A new, efficient, and general semisynthesis of hydnocarpin-type flavonolignans was developed and optimized, enabling gram-scale production of hydnocarpin D (2). Moreover, the syntheses of optically pure hydnocarpin isomers [(10R,11R)-hydnocarpin (1a), (10R,11R)-hydnocarpin D (2a), and (10S,11S)-hydnocarpin D (2b)], as well as the synthesis of isohydnocarpin (8), were achieved for the first time utilizing this new method. The synthesis is based on the two-step transformation of the readily available flavonolignans from milk thistle (Silybum marianum), accessible by isolation from the commercial extract silymarin. The first step relies on the regioselective formylation of the C-3 hydroxy group of the dihydroflavonol-type precursor using the Vilsmeier-Haack reagent, followed by formic acid elimination by triethylamine in the second step. The synthesized compounds were effective inhibitors of Staphylococcus aureus biofilm formation, with (10S,11S)-hydnocarpin D (2b) being the most potent inhibitor. Furthermore, the effect of glucose on biofilm formation was tested, and glucose decreased the biofilm inhibitory activity of 2b. Moreover, 2b increased the susceptibility of Staph. aureus to enrofloxacin.

Department of Biochemistry Faculty of Science Charles University Prague Hlavova 8 Prague 2 CZ 128 40 Czech Republic

Institute of Microbiology of the Academy of Sciences of the Czech Republic v v i Vídeňská 1083 Prague 4 CZ 142 20 Czech Republic

Institute of Organic Chemistry and Biochemistry of the AS CR v v i Flemingovo náměstí 2 Prague 6 CZ 166 10 Czech Republic

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Unconventional application of the Mitsunobu reaction: Selective flavonolignan dehydration yielding hydnocarpins