Medicinal Flowers. XXXX.¹⁾ Structures of Dihydroisocoumarin Glycosides and Inhibitory Effects on Aldose Reducatase from the Flowers of Hydrangea macrophylla var. thunbergii

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Six dihydroisocoumarin glycosides, florahydrosides I and II, thunberginol G 8-O-β-d-glucopyranoside, thunberginol C 8-O-β-d-glucopyranoside, 4-hydroxythunberginol G 3′-O-β-d-glucopyranoside, and thunberginol D 3′-O-β-d-glucopyranoside, have been isolated from the flowers of Hydrangea macrophylla Seringe var. thunbergii Makino (Saxifragaceae) together with 20 known compounds. The chemical structures of the new compounds were elucidated on the basis of chemical and physicochemical evidence. Among the constituents, acylated quinic acid analog, neochlorogenic acid, was shown to substantially inhibit aldose reductase [IC₅₀=5.6 µm]. In addition, the inhibitory effects on aldose reductase of several caffeoylquinic acid analogs were examined for structure–activity relationship study. As the results, 4,5-O-trans-p-dicaffeoyl-d-quinic acid was found to exhibit a potent inhibitory effect [IC₅₀=0.29 µm].