Microbial transformations of chalcones to produce food sweetener derivatives

Biotransformations of two substrates: chalcone (1) and 2′-hydroxychalcone (4) were carried out using four yeast strains and five filamentous fungi cultures. Substrate 1 was effectively hydrogenated in all of tested yeast cultures (80–99% of substrate conversion after 1 h of biotransformation) affording dihydrochalcone 2. In the cultures of filamentous fungi the reaction was much slower, however, Chaetomium sp. gave product 2 in 97% yield. After 12 h of incubation a reduction of dihydrochalcone 2 to alcohol 3 was additionally observed. After 3 days of biotransformation in the culture of Rhodotorula rubra product (S)-3 was obtained with 75% ee (enantiomeric excess) and 99% of conversion. Also after a 3-day biotransformation using the strain Fusarium culmorum product (R)-3 was obtained with 98% ee and 97% of conversion. In most of the tested strains a change in enantiomeric excess of compound 3 during the biotransformation process was noticed. In the culture of Rhodotorula glutinis after 3 h of transformation alcohol (R)-3 was formed with 47% ee and 31% of substrate conversion, whereas after 6 days the (S)-3 enantiomer was obtained with 99% ee and 91% of conversion. In the case of 2′-hydroxychalcone (4), the hydrogenation proceeded much slower and led to 2′-hydroxydihydrochalcone (5) – in the culture of Yarrowia lipolytica 97% of conversion was observed after 3 days. In all cultures of the tested strains no products of 2′-hydroxydihydrochalcone reduction were detected.