Synthesis of (-)-albrassitriol and (-)-6-epi-albrassitriol from (+)-larixol.

A novel synthesis of natural drimanic compounds, (-)-albrassitriol (2) and (-)-6-epi-albrassitriol (3), has been carried out starting from an easily available labdane diterpenoid, (+)-larixol (1). In a two-step procedure, (+)-larixol (1) was converted into 14,15-bisnorlab-7-ene-6,13-dione (9), which was then submitted to a Norrish type II photochemical degradation yielding drim-7,9(11)-diene-6-one (10), whose treatment with OsO4 led selectively to the formation of drim-7-ene-9α,11-diol-6-one (12). The same compound was obtained by selective epoxidation of the C(9)-C(11) double bond in drim-7,9(11)-diene-6-one (10) with monoperphtalic acid. Treatment of the resulting mixture of α- and ß-epoxides (13 and 14) with HClO4 yielded drim-7-ene-9α,11-diol-6-one (12). Reduction of the C6-carbonyl group in drim-7-ene-9α,11-diol-6-one (12) with LiAlH4 afforded (-)-albrassitriol (2) and (-)-6-epi-albrassitriol (3), 12.4% and 13.6% overall yields, respectively.

Synthesis of 5,10-seco analogs of testosterone.

A number of 5,10-seco analogs of testosterone has been synthesized starting from products of the radical oxidation of 3beta,17beta-diacetoxy-5alpha-androstan-5alpha-ol. The obtained compounds possess a flexible 10-membered ring with substituents (O, -OH) at C-3 and C-5. Similar derivatives with an (E)- and (Z)-Delta(1(10))-double bond have been prepared also. X-ray analysis and a combination of NMR experiments have been used for their structure elucidation and conformation analysis.

Saponins, classification and occurrence in the plant kingdom.

Saponins are a structurally diverse class of compounds occurring in many plant species, which are characterized by a skeleton derived of the 30-carbon precursor oxidosqualene to which glycosyl residues are attached. Traditionally, they are subdivided into triterpenoid and steroid glycosides, or into triterpenoid, spirostanol, and furostanol saponins. In this study, the structures of saponins are reviewed and classified based on their carbon skeletons, the formation of which follows the main pathways for the biosynthesis of triterpenes and steroids. In this way, 11 main classes of saponins were distinguished: dammaranes, tirucallanes, lupanes, hopanes, oleananes, taraxasteranes, ursanes, cycloartanes, lanostanes, cucurbitanes, and steroids. The dammaranes, lupanes, hopanes, oleananes, ursanes, and steroids are further divided into 16 subclasses, because their carbon skeletons are subjected to fragmentation, homologation, and degradation reactions. With this systematic classification, the relationship between the type of skeleton and the plant origin was investigated. Up to five main classes of skeletons could exist within one plant order, but the distribution of skeletons in the plant kingdom did not seem to be order- or subclass-specific. The oleanane skeleton was the most common skeleton and is present in most orders of the plant kingdom. For oleanane type saponins, the kind of substituents (e.g. -OH, =O, monosaccharide residues, etc.) and their position of attachment to the skeleton were reviewed. Carbohydrate chains of 18 monosaccharide residues can be attached to the oleanane skeleton, most commonly at the C3 and/or C17 atom. The kind and positions of the substituents did not seem to be plant order-specific.

A new type of steroids with a cyclobutane fragment in the AB-ring moiety.

The synthesis of a 5,10-seco steroid containing two double bonds in a AB-macrocycle as well as the preparation of a steroidal skeleton with a cyclobutane fragment is described. The structures of these compounds are different from those of natural steroids, but they are very similar with respect to conformation of the carbon skeleton.

Isolation and structure elucidation of new radical oxidation products of 5-hydroxy steroids.

