Acid Rearrangement of Secoiridoids Related to Oleuropein and Secologanin

Abstract

Acid treatment of an iridoid glycoside results in the cleavage of the acetal bond between the sugar unit and the monoterpenoid aglycon. Iridoids possessing non‐conjugated enol ether systems, however, undergo the hydration of the iridoid enol ether functionality in acid medium, as well as the hydrolysis of the bond. We examined the acid rearrangement of secoiridoids such as oleuropein (1) and secologanin (2) and their reduction products oleuropeinol (3) and secologaninol (4), to examine whether similar behaviour also occurs in this case. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Oleuropein: A Valuable Bio‐Renewable Synthetic Building Block

“…This means that the equilibrium shifts towards the open form, which is further acetylated. The same mixture was also reduced with sodium borohydride in methanol, transforming the aldehyde into the alcohol 12 (98 % yield; Figure ) …”

“…This reaction produces a mixture of compounds 14 and 15 , with compound 14 being formed in larger amounts (40 % yield). This suggests the existence of an equilibrium between these two compounds, with compound 15 converting slowly into the more stable compound 14 (Figure ) …”

“…The same mixture was also reduced with sodium borohydride in methanol, transforming the aldehyde into the alcohol 12 (98 % yield; Figure 11). [44] The only reported modification of oleuropein in methanol has been the transesterification of the hydroxytyrosyl ester to a methyl ester. In contrast, the acid rearrangement of oleuropeinol in methanol gave two major products.…”

“…This suggests the existence of an equilibrium between these two compounds, with compound 15 converting slowly into the more stable compound 14 (Figure 12). [44] An efficient procedure for a semisynthesis of oleacein (16) from oleuropein through Krapcho demethoxycarbonylation was developed. The possible conversion of oleuropein into oleacein involves three main transformations: cleavage of the glucose moiety, secoiridoid ring-opening followed by the formation of the two aldehyde groups and demethoxycarbonylation.…”

Valorization of Oleuropein via Tunable Acid‐Promoted Methanolysis

“…[8] These transformations are summarized in Scheme 1, and include selectiveh ydrolysis of the hydroxytyrosol ester (A, Scheme 1); [9] formation of oleuropeinol( 3) through reduction of both methyl and hydroxytyrosol esters (B, Scheme 1); [10] enzymatic acetal cleavage by b-glucosidase to form either pyridine alkaloid jasminine (4;C ,Scheme 1) [11] or compound 5 (D, Scheme1), [12] depending on the ammonium salt used;a nd formation of oleacein (6)t hrough Krapcho decarbomethoxylation (E, Scheme 1). [8] These transformations are summarized in Scheme 1, and include selectiveh ydrolysis of the hydroxytyrosol ester (A, Scheme 1); [9] formation of oleuropeinol( 3) through reduction of both methyl and hydroxytyrosol esters (B, Scheme 1); [10] enzymatic acetal cleavage by b-glucosidase to form either pyridine alkaloid jasminine (4;C ,Scheme 1) [11] or compound 5 (D, Scheme1), [12] depending on the ammonium salt used;a nd formation of oleacein (6)t hrough Krapcho decarbomethoxylation (E, Scheme 1).…”