Pyruvate Aldol Condensation Product: A MetaboliteThat Escaped Synthetic Preparation for Over a Century
2020
The homoaldol condensation product
of pyruvate, 2-methyl-4-oxopent-2-enedioic acid (OMPD), has been
recently implicated as a catabolic intermediate in the bacterial degradation
of lignin and previously identified from other biological sources
in reports ranging over 60 years. Yet, while a preparation of the
pyruvate homoaldol product precursor, 4-hydroxy-4-methyl-2-oxoglutaric
acid (HMOG/Parapyruvate), was first reported in 1901, there has not
been a complete published synthesis of OMPD. Analyses of reaction mixtures have helped identify
zymonic acid, the lactone of HMOG, as the direct precursor to OMPD.
The reaction appears to proceed through an acid- or base-mediated
ring opening that does not involve formal lactone hydrolysis. In addition
to a preparative protocol, we provide a proposed mechanism for the
formation of methylsuccinic acid that arises from the nonoxidative
decarboxylation of OMPD. Finally, we calculated the relative stability
of the isomers of OMPD and found Z-OMPD to be the lowest in energy.
These computations also support our observations that Z-OMPD is the
most abundant isomer across a range of pH values.
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