Efficient Chemical Synthesis of (Epi)catechin Glucuronides: Brain-Targeted Metabolites for Treatment of Alzheimer's Disease and Other Neurological Disorders

Abstract

Grape seed extract (GSE) is rich in flavonoids and has been recognized to possess human health benefits. Our group and others have demonstrated that GSE is able to attenuate the development of Alzheimer's disease (AD). Moreover, our results have disclosed that the anti-Alzheimer's benefits are not directly/solely related to the dietary flavonoids themselves, but rather to their metabolites, particularly to the glucuronidated ones. To facilitate the understanding of regioisomer/stereoisomer-specific biological effects of (epi)catechin glucuronides, we here describe a concise chemical synthesis of authentic standards of catechin and epicatechin metabolites 3-12. The synthesis of glucuronides 9 and 12 is described here for the first time. The key reactions employed in the synthesis of the novel glucuronides 9 and 12 include the regioselective methylation of the 4'-hydroxyl group of (epi)catechin (≤1.0/99.0%; 3'-OMe/4'-OMe) and the regioselective deprotection of the tert-butyldimethylsilyl (TBS) group at position 5 (yielding up to 79%) over the others (3, 7 and 3' or 4').

Figure 1

General chemical structure of flavonoids and molecular structures of catechin (1), epicatechin (2), and their methylated and/or glucuronidated forms.

Figure 2

(A) HPLC analysis of crude reaction products from methylation of catechin: 3′-O-methyl catechin (3) and 4′-O-methyl catechin (4). (B) HPLC of crude reaction products from methylation of epicatechin: 3′-O-methyl epicatechin (5) and 4′-O-methyl epicatechin (6).

Scheme 1. Methylation of (Epi)catechin

Figure 3

(A) HPLC of the crude reaction product from the regioselective methylation of catechin: 4′-O-methyl catechin (4). (B) HPLC of the crude reaction product from the regioselective methylation of epicatechin: 4′-O-methyl epicatechin (6).

Scheme 2. Synthetic Procedure to Prepare (Epi)catechin-5-O-glucuronides

Reagents and conditions: (a) TBSCl, imidazole, DMF, rt 72 h; (b) trifluoroacetic acid (TFA), dichloromethane (DCM), 0 °C; (c) methyl 2,3,4-tri-O-(trichloroacetamidoyl)-α-d-glucuronate, BF₃·Et₂O, DCM, 4 Å molecular sieves, 0 °C, 24 h; (d) MeONa/MeOH (5.4 M), NaOH (0.5 M), tetrahydrofuran (THF)/MeOH (4:1, v/v), 0 °C, 3.5 h; (e) tetra-n-butylammonium fluoride (TBAF)/THF (1 M), THF/H₂O (8:2, v/v), rt, 3 h.

Scheme 3. Mechanism of Epimerization of Epicatechin via Quinone Methide

Figure 4

¹H NMR spectrum of 11 in DMSO-d₆ performed at different temperatures. The inset shows an expanded region of H6 signals from the pair of conformational isomers present in a ratio of approximately 1:0.47 (A, 25 °C) or 1:0.23 (B, 10 °C).