Iridoids acid hydrolysis

Following the isolation and structure elucidation of rhexifoline (23) from Castilleja rhexifolia (35), further work on the flower heads yielded three iridoid glucosides, one of which was penstemonoside (302). Recognizing the potential relationship between 23 and 302, Roby and Stermitz carried out the synthetic transformation of 302 to 23 (81). Treatment of penstemonoside (302) with /3-glucosidase afforded a 2 1 epimeric mixture of the lactol 303. Reaction with methanolic HC1 followed by ammonia afforded rhexifoline (23) in 31% overall yield (Scheme 30) (81). Euphroside (304), the principal iridoid of Orthocarpus spp., was used for the structure confirmation of euphrosine (25) (40). Acid hydrolysis of 304 in methanol afforded 305, which was treated with aqueous acid followed by basification with ammonia to afford a low yield of euphrosine (25), identical with the natural product (Scheme 31) (40). 

Valepotriatas iridoids from Valeriana and Ken-tranthus spp. Valeriana officinalis contains up to 5 % V., which are responsible for the sedative properties of this drug. Hydroxyl groups on the iridoid structure are esterified with isovaleric acid. Hydrolysis of the esters with HCl causes decomposition of the unstable alcohol moiety with production of a blue color. The most important representative is Valtratum (see). 

The solubility of iridoids depends on their state (free, glycosylated, acetylated), but usually they are extracted with polar solvents methanol, ethanol, aqueous alcohols, and rarely acetone. Iridoid glycosides are more or less stable some of them are very sensitive to acids and alkalis. Some iridoid glycosides such as aucubin suffer color modification after chemical or enzymatic hydrolysis they give first a blue to green... 

The full paper on the synthesis of onikulactone and mitsugashiwalactone (Vol. 7, p. 24) has been published.Whitesell reports two further useful sequences (cf. Vol. 7, p. 26) from accessible bicyclo[3,3,0]octanes which may lead to iridoids (123 X=H2, Y = H) may be converted into (124) via (123 X = H2, Y = C02Me), the product of ester enolate Claisen rearrangement of the derived allylic alcohol and oxidative decarboxylation/ whereas (123 X = 0, Y = H) readily leads to (125), a known derivative of antirride (126) via an alkylation-dehydration-epoxi-dation-rearrangement sequence. Aucubigenin (121 X = OH, R = H), which is stable at —20°C and readily obtained by enzymic hydrolysis of aucubin (121 X = OH, R = j8-Glu), is converted by mild acid into (127) ° with no dialdehyde detected sodium borohydride reduction of aucubigenin yields the non-naturally occurring isoeucommiol (128 X=H,OH) probably via the aldehyde (128 X = O). ... 

In conclusion, we could say that on a wide sense, the dialdehydes generated by the metabolism and hydrolysis of the aglycones of iridoids, could be in part responsible for the described activities. The non-saturated carbonyl group can react with the SH groups, from amino acids and nucleic acids, causing different effects as the cellular growth inhibition [38,77]. 

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