Abstract
The sweet natural compound monatin 1 has two stereogenic centres and the (2__S__,4__S__) absolute configuration has been attributed to the natural isomer. We obtained all four stereoisomers as pure compounds by a sixβstep synthetic sequence. The stereogenic centre at Cβ4 was introduced stereoselectively by a regioβ 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 Xβray diffraction of chiral derivatives. The stereogenic centre at Cβ2 was introduced nonβspecifically, and the resulting diastereomeric mixtures were separated by RPβHPLC. The absolute configurations of the final products were established by comparing retention times on a chiral HPLC column with those of known samples. The four stereoisomers were submitted to tasting trials and three of them, particularly the (2__R__,4__R__) isomer, were found to be intensely sweet. A sample of natural monatin analysed under the same conditions is shown to contain all the four stereoisomers. The relative stereoisomeric content in the plant, as well as the possible isomerisation of the chiral centres during extraction and manipulation of monatin samples, are important points that need to be clarified by extensive analysis of the natural extracts. (Β© WileyβVCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)