Bent Juhl Nielsen

13C and1H NMR spectroscopy as a tool in the configurational analysis of iridoid glucosides

Abstract The 13 C NMR data of 51 iridoid glucosides or glucoside acetates are tabulated. The collection includes 20 pairs of C-6, C-7 or C-8 epimers. Three parameters in using the data for the configurational assignment of 6- O -substituents are given. The chemical shift for C-9 in a range of substituted compounds is shown to be numerically related to the stereochemistry at C-8. This allows the determination of the configuration at this centre for most types of substitution patterns by calculation of the C-9 shift using increments for each substituent. Such increments are given for 25 substituents in three different solvents. A method for simulation of spectra of unknown iridoid glucosides is presented. By this method, the structures of five novel iridoid glucosides have been elucidated, and that of tecomoside has been revised. The methods used to assign the configurations to C-6 and C-8 epimeric iridoid glucosides by 1 H NMR spectroscopy are discussed and a table with selected data is presented. It is suggested that the structures in the literature for ajugol and myoporoside should be interchanged. Consequently, Horeaus method has failed in these instances. Finally, the differences in the 13 C NMR spectra of pairs of C-6 and C-8 epimeric iridoid glucosides have been interpreted as originating from cis/trans -interactions.

NOVEL BIS-IRIDOID GLUCOSIDES FROM DIPSACUS SYLVESTRIS

Abstract In addition to the known iridoid glucosides loganin, sweroside and cantleyoside, Dipsacus sylvestris has provided 4 novel bis -iridoid glucosides named sylvestrosides I–IV, composed of swerosidic acid, secologanic acid, loganin and loganin aglucone. Sylvestroside I, II and III have been fully characterized by chemical conversions and by 13 C NMR spectroscopy. A probable formula for sylvestroside IV is presented.

Iridoid glucosides from Melampyrum

Abstract Melampyrum arvense and M. cristatum contain, besides aucubin, 8-epiloganin and melampyroside, a new natural iridoid glucoside: gardoside methyl ester. In addition, M. arvense contains mussaenoside and M. cristatum mussaenosidic acid, another novel iridoid glucoside.

Iridoid allosides from Viburnum opulus

Abstract Arbutin and four novel iridoid glycoside esters, named opulus iridoids I–IV, have been isolated from foliage of Viburnum opulus (Caprifoliaceae). Each opulus iridoid constitutes an inseparable mixture of two compounds, differing by containing either 2-methyl- or 3-methylbutyric acid in ester linkage at the 1-OH-group in an iridoid glycoside. In all glycosides 2′,3′-di- O -acetyl-β- D -allopyranose is linked through a glycosidic bond to C-11 in the iridoid aglycone. The opulus iridoids differ by the degree of acetylation of the aglycone and by the attachment, in III and IV, of a β- D -xylopyranosyl group at C-4 of the allose moiety. The structures have been elucidated by 1 H and 13 C-NMR spectroscopy and by cleavage of the glycosidic linkage with boron trifluoride etherate in acetic anhydride, yielding the acetates of the cyclized aglycone and of the appropriate mono- or disaccharide. This is the second report of an iridoid attached to a sugar other than glucose and the second time allose has been encountered in higher plants. The systematic position of Viburnum is briefly discussed.

Iridoid mono- and di-glycosides in Mentzelia

Abstract Eight species of Mentzelia (Loasaceae) have been investigated for iridoid glycosides. In addition to the known glucosides deutzioside, decaloside, mongolioside, loganin and sweroside, several novel compounds have been isolated and characterized by chemical and spectroscopic means. 6′- O -Acetyl deutzioside was found in a single species, while the diglycosidic compounds glucosyl-decaloside, allosyl-decaloside and quinovosyl-decaloside were each isolated from one or more species. In addition, a novel compound, epoxydecaloside (= 11-hydroxy-deutzioside), together with glucosyl-epoxydecaloside, allosyl-epoxydecaloside and mentzelosyl-epoxydecaloside are described. The last compound contains a 4-deoxy-α- l - erythro -pentopyranosyl moiety, whose parent sugar, named mentzelose, has not been encountered so far in nature. A non-glycosidic iridoid, mentzetriol, has been characterized solely by spectroscopic means and a structure is proposed. The secoiridoid secoxyloganin has been found for the first time in a plant source, and the coumarin glucoside scopolin has been isolated from two species of Mentzelia . 13 C and 1 H NMR spectra of several iridoid compounds are presented. The biosynthesis of the compounds is considered and the systematic position of Loasaceae discussed concluding in a possible derivation from Cornalean ancestors.