Three new products have been isolated from the lead-tetraacetate version of the hypoiodite oxidation of 3beta,17beta-diacetoxy-5-hydroxy-5 alpha-androstane. Along with the expected 1(10)-unsaturated 5,10-seco steroidal 5-ketones, the fragmentation reaction gave two epimeric C-4 iodides. Their structural assignment was based on X-ray data of one of them ((4R,10S)-4-iodo-3beta,17beta-diacetoxy-5,10-secoandrostan-5-one). The third new product was found to be the 5 beta,6 beta-epoxide resulting from the dehydration of the tertiary alcohol followed by epoxidation of the intermediate Delta(5)-olefin.

The synthesis of functionalized 13,14-seco-steroids via Grob fragmentation.

A synthetic methodology for the synthesis of 13,14-seco-steroids with substituents at C-14 and C-17 is described. The approach involves Grob fragmentation of 14beta-hydroxy-17beta-tosylates, hydroboration-oxidation of the intermediate delta13(17)-olefin, and hydride reduction of the 14-ketone. An unambiguous structural assignment of (13R,14S,17S)-14,17-diacetoxy-3-methoxy-7alpha-methyl-13,14-secoestra-1,3,5(10)-triene was determined by X-ray analysis.

Synthesis of 13,14-secotestosterone derivatives.

A number of testosterone analogs with a 13,14-secosteroidal fragment have been prepared from (13S)-13-iodo-6beta-methoxy-3alpha, 5-cyclo-13,14-seco-5alpha-androstan-14,17-dione. The key steps involved stereoselective deiodination of the starting compound with triphenylphosphine and selective protection of the 17-keto group with trimethylsilylcyanide. Removal of iodine at C-13 proceeded with inversion of the configuration at C-13, which has been established by X-ray crystallography. 13,14-Secotestosterone analogues substituted and non-substituted at C-14 have been prepared. The obtained compounds containing flexible CD ring fragments are of great interest for comparative studies in biological tests together with testosterone and other steroids with a rigid tetracyclic skeleton.

Reaction of (13S)-13-iodo-6beta-methoxy-3alpha,5-cyclo-13,14-seco-5alpha-androstane-14,17-dione with hydroxylamine and its application to the synthesis of new 13,14-seco steroids.

The synthesis of 13,14-seco steroids starting from easily available (13S)-13-iodo-6beta-methoxy-3alpha,5-cyclo-13,14-seco-5alpha-androsta-14,17-dione is described. The C-17 ketone was converted regioselectively into its oxime with simultaneous stereoselective deiodination at C-13. The remaining C-14 carbonyl group was then reduced stereoselectively with Ca(BH4)2. The configurations at the relevant stereocenters of the thus obtained hydroxy oxime were determined by X-ray analysis. Successful regeneration of the C-17 carbonyl group was achieved by treatment of the corresponding oxime acetate with TiCl3.

Qualitative and quantitative variation among volatile profiles induced by Tetranychus urticae feeding on plants from various families.

Many plant species are known to emit herbivore-induced volatiles in response to herbivory. The spider mite Tetranychus urticae Koch is a generalist that can feed on several hundreds of host plant species. Volatiles emitted by T. urticae-infested plants of 11 species were compared: soybean (Glycine max), golden chain (Laburnum anagyroides), black locust (Robinia pseudo-acacia), cowpea (Vigna unguiculata), tobacco (Nicotiana tabacum), eggplant (Solanum melalonga), thorn apple (Datura stramonium), sweet pepper (Capsicum annuum), hop (Humulus lupulus), grapevine (Vitis vinifera), and ginkgo (Ginkgo biloba). The degree to which the plant species produced novel compounds was analyzed when compared to the odors of mechanically damaged leaves. Almost all of the investigated plant species produced novel compounds that dominated the volatile blend, such as methyl salicylate, terpenes, oximes, and nitriles. Only spider mite-infested eggplant and tobacco emitted a blend that was merely quantitatively different from the blend emitted by mechanically damaged or clean leaves. We hypothesized that plant species with a low degree of direct defense would produce more novel compounds. However, although plant species with a low direct defense level do use indirect defense to defend themselves, they do not always emit novel compounds. Plant species with a high level of direct defense seem to invest in the production of novel compounds. When plant species of the Fabaceae were compared to plant species of the Solanaceae, qualitative differences in spider mite-induced volatile blends seemed to be more prominent in the Fabaceae than in the Solanaceae.

