Iridoid glycoside biosynthesis in Penstemon secundiflorus. Another H-5, H-9 trans-iridoid glycoside

Isolation and characterization of the new iridoid 10-hydroxy-(5 alpha H)-6- epidihydrocornin from Penstemon secundiflorus (Scrophulariaceae) is describ ed. In biosynthetic experiments, deoxyloganic acid was incorporated into th e transfused iridoid glycosides (5 alpha H)-6-epidihydrocornin and 10-hydro xy-(5 alpha H)-6-epidihydrocornin in P. secundiflorus. Formation of the tra ns-fused compounds is therefore a late event in the biosynthesis and does n ot occur during iridoid formation by cyclization of the open chain monoterp ene precursor. In the same plant, 8-epideoxyloganic acid was not incorporat ed into the trans-iridoids. Deoxyloganic acid was also incorporated into 10 -hydroxyhastatoside (which bears an 8 beta-methyl group), while 8-epideoxyl oganic acid was incorporated into penstemoside (with an 8 alpha-methyl grou p). Thus, iridoid biosynthetic pathways leading from both deoxyloganic acid and 8-epideoxyloganic acid were found in the same plant. (C) 1998 Elsevier Science Ltd. All rights reserved.