Studies in Polyphenol Chemistry and Bioactivity. 4.¹ Synthesis of Trimeric, Tetrameric, Pentameric, and Higher Oligomeric Epicatechin-Derived Procyanidins Having All-4β,8-Interflavan Connectivity and Their Inhibition of Cancer Cell Growth through Cell Cycle Arrest¹

Abstract

We report an improved synthesis of bis(5,7,3',4'-tetra-O-benzyl)epicatechin 4beta,8-dimer (3) from 5,7,3',4'-tetra-O-benzylepicatechin (1) and 5,7,3',4'-tetra-O-benzyl-4-(2-hydroxyethoxy)epicatechin (2) by replacing the previously employed Lewis acid, titanium tetrachloride, with the clay mineral Bentonite K-10. Under the same conditions, the benzyl-protected all-4beta,8-trimer, -tetramer, and -pentamer were obtained regioselectively from their lower homologues, albeit in rapidly decreasing yields. Reaction of 2 with an organoaluminum thiolate generated from 2-mercaptobenzothiazole and trimethylaluminum followed by acetylation produced 3-O-acetyl-4-[(2-benzothiazolyl)thio]-5,7,3',4'-tetra-O-benzylepicatechin (12). Medium-sized protected oligomers with 4beta,8-interflavan linkages are obtained in improved yields by using this compound as the electrophile and silver tetrafluoroborate as activator and are isolated by reversed-phase HPLC. Their deprotection by ester saponification followed by hydrogenolysis yielded the free procyanidins, which were characterized as their peracetates. The synthetic procyanidins are identical by normal-phase HPLC with fractions isolated from cocoa. The principle of chain extension by two members was demonstrated using a dimeric electrophile obtained by self-condensation of compound 12. Both the synthetic and natural pentamer 32 inhibit the growth of several breast cancer cell lines. Using the MDA MB 231 line, it was established that this outcome is based on the induction of cell cycle arrest in the G0/G1 phase. Subsequent cell death is more likely necrotic rather than apoptotic. Control experiments demonstrate that the polyphenol itself, rather than hydrogen peroxide potentially formed by its autoxidation, is the causative agent.

Synthesis of Procyanidins C2 and C1 Using Lewis Acid Mediated Equimolar Condensation

“…9 However, the yield of transformation from 5 to 6 was not high enough, we tried the Kozikowski's method using n-Bu 4 NOH to afford 6 in 66% yield. 14 The 1 H and 13 C NMR spectral data of 6 were identical with those of the reported values. 9 The 3,4-cis trimer could not be detected.…”

“…As to the purification of 1, when the crude products were directly evaporated, partially insoluble materials remained. 14 We met with some difficulty to purify 1 after debenzylation. We tried separation by ODS cartridge column chromatography and preparative HPLC, however, we could not obtain 1 in a satisfying yield.…”

Oligomeric catechins: An enabling synthetic strategy by orthogonal activation and C(8) protection

“…However, because the catechin unit I also maintains a nucleophilic center itself, it can also serve as a nucleophile, leading to self-condensation products and multiple undesired reactions. A possible, but painstaking and inelegant solution to this conundrum is the use of nucleophilic partner II in excess for avoiding self-reactions of I (7,(11)(12)(13).With this issue in mind, we embarked on the development of a synthetic strategy to prepare oligomeric catechins. Our basis for innovation was the ''sugar-flavonoid analogy'' inferred by the S N 1 reactivity shared by carbohydrates and polyphenols ( Fig.…”

“…These exciting properties have inspired synthetic efforts aimed at producing and characterizing these challenging molecules and elucidating their biological mode of action. Noteworthy are the elegant contributions by Kozikowski, Tückmantel, and coworkers (7)(8)(9)(10), who have shown the potential biological activities of higher epicatechin oligomers prepared by nonselective oligomerization and separation.…”

“…However, because the catechin unit I also maintains a nucleophilic center itself, it can also serve as a nucleophile, leading to self-condensation products and multiple undesired reactions. A possible, but painstaking and inelegant solution to this conundrum is the use of nucleophilic partner II in excess for avoiding self-reactions of I (7,(11)(12)(13).…”