Scale-Up of a Chemo-Biocatalytic Route to (2R,4R)- and (2S,4S)-Monatin

Monatin, a natural sweetener, refers to a collection of four isomers of 2-((1H-indol-3-yl)methyl)-4-amino-2-hydroxypentanedioic acid. A chemo-biocatalytic approach to kilogram quantities of enantiopure 2S,4S-monatin and 2R,4R-monatin from indole is described. Key steps in the process include a (2 + 3) cycloaddition reaction followed by nickel-catalysed reduction to construct the monatin backbone, and a highly selective enzyme resolution of the 2S,4S- and 2R,4R-monatin diastereomeric pair to afford each enantiomer in 99% ee.