Esters Organic Syntheses procedures

Conventional organic synthesis procedures are sometimes employed to prepare the simpler types of those molecules included here. Thus, (v/c-dihydroxyalkyl)phosphonic diesters are obtainable through the hydroxylation of the corresponding unsaturated acid esters using OSO4 and standard techniques. The isolation and purification of a diol may prove to be difficult because of possible intramolecular transesterification, sometimes accompanied by dehydration (reaction 12)" ... 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

The use of ester-cleaving enzymes is probably going to be one of the most useful biological-chemical methods in the synthetic laboratory. No example of this type of reaction has hitherto been published in the Organic Syntheses series of procedures. So far, the only biological-chemical Organic Synfheses-procedures are two yeast reductions,4 5 one oxidation with horse-liver-alcohol-dehydrogenase,6 and a disaccharide synthesis catalyzed by emulsin.7 The procedure described here is... 

Acyloxyborane as an Activating Device for Carboxylic Acids . The reduction of carboxylic acids by borane is an important procedure in organic synthesis. The remarkable reactivity of borane towards carboxylic acids over esters is characteristic of this reagent. Such selectivity is rarely seen with other hydride reagents. 

Reaction of lithium alkynolates with alcohols is especially useful for organic synthesis as a method of ester homologation . In this case ethyl esters of carboxylic acids 27 are used for the generation of lithium alkynolates in a multistep one-pot procedure (equation 19). Subsequent ethanolysis of lithium alkynolate in the presence of HCl gives homologated esters 28 in good yield. 

Transesterification is an important transformation in organic synthesis in industrial as well as in academic laboratories [138]. There are many catalysts available for such reactions and the most common procedure is to reflux the ester with a catalytic amount of Ti(0-/-Pr)4 in an alcohol solvent. Other catalysts include DBU/LiBr [139] as well as several organometallic compounds [138]. 

It is well known that I is one of the softest common nucleophiles. The use of I in deblocking methyl esters (Taschner-Eschenmoser method) (48-51) has become a conventional procedure in organic synthesis. Nucleophilic attack by I on the methyl group [Sn2(C)1 is undisputedly operative. 

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