IRIDOID GLUCOSIDES IN FOUQUIERIACEAE

Abstract From three Fouquieria sp. 12 iridoid glucosides were isolated and identified. Eight of these were structurally related to galioside (monotropein methylester), while four were hydroxy substitution products of deoxyloganin. In three cases the glucoside occurred together with the corresponding 10- O -acetate.

Biosynthesis of the iridoid glucoside cornin in Verbena officinalis

Abstract The biosynthesis of cornin (verbenalin) and dihydrocornin in Verbena officinalis has been investigated. The incorporation of [2H] deoxyloganin was found to be largely independent of the incubation time between one day and a week. An improved method for the preparation of deoxygeniposide from gardenoside is reported and [2H]-iridodial glucoside and [2H]-iridotrial glucoside were prepared from the former. Feeding experiments with young plants using these glucosides, as well as the aglucones, showed much better incorporations for the latter compounds as measured by 2H NMR spectroscopy. 13 C-labelled 10-hydroxygeraniol, 10-hydroxycitronellol, iridodial, iridotrial, iridodial glucoside, iridotrial glucoside, deoxyloganic acid, and deoxyloganic acid aglucone were prepared. [13C]-Mevalonic acid and the above compounds were fed to plants of medium age, and all gave incorporations measurable by 13C NMR spectroscopy into dihydrocornin. The postulated existence of two different metabolic pathways in the biosynthesis of cornin in young and old plants, respectively, could not be established as complete scrambling between the C-3 and C-11 in the iridoid skeleton apparently takes place with all the early precursors. The complete pathway from iridodial forwards to hastatoside has been elucidated.

Chemotaxonomy of the acanthaceae. Iridoids and quaternary amines

Abstract Fourteen of the 40 investigated species of Acanthaceae sensu lato contain iridoid glucosides. A total of 20 iridoids are now reported from the family; most have been isolated in the present work. Seven of the compounds are so far unique for the family, namely 6- O -acetylshanzhiside methyl ester, 6- O -acetylbarlerin, 8( S )-7,8-dihydroaucubin, eranthemoside, hygrophiloside, 6- epi -stilbericoside, and thunbergioside. A plausible biogenetic scheme interrelating the iridoid glucosides is presented. Notable amounts of betaine were isolated or detected by 1 H NMR spectroscopy in crude extracts of virtually all (31/32) species belonging to subfamilies Acanthoideae and Ruellioideae. Trigonelline was similarly found in 14 species belonging to all five subfamilies. Other, unidentified quaternary methylammonium compounds were detected in the subfamilies Thunbergioideae, Mendoncioideae and Nelsonioideae. Quaternary methylammonium compounds are not usual in the order Scrophulariales and therefore the common occurrence of such compounds in the five subfamilies confirms their positioning within the Acanthaceae; this contrasts with the opinion of many taxonomists who prefer to raise one or more of these subfamilies to family rank. The common occurrence of the rare iridoid glucoside stilbericoside in Thunbergia , Stilbaceae and Retziaceae supports Dahlgrens placing of the two latter families in Scrophulariales.

9-hydroxy substituted iridoids from Gelsemium sempervirens

Abstract Two chemotypes of Gelsemium sempervirens were investigated. One, cultivated in Copenhagen, contained six iridoids in appreciable amounts, namely gelsemide, gelsemide 7-glucoside, gelsemiol, gelsemiol 1- and 3-glucoside, and 9-hydroxysemperoside. The other sample, collected in the wild in North Carolina, U.S.A., contained semperoside and the known brasoside as well as the coumarin glycoside fabiatrin. The two iridoids were found only in trace amounts in the cultivated specimen. The structures of the seven new iridoids were established mainly by spectroscopic methods and that of gelsemide was confirmed by X-ray analysis. Two new structural features were noted: three of the compounds carried a hydroxyl group at the 9-position, while two contained the glucosyl moiety at C-3. All iridoids were lactones of the asperuloside type with or without a 3,4-double bond.

Iridoid glucosides from Asystasia bella

Abstract Five new iridoid glucosides named asystasioside A–E together with the known compounds catalpol, gardoside methyl ester, 8-epiloganin and mussaenoside were isolated from Asystasia bella (= Mackaya bella ). Asystasiosides A–D are 1-β-glucopyranosyl esters of the four iridoids 8-epideoxyloganic acid, 7-deoxygardoside, 10-deoxygeniposidic acid and geniposidic acid, respectively. Asystasioside E is the chlorohydrin of catalpol with the chlorine atom in the 7α-position. The structural elucidations were mainly performed by NMR spectroscopy, and the structure of asystasioside E was proved by conversion to catalpol. The nine known chloride containing iridoids are all chlorohydrins.