Biosynthesis of the iridoid glucosides cornin, hastatoside, and griselinoside in Verbena species

Abstract

² H N.m.r. spectroscopy has been used to demonstrate that in specifically labelled deoxyloganin the label at C-8 is retained during its conversion into cornin in Verbena officinalis, thus excluding deoxygeniposide as an intermediate. Deoxygeniposide, loganin, and mussaenoside produced no measurable incorporation into cornin. In V. hastata deoxyloganin and dihydrocornin were incorporated into cornin and hastatoside while deoxygeniposide again gave no observable incorporation. Forsythide dimethyl ester was an efficient precursor for griselinoside in V. hispida, while no incorporation was observed with deoxygeniposide, geniposide, deoxyloganin, and dihydrocornin. However, the last two compounds were metabolized to cornin, otherwise absent in this species. Incorporation of forsythide dimethyl ester and geniposide into griselinoside in Griselinia littoralis was not observed.