Functionality by esterification

The hyperbranched polyesteramides described above can also easily be functionalized by esterification with various mono carboxylic acids like acetic acid, benzoic acid, 2-ethylhexanoic acid, stearic acid, (un)satuxated fatty acids, or (meth)acrylic acid. With the exception of the latter mentioned acids, which give highly temperature sensitive products, the synthesis of these functionalized hyperbranched polyesteramides can be performed in two different ways ... 

Protection of an alcohol function by esterification sometimes offers advantages over use of acetal or ether groups. Generally, ester groups are stable under acidic conditions. Esters are especially useful in protection during oxidations. Acetates and benzoates are the most commonly used ester derivatives. They can be conveniently prepared by reaction of unhindered alcohols with acetic anhydride or benzoyl chloride, respectively, in the presence of pyridine or other tertiary amines. 4-Dimethylaminopyridine (DMAP) is often used as a catalyst. The use of A-acylimidazolides (see Section 3.4.1) allows the... 

In search of a convenient procedure for preparing diazo substrates for the cycloaddition to Cgg, Wudl introduced the base-induced decomposition of tosyl-hydrazones [116]. This procedure allows the in situ generation of the diazo compoimd without the requirement of its purification prior to addition to Cgg. Since they are rapidly trapped by the fullerene, even unstable diazo compounds can be successfully used in the 1,3-dipolar cycloaddition. In a one-pot reaction the tosyUiydrazone is converted into its anion with bases such as sodium methoxide or butylHfhium, which after decomposition readily adds to Cgg (at about 70 °C). This method was first proven to be successful with substrate 142. Some more reactions that indicate the versatility of this procedure are shown in Table 4.4. Reaction of 142 with CgQ under the previously described conditions and subsequent deprotection of the tert-butyl ester leads to [6,6]-phenyl-C5j-butyric acid (PCBA) that can easily be functionalized by esterification or amide-formation [116]. PCBA was used to obtain the already described binaphthyl-dimer (obtained from 149 by twofold addition) in a DCC-coupling reaction [122]. 

Magnesium ascorbyl phosphate (MAP) is more effective than ascorbyl palmitate (AP), as the protection of its active function by esterification protects the MAP against oxidation and provides an antioxidant effect that is of better quality and longer lasting than that of ascorbyl palmitate, a molecule in which the enediol function of the 2 carbon atom is not protected by esterification. 

These acid-catalyzed C-glycosylations were successfully extended to the D-ribofuranose series by Sorm and coworkers,148 who utilized the reaction in the first reported synthesis of showdomycin. Thus, treatment of 2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl bromide (81) with 1,2,5-trimethoxybenzene in the presence of zinc oxide gave 2,4,6-trimethoxy-l-(2,3,5-tri-0-benzoyl-/3-D-ribofuranosyl)benzene (196). Ozonolysis of the corresponding acetate derivative, followed by esterification, gave the highly functionalized C-/3-I>ribofuranosyl derivative (197), which was used as a key intermediate in the synthesis of showdomycin (see Section III,l,b). 

Temporary protection of the aldehyde function and reduction with aluminum hydride gave ( )-3-epigeissoschizal (275), from which silver oxide oxidation, followed by esterification, resulted in methyl ( )-epigeissoschizoate (277). 

TFEMA), necessary for preparation of functional water repellent paints and optical fiber coating agents. TFEMA can be manufactured by esterification of TFEA and methacrylic acid (MA) in the presence of an acid catalyst, at 70 °C. To obtain a higher conversion rate it is necessary to remove the water from the system, avoiding the formation of the thermodynamic equilibrium composition. 

Allylic acetates are usually prepared by esterification from allylic alcohols. However, the corresponding alcohols are often only accessible by the fairly expensive hydride reduction of carbonyl compounds. Consequently, direct allylic functionalization of easily available olefins has been intensively investigated. Most of these reactions involve peroxides or a variety of metal salts.However, serious drawbacks of these reactions, (e.g. toxicity of some metals, stoichiometric reaction conditions, or nongenerality) may be responsible for their infrequent use for the construction of allylic alcohols or acetates. 

Lipases are enzymes of the hydrolase family and, in nature, hydrolyze fatty acid esters in aqueous environment. It is worth recalling that the hydrolysis of esters is a reversible reaction. Chemists thus often use lipases to catalyze the reverse reaction, i.e., the esterification and the ROP of lactones. In 1993, the groups of Kobayashi [91] and Knani [92] reported independently the hpase-catalyzed ROP of sCL and 8-valerolactone. The aliphatic polyesters were functionalized by a carboxylic group at one chain-end and a hydroxyl group at the other chain-end. Accordingly, the polymerization was initiated and terminated by water present in the reaction media. 

For the synthesis of peptides, the phosphonic moiety in most cases should be masked as a diester. Diesters of 1-aminoalkylphosphonic acids can be synthesized directly or by esterification of 1-aminoalkylphosphonic acids. If peptides with the free phosphonic moiety are the desired products, then methods are available for the selective removal of both ester groups. Peptides with a free C-terminal phosphonic acid functionality can be synthesized directly from the free 1-aminoalkylphosphonic acids. In addition, methods for synthesis of the peptides with C-terminal phosphonates directly from the peptides are also available. In general, most methods for the synthesis of peptide bonds work well for the synthesis of peptides with C-terminal phosphonates if diesters of 1-aminoalkylphosphonic acids are used. Bulky diaryl esters give yields similar to the diethyl esters. Therefore, the most challenging step in the synthesis of peptide phosphonates is the synthesis of 1-aminoalkylphosphonic acids and/or their esters. It is not possible in this section to review all of the literature data and only examples of several general methods are included. This will still provide a variety of methods for the efficient synthesis 1-aminoalkylphosphonic acids, their esters, and related peptide derivatives. 

Functional group protection. The NH— group in proline is protected by acylation in the usual Schotten-Baumann manner with benzyl chloroformate to yield the benzyloxycarbonyl derivative (42). Correspondingly the —C02H group in glycine is protected by esterification in ethanol to form the ethyl ester, obtained as the hydrochloride (43) under Fischer-Speier conditions. 

In the event, iodolactonization of the carboxylate salt derived from the ester 458 afforded 459, and subsequent warming of the iodo lactone 459 with aqueous alkali generated an intermediate epoxy acid salt, which suffered sequential nucleophilic opening of the epoxide moiety followed by relactonization on treatment with methanol and boron trifluoride to deliver the methoxy lactone 460. Saponification of the lactone function in 460 followed by esterification of the resulting carboxylate salt with p-bromophenacylbromide in DMF and subsequent mesylation with methanesulfonyl chloride in pyridine provided 461. The diazoketone 462 was prepared from 461 by careful saponification of the ester moiety using powdered potassium hydroxide in THF followed by reaction with thionyl chloride and then excess diazomethane. Completion of the D ring by cyclization of 462 to the keto lactam 463 occurred spontaneously on treatment of 462 with dry hydrogen chloride. 

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