Studies in natural products Part I. The biosynthesis of erythrina alkaloids Part II. An attempted in vitro demethylation of lanosterol
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Notice bibliographique
Résumé
In Part I, hypotheses for the biogenesis of Erythrina alkaloids are discussed. Di-(β-3,4-dihydroxyphenyl)-ethylamine the theoretical precursor predicted by the biogenetic theory, was prepared and ring closure to the erythrinane ring system by oxidative coupling was attempted under various conditions. Consequently, the biogenesis of the Erythrina alkaloids was re-examined and a new proposal is advanced for the biosynthesis of these alkaloids. Synthetic routes to a hypothetical precursor, proposed here for the first time as a potential intermediate, are described. The biogenetic-type synthesis of the spiro-amine ring system present in the Erythrina alkaloids was achieved by oxidative coupling of the blocked diphenolic precursor, as predicted by the proposed biosynthetic scheme. Oxidation of di-(β-3-hydroxy-4-methoxyphenyl)-ethylamine by alkaline potassium ferricyanide afforded 3,15-dimethoxy-16-hydroxy-2-oxo-erythrina-1(6),3-diene in 15% yield. Reduction of the latter by sodium borohydride gave 3,15-dimethoxy-2,16-dihydroxyerythrina-l(6),3-diene. Acetylation of the dienone yielded 3,15-dimethoxy-16-acetoxy-2-oxoerythrina-1(6),3-diene. The total biogenetic-type synthesis of erysodine is therefore but two steps from completion. The results as a whole confirm the hypothesis that Erythrina alkaloids are produced in Nature by oxidative coupling of diphenols. They also demonstrate the directing role of the protective groups in the phenolic precursor. The evidence allows a biosynthetic pathway for the aromatic Erythrina alkaloids to be considered, and the mechanism for the ring closure process is discussed. The isotopically labelled precursor 3-hydroxy-4-methoxy-N-(3-hydroxy-4-methoxyphen[1-¹⁴C]ethyl)-phenethylamine was prepared to test the biosynthetic hypothesis in the plant. Feeding experiments are in progress. In Part II, the biogenesis of cholesterol and methods for functionalising inert methyl groups are reviewed, and a new theoretical approach to removal of the 14α-methyl group from lanosterol is described. The removal of this methyl group in vitro could not be achieved, but a series of interesting compounds was obtained. Evidence for the structures of these compounds is presented. Thus, photosensitized oxygenation of dihydrolanosteryl acetate in the presence of para-nitrobenzenesulphonyl chloride yielded 3β-acetoxylanosta-7,9(ll)-diene, 3β-acetoxylanost-8-ene-7-one and 3β-acetoxylanost-8-ene-7a-hydroperoxide. In addition a compound having an ambiguous structure and designated as IP1 was obtained. The dibromo-derivative of the latter is 33-acetoxy-7a,lla-dibromolanostane-8 a,9α-epoxide, the structure of which was determined by X-ray crystallographic study. A working structure for compound IP1 based on the physical and chemical evidence is discussed.
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Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découle