Abstract
We herein report our studies into the effects of microwave irradiation on the solvent-free esterification of L-leucine with alcohols. In the absence of solvent, microwave irradiation accelerated the reaction compared to conventional oil bath heating. Measurement of the dielectric properties under the reaction conditions revealed that the high dielectric loss factor of the reaction mixture containing L-leucine, n-butanol, and a p-toluene sulfonic acid catalyst could be attributed to the acceleration of the reaction. The depth of microwave penetration into the reaction mixture as derived from the in-situ measurement of the dielectric properties was ~13 mm, which suggested that a thinner reaction vessel was favorable for the esterification of L-leucine. In addition to the batch reaction using a desktop microwave reactor, two types of flow reaction were also performed using a desktop tubular reactor and a semi-bench-scale tubular reactor. These flow reactions also exhibited high performances, thus allowing the scale-up of this reaction system for industrial use.
Highlights
Microwaves are electromagnetic waves with frequencies ranging between 300 MHz and 30 GHz
In the context of organic synthesis, amino acid esters are widely employed as inter-mediates for a wide range of applications, such as medicinal chemistry [12], peptide synthesis [13], asymmetric synthesis [14], dye-sensitized solar cells [15], and polymer chemistry [16]
We discussed our studies into the application of microwave irradiation to the solvent-free esterification of L-leucine with alcohols, namely ethanol and n-butanol
Summary
Microwaves are electromagnetic waves with frequencies ranging between 300 MHz and 30 GHz. As the amino group present in amino acids can form an amide moiety through bonding with the carboxylic group of another molecule, the amino group must be protected prior to esterification. This is typically carried out using an equimolar amount of acid to produce the species shown in Eq (1). In terms of the esterification reaction itself, amino acids can be esterified using a combination of methanol and thionyl chloride, where the acid chloride reacts with methanol to form the ester [26]. Safer and more efficient methods of amino acid esterification are required for application in industry
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