The biosynthesis of the aroma volatile 2-methyltetrahydrothiophen-3-one in the bacterium Chitinophaga Fx7914.

2-Methyltetrahydrothiophen-3-one (3) is a volatile compound that plays an important role especially in food and flavour chemistry because it contributes to the aroma of several foodstuffs including wine. Although 3 can be formed by chemical reactions during food preparation, it is also produced by microorganisms. Recent studies with yeasts showed that methionine (1) is a potential precursor of 3, but the mechanism of the transformation is unknown. The biosynthetic pathway leading to 3 in the bacterium Chitinophaga Fx7914 was probed. Extensive feeding experiments with differently labelled precursors by using liquid cultures of Chitinophaga Fx7914 were performed. The volatiles released by the bacterium were collected by using a closed loop stripping apparatus (CLSA) and analysed by GC-MS. The observed incorporation pattern of the precursors into 3 led to the elucidation of the biosynthetic pathway. One part of the compound 2 originates from homocysteine (15), which is transformed into 3-mercaptopropanal (17). The second biosynthetic building block is pyruvate (14). An acyloin-forming reaction furnishes the key intermediate 21, which cyclises intramolecularly to a diol. Dehydration followed by tautomerisation lead to the cyclic ketone 3, which is produced by the bacterium in racemic form.