Pruinosanones A-C, anti-inflammatory isoflavone derivatives from Caragana pruinosa

Pruinosanone A (1), a novel spirochromone, was isolated from the roots of Caragana pruinosa. Two biogenetically related isoflavone intermediates, pruinosanones B and C (2 and 3), were also isolated, together with five known analogs identified as 3-hydroxy-9-methoxypterocarpan (4), 7,2'-dihydroxy-4'-methoxyisoflavanol (5), retusin-8-methylether (6), 7,2'-dihydroxy-8,4'-dimethoxy isoflavone (7) and 7,3'-dihydroxy-8,4'-dimethoxy isoflavone (8). The structures of 1-3 were elucidated based on extensive spectroscopic methods. Notably, 1 is the first example of a spirochromone possessing an unprecedented pentacyclic skeleton containing a spiro[benzo[d][1,3]dioxole-2,3'-chroman]-4'-one motif, which was confirmed by X-ray diffraction analysis. A plausible biosynthetic pathway for 1 was also proposed. Compounds 1-8 were tested for their ability to inhibit nitric oxide (NO) production in LPS-induced RAW 264.7 macrophages, and compounds 1-3 were the most potent inhibitors of NO production, with IC50 values of 1.96, 1.93 and 1.58 μM, respectively. A structure-activity relationship analysis revealed that the fused 2-isopropenyl-2,3-dihydrofuran moiety plays a vital role in the potency of these compounds. Moreover, 1 was found to significantly inhibit inducible nitric oxide synthase (iNOS) protein expression, which accounts for the potent inhibition of NO production by this spirochromone.