Optically active polymers of 3-alkylmalic acids: Contribution of the bioconversion for diversifying the chiral precursors

The need for new optically active monomers and polymers is conducive to the setting up of stereospecific synthesis routes starting from chiral precursors. The biomass can be considered as a major source for extracting such biomolecules aimed at chemoenzymatic transformation and further polymerization. Due to its versatility, beta-methylaspartate ammonia-lyase, from cell-free extracts of Clostridium tetanomorphum, has been used in the bioconversion of alkylfumarates into optically active pure 3-alkylaspartic acids with alkyl=methyl, ethyl, isopropyl. These amino acids have been transformed in several steps into optically active benzyl 3-alkylmalolactonates leading to semi-crystalline polyesters. 3-Methylaspartic acid includes two chiral centers and the racemic compound containing the four stereoisomers can be prepared by a multiple step synthesis. The ability of beta-methylaspartase to catalyse both syn- and anti-elimination of ammonia from natural 3-methylaspartic acid has been expressed to retain one stereoisomer and this bioconversion is a preparative method for obtaining unnatural stereoisomers. Moreover, the catalytic hydrogenolysis of the benzyl alpha,beta-substituted beta-lactone yields stable 3-alkylmalolactonic acid which can be coupled with functional alcohols and copolymerized. At last the introduction of (2S)-3,3-dimethyl-1-2-butanol, using Rhodotorula glutinis as microorganism in a biological synthesis step, as chiral ester pendant group, has conducted to optically active polyesters with very high melting transition temperatures. The combination of bioconversion and chemical synthesis is a very useful tool for building hydrolyzable functionalized polyesters required for temporary applications.