Chemical and enzymatic hydrolysis of anthraquinone glycosides from madder roots.

For the production of a commercially useful dye extract from madder, the glycoside ruberythric acid has to be hydrolysed to the aglycone alizarin which is the main dye component. An intrinsic problem is the simultaneous hydrolysis of the glycoside lucidin primeveroside to the unwanted mutagenic aglycone lucidin. Madder root was treated with strong acid, strong base or enzymes to convert ruberythric acid into alizarin and the anthraquinone compositions of the suspensions were analysed by HPLC. A cheap and easy method to hydrolyse ruberythric acid in madder root to alizarin without the formation of lucidin turned out to be the stirring of dried madder roots in water at room temperature for 90 min: this gave a suspension containing pseudopurpurin, munjistin, alizarin and nordamnacanthal. Native enzymes are responsible for the hydrolysis, after which lucidin is converted to nordamnacanthal by an endogenous oxidase.

Insect antifeedant activity of clerodane diterpenes and related model compounds.

A comprehensive compilation of all test results on the insect antifeedant activity of clerodane diterpenes and related model compounds is reported. To increase the compatibility of data from different sources, some of the results reported in the literature have been converted into a standardized form. The compounds were sorted into groups according to the different types of sidechain attached to C-9. Despite the wealth of information, collected in 15 tables, it remains difficult to assign importance to separate structural elements in relation to the observed antifeedant activity. A detailed analysis of the structure-activity relationships could not be presented, but some interesting trends can be distinguished based on the structures of the strongest antifeedants. The compilation covers the literature up to December 2001.

Isolation and structure elucidation of radical scavengers from Thymus vulgaris leaves.

2,2-Diphenyl-1-picrylhydrazyl radical (DPPH*) scavenging activity-guided fractionation of a leaf extract of Thymus vulgaris led to the isolation of the radical scavengers rosmarinic acid 1, eriodictyol, taxifolin, luteolin 7-glucuronide, p-cymene 2,3-diol, p-cymene 2,3-diol 6-6'-dimer, carvacrol, thymol, and a new compound, 2. The fractionation was considerably facilitated by using an on-line HPLC detector for radical scavenging activity. In this detector activity is monitored as the disappearance of the color of a postcolumn added stable radical after reacting with radical scavengers in a reaction coil. Compound 2, which consists of rosmarinic and caffeic acid moieties linked via a C-3'-C-8' ' ether bridge, was mainly elucidated by various NMR techniques and CD. Phenylpropanoid trimer 2 was a weaker and stronger radical scavenger than rosmarinic acid 1 in off-line TEAC and DPPH* assays, respectively.

Biosynthesis of costunolide, dihydrocostunolide, and leucodin. Demonstration of cytochrome p450-catalyzed formation of the lactone ring present in sesquiterpene lactones of chicory.

