Monatin and Its Stereoisomers: Chemoenzymatic Synthesis and Taste Properties

Abstract

The sweet natural compound monatin 1 has two stereogenic centres and the (2S,4S) absolute configuration has been attributed to the natural isomer. We obtained all four stereoisomers as pure compounds by a six-step synthetic sequence. The stereogenic centre at C-4 was introduced stereoselectively by a regio-and enantiospecific enzymatic hydrolysis of the racemic ethyl dicarboxylate 4 using a protease from Aspergillus oryzae. The absolute configuration of the intermediate products was assigned by X-ray diffraction of chiral derivatives. The stereogenic centre at C-2 was introduced non-specifically, and the resulting diastereomeric mixtures were separated by RP-HPLC. The absolute configurations of the final products were established by comparing retention

Preparation and Characterization of Four Stereoisomers of Monatin

“…However, these results are difficult to reconcile with the stereochemical assignments made by Vleggaar et al and Bassoli et al, in which they reported that the natural monatin they obtained was a mixture of the four stereoisomers. 12) We do not have any information regarding their detailed extraction methodology; thus, it is not possible to conclude whether these isomers were already present in the plant or if they occurred as a result of racemization during isolation. Taking the sweetness potency of each isomer into account, there is a possibility that the monatin isolated by Vleggaar et al may be a mixture of 1a and the (2R,4R)-isomer (1c).…”

“…7,8) Bassoli et al disclosed the isolation of the four stereoisomers of monatin and sensory analysis of each stereoisomer; however, the chemical and optical purities of each stereoisomer were not sufficiently indicated. 12) In their study, the assignment of stereochemistry of the products was justified by HPLC analysis using a chiral column and carried out by our laboratories. 12) We have been interested in the taste profiles of other stereoisomers of monatin, particularly that of (2R)-isomers, as it is known that (R)-amino acids (D-amino acids) such as (R)-tryptophan exhibit a sweet taste, while (S)-amino acids do not, with the exception of (S)-alanine, (S)-proline, and (S)-serine.…”

“…12) In their study, the assignment of stereochemistry of the products was justified by HPLC analysis using a chiral column and carried out by our laboratories. 12) We have been interested in the taste profiles of other stereoisomers of monatin, particularly that of (2R)-isomers, as it is known that (R)-amino acids (D-amino acids) such as (R)-tryptophan exhibit a sweet taste, while (S)-amino acids do not, with the exception of (S)-alanine, (S)-proline, and (S)-serine. 13,14) As mentioned above, there has been no clear data for the intensity of sweetness of each monatin stereoisomer in practical concentrations corresponding to 5-10% sucrose concentration, with the exception of natural monatin.…”

Concise Synthesis of (2<i>R</i>,4<i>R</i>)-Monatin

“…1) We previously reported that all isomers exhibit a sweet taste; however, (2S,4S)-monatin was found to be the least sweet isomer, whereas the other three isomers, especially (2R,4R)-monatin, were found to be intensely sweet (1700 times at 10% sucrose equivalent). 2,3) A major problem in the synthesis of (2R,4R)-monatin is the highly diastereoselective formation of a tetra-substituted carbon center at C4. Various chemical synthetic methods have been reported for the formation of the (2S,4S)-isomer, some of which produce a mixture of stereoisomers, [4][5][6][7] whereas others describe stereoselective syntheses.…”

“…3,14) Since the racemization at C4 of these ester derivatives is impossible, these methods are not efficient for obtaining a single stereoisomer of monatin.…”