3,3',4-Tri-O-methylellagic acid 4'-sulfate potassium salt (I, R1 = R2 = R3 = Me, R4 = SO3K) was isolated from Sinfito (Potentilla candicans) and shown to be an aldose reductase (AR) inhibitor (IC50 8.0 × 10-8M) and more potent than ellagic acid itself. Other derivatives (I; R1 = e.g., H, Ac, Me, Et, Pr; R2 = H, Ac, Me or SO3K, R3 = H, Me, Et, Ac or SO3K and R4 = H, Me, Ac or SO3K) were synthesized and their AR inhibitory activity was determined The activity decreased with increasing number of alkyl groups in I. However, all sulfonylated compounds increased the activity and thus, the sulfonyl group was shown to be very important and necessary for the activity.