Absolute Structures of 3-Hydroxy-2-prenylflavanones with an Ether Linkage between the 2’- and 3-Positions from Moraceous Plants

Absolute configurations at C-2 and C-3 of eight sanggenon-type flavanones (3-hydroxy-2-prenylflavanones with an ether linkage between the C-2′ and C-3 positions, i.e., sanggenons A, C, L, and M, sanggenols F, and G, and soroceins D and F.) were determined as (2R,3S)-configurations by chemical and spectroscopic methods.Many 3-hydroxy-2-prenylflavanones with an ether linkage between the 2′-and 3-positions have been isolated from moraceous plants, e.g., Morus cathayana, Sorocea bonplandii, S. ilicifolia, and the traditional Chinese herbal medicine, "Sang-bai-pi". 2-16Among these compounds, sanggenons C (1) and D(2) were isolated as hypotensive constituents of sang-bai-pi. 5, 6 In addition, sanggenon D (2) inhibited the specific 3 H-12-O-tetradecanoylphorbol-13-acetate binding to mouse skin particulate, the activation of protein kinase C with teleocidin, and the induction of ornithin decarboxylase activity by teleocidin in mouse skin. 14These agents (1 and 2) are Diels-Alder type adducts derived from a chalcone (A′-ring-C-8″-C-4″-C-5″-B′-ring of 1) and a sanggenon-type flavanone 17 with a 6-dehydroprenyl group by an enzymatic reaction.Recently, we reported that these sanggenon-type flavanones showed higher cytotoxic action against human oral tumor cell lines (HSC-2 and HSG) than against normal human gingival fibroblast. 18However, absolute structures of these sanggenon-type flavanones have not yet been determined.We report here the elucidation of the absolute configurations at C-2 and C-3 of eight of these 2-prenyl-3-hydroxyflavanones.Previously, Nomura et al. reported that the relationship between the 2prenyl and 3-hydroxyl groups of sanggenon A (3) was a cis-configuration in relative. 7,16 lkaline treatment of the compound (3, [α] D + 176°) produced an angular type flavanone, sanggenon M [(±)-4],