Amino salt-free esterification

MA Wegman, JM Elzinga, E Neeleman, F van Rantwijk, RA Sheldon. Salt-free esterification of alpha-amino acids catalysed by zeolite H-USY. Green Chemistry 3 (2) 61-64,2001. 

As any organic chemist will tell you, the conversion of an amino acid to the corresponding ester also requires more than one equivalent of a Bronsted acid. This is because an amino acid is a zwitterion and, in order to undergo acid catalysed esterification, the carboxylate anion needs to be protonated with one equivalent of acid. However, it was shown [38] that amino acids undergo esterification in the presence of a catalytic amount of zeolite H-USY, the very same catalyst that is used in naphtha cracking, thus affording a salt-free route to amino acid esters (Fig. 1.11). This is a truly remarkable reaction in that a basic compound (the amino ester) is formed in the presence of an acid catalyst. Esterification of optically active amino acids under these conditions (MeOH, 100 °C) un-... 

The exchange resins 6nd application in (i) the purification of water (cation-exchange resin to remove salts, followed by anion-exchange resin to remove free mineral acids and carbonic acid), (ii) removal of inorganic impurities from organic substances, (iii) in the partial separation of amino acids, and (iv) as catalysts in organic reactions (e.g., esterification. Section 111,102, and cyanoethylation. Section VI,22). 

Amino acylations.1 This salt is far more reactive, particularly for O-acylation, than N,N -carbonyldiimidazole. Thus it effects esterification of N-Cbz protected amino acids with even hindered alcohols such as /-menthol in 98% yield without need of a base and, consequently, free from racemization. It also can effect coupling of amino acids in high yields and without racemization. 

The presence of ammonium salts either formed or added often makes it difficult to isolate free amino acids their separation is aided by esterification. 

V-Benzylidene-yV-oxides of a-amino acid esters may be hydrolysed to the corresponding A-hydroxyamino acid esters by treatment with hydroxylamine salts, in contrast to hydroxylamine which also causes ester cleavage.The A-oxide esters can be obtained from sodium oA7t/-benzaldoximate and a-bromoesters, or by esterification of the corresponding free acid (Scheme 54). 

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