AMDase catalyzes the decarboxylation of disubstituted aryl-and alkenylmalonic acids with outstanding stereoselectivity. The selectivity is outstanding, giving access to a large series of the corresponding a-arylpropionic acids and a-alkenylpropionic acids. a-Chiral carboxylic acids play a fundamental role as structural components in many active pharmaceutical ingredients. Few enzymatic reactions provide access to these building blocks, and their selectivity is often limited. Consequently, AMDase has received considerable attention. The enzyme has been used for the asymmetric synthesis of non-steroidal 'profen' anti-inflammatory drugs such as (S)-flurbiprofen and (S)-naproxen, a-heterocyclic propionic acids, and a-alkenylcarboxylic acids that are building blocks for natural products and pesticides. The substrate scope of the enzyme is on the one hand very wide in respect of the larger substituent, with the main requirement that it should bear a p-electron system. In contrast, the size of the smaller substituent is limited; H, CH3, Cl, Br, OH, and NH2 groups are accepted, but 2-aryl-2-ethyl malonates are not converted. Another challenge for the application of AMDase lies in the spontaneous decarboxylation of a-arylmalonate substrates during their synthesis from the corresponding dialkyl esters. We solved this problem by mild hydrogenolysis of the corresponding dibenzyl esters, a reaction that was scaled-up by Pfizer and applied for the asymmetric synthesis of optically pure a-(N-heterocyclic) propionic acids. The outstanding stereoselectivity of AMDase originates from its mechanism.