Chicory (Cichorium intybus) is known to contain guaianolides, eudesmanolides, and germacranolides. These sesquiterpene lactones are postulated to originate from a common germacranolide, namely (+)-costunolide. Whereas a pathway for the formation of germacra-1(10),4,11(13)-trien-12-oic acid from farnesyl diphosphate had previously been established, we now report the isolation of an enzyme activity from chicory roots that converts the germacrene acid into (+)-costunolide. This (+)-costunolide synthase catalyzes the last step in the formation of the lactone ring present in sesquiterpene lactones and is dependent on NADPH and molecular oxygen. Incubation of the germacrene acid in the presence of 18O2 resulted in the incorporation of one atom of 18O into (+)-costunolide. The label was situated at the ring oxygen atom. Hence, formation of the lactone ring most likely occurs via C6-hydroxylation of the germacrene acid and subsequent attack of this hydroxyl group at the C12-atom of the carboxyl group. Blue light-reversible CO inhibition and experiments with cytochrome P450 inhibitors demonstrated that the (+)-costunolide synthase is a cytochrome P450 enzyme. In addition, enzymatic conversion of (+)-costunolide into 11(S),13-dihydrocostunolide and leucodin, a guaianolide, was detected. The first-mentioned reaction involves an enoate reductase, whereas the formation of leucodin from (+)-costunolide probably involves more than one enzyme, including a cytochrome P450 enzyme.

Identification of radical scavengers in sweet grass (Hierochloe odorata).

Extracts from aerial parts of sweet grass (Hierochloe odorata) were active DPPH free radical scavengers. The active compounds were detected in extract fractions using HPLC with on-line radical scavenging detection. After multistep fractionation of the extract, two new natural products possessing radical scavenging activity were isolated, and their structures were elucidated by NMR and MS. They were identified as 5,8-dihydroxybenzopyranone and 5-hydroxy-8-O-beta-D-glucopyranosyl-benzopyranone. Activities of the compounds isolated were tested by DPPH and ABTS free radical scavenging assays, and compared with the known natural antioxidant rosmarinic acid and Trolox.

Screening of plant extracts for antioxidant activity: a comparative study on three testing methods.

Three methods widely employed in the evaluation of antioxidant activity, namely 2,2'-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging method, static headspace gas chromatography (HS-GC) and beta-carotene bleaching test (BCBT), have been compared with regard to their application in the screening of plant extracts. The strengths and limitations of each method have been illustrated by testing a number of extracts, of differing polarity, from plants of the genus Sideritis, and two known antioxidants (butylated hydroxytoluene and rosmarinic acid). The sample polarity was important for the exhibited activity in the BCBT and HS-GC methods but not for the DPPH method. The complex composition of the extracts and partition phenomena affected their activity in each assay. The value of the BCBT method appears to be limited to less polar samples. Although slow, the HS-GC method is preferable for assessing the antioxidant inhibitory properties on the formation of unwanted secondary volatile products. Being rapid, simple and independent of sample polarity, the DPPH method is very convenient for the quick screening of many samples for radical scavenging activity.

Effect of acyl chain length and branching on the enantioselectivity of Candida rugosa lipase in the kinetic resolution of 4-(2-difluoromethoxyphenyl)-substituted 1,4-dihydropyridine 3,5-diesters.

Since 2,6-dimethyl-4-aryl-1,4-dihydropyridine 3,5-diesters themselves are not hydrolyzed by commercially available hydrolases, derivatives with spacers containing a hydrolyzable group were prepared. Seven acyloxymethyl esters of 5-methyl- and 5-(2-propoxyethyl) 4-[2-(difluoromethoxy)phenyl]-2,6-dimethyl-1,4-dihydro-3,5-pyridinedicarboxylate were synthesized and subjected to Candida rugosa lipase (CRL) catalyzed hydrolysis in wet diisopropyl ether. A methyl ester at the 5-position and a long or branched acyl chain at C3 gave the highest enantiomeric ratio (E values). The most stereoselective reaction (E = 21) was obtained with 3-[(isobutyryloxy)methyl] 5-methyl 4-(2-difluoromethoxyphenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate, and this compound was used to prepare both enantiomers of 3-methyl 5-(2-propoxyethyl) 4-[2-(difluoromethoxy)phenyl]-2,6-dimethyl-1,4-dihydro-3,5-pyridinedicarboxylate. The absolute configuration of the enzymatically produced carboxylic acid was established to be 4R by X-ray crystallographic analysis of its 1-(R)-phenylethyl amide.

