Triacetic acid lactone (TAL;[1] 4-hydroxy-6-methyl-2-pyrone) is an organic compound derived enzymatically from glucose. It is a light yellow solid that is soluble in organic solvents.
Triacetic acid lactone consists of two main tautomers.
The tautomer on the left, featuring a 4-hydroxy group, the C4 carbon, is dominant. Triacetic acid lactone is classified as a 2-pyrone compound owing to the ketone group on the C2 carbon in its dominant form.
Triacetic acid lactone is synthesized either from dehydroacetic acid, another 2-pyrone derivative, or from glucose by enzymatic catalysis. In its original synthesis, triacetic acid lactone was obtained by treatment of dehydroacetic acid with sulfuric acid at 135 °C. Dehydroacetic acid undergoes ring-opening and hydration to form "tetracetic acid".[2] Upon cooling, triacetic acid reverts to a lactone ring similar to the dehydroacetic acid structure, and the triacetic acid lactone is recovered by crystallization in cold water.
The microbial synthesis of triacetic acid lactone requires the enzyme 2-pyrone synthase (2-PS).[3] This enzyme has been examined in two hosts Escherichia coli and Saccharomyces cerevisiae. The Saccharomyces cerevisiae host being used during the synthesis produces a higher yield (70%) compared with the Escherichia coli host, which produces a yield of 40% of triacetic acid lactone. This enzyme catalyzes the synthesis of triacetic acid lactone from acetyl-CoA via two subsequent condensations with malonyl-CoA. This produces an intermediate of 3,5-diketohexanoate thioester, which undergoes ring closure to produce triacetic acid lactone.
The lactone is a versatile intermediate in organic synthesis. Substantial negative charge accumulates on the C3 carbon, rendering it nucleophilic, but the C5 carbon is inert.
It has also been described as a platform chemical, meaning that it could be the precursor to other fine chemicals. The lactone undergoes decarboxylation to acetylacetone. It is also a precursor to sorbic acid, dienoic acid, and hexenoic acid. Dienoic acid is used to inhibit the growth of various molds and hexenoic acid is used as a flavoring agent.[4] Acetylacetone is used for metal extraction and plating and as a food additive.[5]