cis–trans Isomerization of silybins A and B

• Michaela Novotná,
• Radek Gažák,
• David Biedermann,
• Florent Di Meo,
• Petr Marhol,
• Marek Kuzma,
• Lucie Bednárová,
• Kateřina Fuksová,
• Patrick Trouillas and
• Vladimír Křen

Beilstein J. Org. Chem. 2014, 10, 1047–1063, doi:10.3762/bjoc.10.105

• ; 10,11-cis-silybin; isomerization; silibinin; silybin; silymarin; Introduction The flavonolignan silybin (alternative name silibinin), occurring in the fruits of Silybum marianum (milk thistle), consists of two stereoisomers – silybin A (1a) and B (1b) – in a ca. 1:1 ratio (Figure 1). Their absolute

• silybin derivatives 2a and 2b, respectively. The unknown compounds exhibit a J10,11 of ca. 3 Hz, whereas natural silybin (1) has a J10,11 of ca. 8 Hz. Based on this evidence, the unknown compounds from the acylation reaction were proposed to be a diastereoisomeric mixture of 23-O-acetyl-10,11-cis-silybin

• A (7) and B (8) (Figure 3) in a ca. 1:1 ratio. As the yields of the two 10,11-cis-silybin isomers were low (ca. 10%), we optimized the reaction conditions for better yields. In the initial screening, aimed at finding the most suitable solvent and Lewis acid (Table 1), we used natural silybin, i.e