Base-Induced Rearrangement of Perhydronaphthalene-1,4-diol Monosulfonate Esters to 11-Oxatricyclo[5.3.1.0(2,6)]undecanes. Total Synthesis of Furanether B.

It is observed that mesylate 1 on exposure to Li(Ot-Bu)(3)AlH in refluxing toluene rearranges selectively to the 11-oxatricyclo[5.3.1.0(2,6)]undecane derivative 3. A similar rearrangement, leading to a bridged tricyclic ether (14 --> 5), has been used as the key to the total synthesis of furanether B (4), a naturally occurring lactarane sesquiterpene, with the readily available ketone 8 as the starting material. Completion of the synthesis of the natural product is accomplished by an annulation method based on a Pummerer-induced cyclization reaction.

Total Synthesis of Neohedycaryol. Its Possible Role in the Biosynthesis of Eudesmane Sesquiterpenes.

The total synthesis of neohedycaryol (4), the C(9)-C(10) double bond regioisomer of the germacrane sesquiterpene hedycaryol, was accomplished in 10 steps from the known dione 6. A Marshall fragmentation of the intermediate mesylate 14 was used to prepare the trans,trans-cyclodeca-1,6-diene ring present in neohedycaryol. During the synthesis of 14, a pronounced example of through-bond interactions (TBI) was observed. The preferred elongated chair conformation of neohedycaryol was demonstrated spectroscopically and by chemical conversion into alpha-, beta-, and gamma-eudesmol. These findings indicate that the occurrence of neohedycaryol as a precursor in the biosynthesis of epi-eudesmanes as proposed in the literature is unlikely. The preference of neohedycaryol for the elongated chair conformation further shows that the compound occupies the meso form. This implies that neohedycaryol may act as a precursor in the biosynthesis of both ent- and usual eudesmanes.

Synthesis of (E,E)-Germacrane Sesquiterpene Alcohols via Enolate-Assisted 1,4-Fragmentation.

An efficient method has been developed for the synthesis of (E,E)-germacrane sesquiterpene alcohols. The key step in these syntheses involves the enolate-assisted 1,4-fragmentation of properly functionalized perhydro-1-naphthalenecarboxaldehydes with 1 equiv of sodium tert-amylate as base, to give the corresponding (E,E)-germacrane aldehydes. These aldehydes are not very stable, and in situ reduction of the aldehyde function with Red-Al is required to obtain high yields of the desired germacrane alcohols. This procedure has led to the successful synthesis of 15-hydroxygermacrene B (4) and 15-hydroxyhedycaryol (35) from the mesylates 6 and 33, respectively. When KHMDS is used instead of sodium tert-amylate in the fragmentation reaction of 6, isomerization of the initially formed C(4)-C(5) E double bond cannot be avoided and results, after in situ reduction with Red-Al, in the formation of the (E,Z)-germacrane alcohol 24. The 15-hydroxy-(E,E)-germacranes are excellent starting materials for the selective synthesis of the corresponding 4,5-epoxides, which in turn can perfectly well be used in studies on the biomimetic formation of guaiane sesquiterpenes.

About the Chiral Stability of Germacrene B and the Biomimetic Synthesis of Guaiane Sesquiterpenes.

The planar chirality of 15-hydroxygermacrene B (2) has been examined by means of the asymmetric Sharpless epoxidation, performed as a kinetic resolution. (1)H NMR experiments with Eu(hfc)(3) demonstrate that the recovered 2 is racemic. Consequently, 2 and most likely also germacrene B (1) are not enantiomerically stable at room temperature. The formation of the optically active cis-fused guaiane 4 of high ee with limited Sharpless reagent shows that the asymmetric epoxidation of 2 proceeds highly enantioselectively. The Sharpless epoxidation methodology applied on the (E,Z)-germacrane 3 results in the formation of the stable optically active epoxide 5. Acid-induced cyclization of 5, leading to a mixture of guaianes, probably proceeds via the trans-fused carbocationic intermediate A.