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In CH 3CN and DMSO silybin decomposed, and in CHCl 3 the reaction failed. The following solvents were tested: EtOAc, DMF, CH 3CN, DMSO, CHCl 3 at 0, 25, and 50 ☌. The choice of solvent was limited by the low solubility of silybin in most organic solvents or by their incompatibility with the Lewis acid BF 3 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. 3 Hz, whereas natural silybin ( 1) has a J 10,11 of ca. The unknown compounds exhibit a J 10,11 of ca. 1H NMR spectra indicated that the two compounds differ only slightly from the silybin derivatives 2a and 2b, respectively. In HPLC they were assigned to two separate peaks with the same molecular mass ( m/ z 524). Separation of the resulting mixture by silica gel column chromatography yielded an inseparable mixture of the two new compounds. Enzymatic alcoholysis ( n-butanol) of this mixture by Novozym 435 led, after a prolonged reaction time, to the removal of the acetyl groups from trans-isomers 2a and 2b to give 1a and 1b ( Figure 1), while the two minor isomers remained acetylated. 90%), but two novel compounds with UV spectra similar to that of silybin. In our previous work on the enzymatic kinetic resolution of silybin, BF 3∙OEt 2 in EtOAc was found to catalyze the transesterification of silybin to yield not only 23- O-acetylsilybin ( 2, ca. The aim of this work was to prepare stereochemically pure 2,3- cis- and 10,11- cis-isomers of silybin A ( 1a) and B ( 1b) by a Lewis acid catalyzed isomerization to determine their absolute configuration and to propose a mechanism for the cis– trans-isomerization processes.
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Therefore, the detailed structural knowledge and availability of (potential) minor impurities is a fundamental requisite, e.g., for master file assembly. Silybin is an important pharmaceutical commodity and a complete understanding of its composition is needed in order to understand its pharmaceutical properties. Therefore, their identification in a particular silymarin preparation would have only limited information value. Variations of the minority content (not only silybin cis-isomers) is even more pronounced. marianum and by the cultivation, harvest and processing conditions. The composition of silymarin strongly depends upon its source, which is influenced by the variety of S. The content of silybin cis-isomers in silymarin is presumably very low. Unfortunately, the conventional identification by LC–MS or UV–vis scanning is inadequate as the major and minor compounds exhibit the same MS and UV profiles.
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The isolation of naturally occurring cis-isomers in the pure form and their complete structure identification would be the only unambiguous way how to prove identity of natural and synthetic cis-isomers. The origin of the cis-isomers either as biosynthetic side products or as artifacts formed during their isolation is also unknown.
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Nevertheless, their absolute configurations remained unknown. 11 Hz in the trans-isomers and 2–3 Hz in the cis-isomers. Other authors reported 2,3- cis-isomers of silybin, the relative configuration of which were corroborated by 1H NMR coupling constants, i.e., J 2, 3 of ca. Another study focusing on the minor components of silymarin identified two new compounds isosilybin C ( 3) and D ( 4), however these were shown to be regioisomers of isosilybins A ( 5) and B ( 6) ( Figure 2). It was speculated that some of them were 10,11- cis-analogues of the major silymarin constituents, but these minor components were never isolated in sufficient amount and purity to enable such unwarranted hypotheses to be verified. Little is known about the structures of the minor components of silymarin. The mechanism of this coupling reaction was described and implied a 10,11- trans relative configuration, as in all the major components of silymarin. marianum) are products of a phenolic oxidative coupling of the flavonoid taxifolin and the lignan coniferyl alcohol. Both silybin isomers as well as other flavonolignans from silymarin (crude defatted extract from the fruits of S. Their absolute configuration is known and their separation was accomplished recently. 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.