitrophenyl esters with 4-methoxybenzyl alcohol in the presence of several equivalents of imidazole, followed by subsequent acidolytic cleavage of the N-protecting group.Alternatively, reaction of N-protected amino acids silver(I) salts with 4-methoxybenzyl halides (TEA/CHCI3, rt, 24 h, 60% yield) or treatment of N-protected amino acids with 4-meth-... 

Transesterification has been carried out with phase-transfer catalysts, without an added solvent. Nonionic superbases (see p. 365) of the type P(RNCH2CH2)3N catalyze the transesterification of carboxylic acid esters at 25°C. ° Silyl esters (R C02SiR3) have been converted to alkyl esters (R C02R) via reaction with alkyl halides and tetrabutylammonium fluoride. Thioesters are converted to phenolic esters by treatment with triphosgene-pyridine and then phenol. 

For the polymerization to proceed at a reasonable rate, the use of a transesterification catalyst is needed. Compounds which are usually used as a catalyst for the preparation of polyesters through transesterification can be used here. These include lithium, sodium, zinc, magnesium, calcium, titanium, maganese, cobalt, tin, antimony, etc. in the form of a hydride, hydroxide, oxide, halide, alcoholate, or phenolate or in the form of salts of organic or mineral acids, complex salts, or mixed salts.(10) In this study, tetrabutyl titanate (TBT) in the amount of 1000 ppm was used normally. 

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. 

In 1982, Evans reported that the alkylation of oxazolidinone imides appeared to be superior to either oxazolines or prolinol amides from a practical standpoint, since they are significantly easier to cleave [83]. As shown in Scheme 3.17, enolate formation is at least 99% stereoselective for the Z(0)-enolate, which is chelated to the oxazolidinone carbonyl oxygen as shown. From this intermediate, approach of the electrophile is favored from the Si face to give the monoalkylated acyl oxazolidinone as shown. Table 3.6 lists several examples of this process. As can be seen from the last entry in the table, alkylation with unactivated alkyl halides is less efficient, and this low nucleophilicity is the primary weakness of this method. Following alkylation, the chiral auxiliary may be removed by lithium hydroxide or hydroperoxide hydrolysis [84], lithium benzyloxide transesterification, or LAH reduction [85]. Evans has used this methology in several total syntheses. One of the earliest was the Prelog-Djerassi lactone [86] and one of the more recent is ionomycin [87] (Figure 3.8). 

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