Synthesis and Evaluation of Influenza Virus Sialidase Inhibitory Activity of Hinokiflavone-Sialic Acid Conjugates

Kunio Takahashi, Kazuhiko Miki, Takayuki Nagai, Takayuki Nakamura, Mitsuru Tuji, Kiyotaka Koyama, Kaoru Kinoshita, Kimio Furuhata, Haruki Yamada
2008 Heterocycles  
The known biflavonoid, hinokiflavone (1) was isolated from the leaves of Metasequoia glyptostroboides Hu et Cheng and displayed influenza A and B virus sialidase inhibitory activity. The unnatural glycoconjugate, hinokiflavone-sialic acid (8) was synthesized and exhibited more potent inhibitory activity. HETEROCYCLES, Vol. 75, No. 4, 2008 879 hinokiflavone-sialic acid conjugates and evaluated their inhibitory activity against influenza virus sialidase. RESULTS AND DISCUSSION The leaves of
more » ... The leaves of Metasequoia glyptostroboides Hu et Cheng (289.4 g), collected in Tokyo prefecture, Japan, were extracted with acetone seven times. The acetone extract (26.2 g) was subjected to silica gel column chromatography using CHCl 3 -MeOH-DMSO to obtain known biflavonoid, hinokiflavone (1) (758.4 mg). Hinokiflavone (1) was identified by comparing its spectroscopic data with reported data. 2 The glycosylation of the arylic hydroxyl group to sialic acid (Neu5Ac) (2) was performed by the Williamson's method. 3, 4 The preparation of the methyl ester of Neu5Ac (3) was carried out by using a dried strong cation-exchange resin (Dowex-50W-X2) with MeOH. Following acetylation of 3 with Ac 2 O and pyridine, the chloro derivative (5) was prepared from 4 by AcCl. Compound 5 was identified by comparing its spectral data with that reported in the literature. 5 O HO OH HO AcHN OH HO O HO OH HO AcHN OH HO O AcO OAc AcO AcHN OAc AcO O AcO Cl AcO AcHN OAc AcO a 2 c CO 2 H CO 2 Me CO 2 Me CO 2 Me 100% 89% 87% b 5 3 4 Scheme 1. Reagents and conditions: (a) MeOH, Dowex-50W-X2, rt, 48 h; (b) Ac 2 O, pyridine, rt, 20 h; (c) AcCl, rt, 48 h. To synthesize hinokiflavone-sialic acid (6), a Williamson's reaction using sodium hydride was carried out to combine the chloro derivative of Neu5Ac (5) with the C-7" position of 1. Compound 6 was assigned the molecular formula of C 50 H 45 NO 22 by its positive HRFABMS spectral data m/z 1012.2524 [M+H] + , (calcd 1012.2512, C 50 H 45 NO 22 , [M+H] + ). The UV spectrum (MeOH) of 6 showed absorption maxima at λ max nm (log ε): 209 (4.80), 269 (4.56) and 336 (4.68) nm, indicating the presence of aromatic rings. The IR spectrum exhibited the presence of hydroxyl or amino groups (3430 cm -1 ) and carbonyl (1750, 1660, 1610 cm -1 ) groups. The 1 H-NMR spectrum showed six singlet peaks at δ 1.71, 1.87, 1.91, 2.03, 2.16 and 3.68 (each 3H, s) each due to a methyl group. The last methyl group was assigned to the methyl ester. The spectrum exhibited signals in the aromatic region (δ 6.20-8.15) due to the hinokiflavone skeleton, and between δ 1.91-5.40 due to the sialic acid part. The carboxyl methyl proton at δ 3.68 showed NOESY 880 HETEROCYCLES, Vol. 75, No. 4, 2008 1H, J = 12.9, 4.6 Hz), 3.68 (s, 3H), 3.94 (q, 1H, J = 10.2 Hz), 4.10 (dd, 1H, J = 12.2, 6.3 Hz), 4.22 (dd, 1H, J = 12.2, 2.9 Hz), 4.45 (dd, 1H, J = 10.7, 1.5 Hz), 4.72 (ddd, 1H, J = 10.9, 10.9, 4.6 Hz), 5.17 (dd, 1H, J = 8.8, 1.5 Hz), 5.40 (m, 1H), 6.20 (d, 1H, J = 2.2 Hz), 6.49 (d, 1H, J = 2.2 Hz), 6.89 (s, 1H), 6.93 (d, 2H, J = 8.8 Hz), 6.99 (s, 1H), 7.07 (d, 2H, J = 8.8 Hz), 7.10 (s, 1H), 7.80 (d, 1H, J = 9.8 Hz), 8.04 (d, 2H, J = 8.8 Hz), 8.15 (d, 2H, J = 8.8 Hz), 10.55 (br. s, 1H), 10.76 (br. s, 1H), 12.87 (s, 1H), 13.23 (s, 1H);
doi:10.3987/com-07-11285 fatcat:jcgnl7pstzcxjngh4hvwnfstwa