Fumaric acid substrate selection

Until now, examples were discussed in which amino acids are produced from mixed organic matter substrates. It is also possible to start with defined chemical compounds. An example is the synthesis of L-alanine from fumaric acid in a two-step reaction. Other examples for a highly selective fermentation are the synthesis of L-Dopa from orthocatechol and of L-tyrosine from phenol. 

In addition, the use of enzyme selective inhibitors has turned out to be very effective. Reductions were performed by adding l,l,l-trifluoro-2,4-pentane-dione 1 to a yeast-water suspension with selected additives such as methylvinyl-ketone, allyl alcohol, alkanoic acids, ethyl chloroacetate or allyl bromide, all of them reported to affect the stereochemical course of baker s yeast reduction. In some cases, both the influence of the yeast/substrate ratio and the influence of the presence of glucose were considered. In the presence of alkanoic acids (acetic, fumaric, or oleic acid), no significant effect was observed. However, addition of methylvinyl-ketone, allyl alcohol, ethyl chloroacetate and allyl bromide to the reaction system affected the stereochemical course of the reduction of 1. In particular, (R)-(+)-2 was produced in the presence of ethyl chloroacetate and allyl bromide as additive. 

This conjugate addition proceeds in excellent yields. High regio- and diastereo-selectivity is observed with lanthanide and other Lewis acids (entries 3, 4 and 6), but little selectivity is seen in the absence of a chelating Lewis acid (entries I and 2) and with substoichiometric amounts of Lewis acid (entry 5). It had previously been established that radical addition to crotonates and cinnamates could be accomplished with catalytic amounts of Lewis acid. However, the fumarate substrate is orders of magnitude more reactive, and hence radical addition to the uncom-plexed substrate presumably competes, leading to lower observed selectivity. 

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