Antibacterial chromene derivatives from Psorothamnus fremontii

In our search for antibacterial compounds from plants, the ethanol extract of the native American plant Psorothamnus fremontii (Fabaceae) was identified as a hit due to its in vitro activity against methicillin-resistant Staphylococcus aureus (MRSA) with an IC₅₀ of < 8 µg/mL. Bioassay-guided fractionation of this extract was thus conducted, leading to the identification of five chromene derivatives including two new compounds. The structures of the new compounds were established by interpretation of spectroscopic data as 6-hydroxy-7-isobutyryl-3,3,5-trimethyl-2,3-dihydrooxireno[2,3-d]chromen-8(1aH)-one (1) and 6-hydroxy-7-isobutyryl-3,3-dimethyl-5-(3-methylbut-2-en-1-yl)-2,3-dihydrooxireno[2,3-d]chromen-8(1aH)-one (2). The known compound 1-(5,7-dihydroxy-2,2-dimethyl-6-(3-methylbut-2-en-1-yl)-2H-chromen-8-yl)ethanone (3) showed potent activity against MRSA with an IC₅₀ of < 0.8 µg/mL and appeared to be responsible for the activity of the crude plant extract.