Synthese von Plectranthonen, diterpenoiden Phenanthren‐1,4‐chinonen

Abstract

Synthesis of Plectranthons, Diterpenoid Phenanthrene-1,4-dionesThe following phenanthrene-l,4-diones have been synthesized by using the photocyclization of the corresponding highly substituted stilbenes as the key step : 3-hydroxy-5,7,8-trimethyl-2-(prop-2-enyl)phenanthrene-1,4-dione (l), (RS)-, (R )-, and (S)-2-[3-hydroxy-5,7,8-trimethyl-1,4-dioxophenanthren-2-y1]-1 -methylethyl acetate (2, 31, and 32, resp.), 3-hydroxy-7,8-dimethyl-2-(prop-2-enyl)phenanthrene-1,4-dione (3), 3-hydroxy-7,8,10-trimethyl-2-(prop-2-enyl)phenanthrene-I ,4-dione (4), 5,7,8-trimethyl-2-(prop-2-enyl)phenanthrene-1 ,4-dione (17), and 3-hydroxy-2-methylphenanthrene-1,4-dione (42). The quinones 1 and 3 proved to be identical with the recently isolated plectranthons A and C. Compounds 2, 31, and 32 exhibited the same UVjVIS, IR, 'H-NMR and mass spectra as natural plectranthon B, but had different melting points. This might be due either to crystal modifications or to diastereoisomerism caused by the helical structure of the phenanthrene-1.4-dione skeleton. The spectral data of synthetic 4 were not compatible with those of natural plectranthon D for which structure 4 had been proposed based mainly on 'H-NMR arguments concerning the chemical shifts of H-C(9) and H-C(10) in 1-3. Extensive 'H-NMR investigations have now revealed that the currently stated assignments of the H-C(9)/ H-C(1O) AB system have to be reversed for highly substituted phenanthrene-l,4-diones: in the model compounds 2-methylphenanthrene-1.4-dione (41) and 2, H-C( 10) resonates at lower field as expected (peri-position), whereas in the highly substituted congeners 1,2,3,31, and 32, H-C(9) is shifted paramagnetically, a fact which had lead to the erroneous assignment of structure 4 for natural plectranthon D. Um die Gruppe der Plectranthone besser zuganglich zu machen, sie weiter zu charakterisieren sowie die Strukturvorschlage abzusichern, haben wir nun einen allgemein anwendbaren Weg zur Synthese von substituierten Phenanthren-1,4-chinonen ausgearbeitet. Uber die erzielten Ergebnisse wird im folgenden berichtet. Aufgrund von Vorversuchen entschieden wir uns fur den Aufbau via eine photochemische Stilben-Cyclisierung

Photochemische Reaktionen als Schlüsselschritte in der Naturstoffsynthese

“…Im vorliegenden Fall entstanden die beiden Regioisomere 22 und 23 in einem Regioisomerenverhältnis r.r. Weitere einfache Phenanthren-Naturstoffe, die durch eine [6p]-Cyclisierung zugänglich wurden, sind die Plectranthone, [29] die Aristolochische Säure, [30] (AE )-Tylophorin, [31] die Phenanthrene TaIV und TaVIII [32] und das diterpenoide Chinon (AE )-Danshexinkun A. [33] Selbst am Ende einer Synthese von komplexen Phenanthrenen kann die [6p]-Cyclisierung effektiv als Schlüsselreaktion eingesetzt werden.…”

Photochemical strategies for the construction of polycyclic molecules

“…(±)-Modephene (163) was synthesized by elaboration of the tetracyclic ketone 162, which itself was accessible by ODPM rearrangement of 161 (50%) (eq 26). 62 The synthesis of 2,8-didehydronoradamantan-9-one (167) was performed starting from brend-4-en-2-one (164) The enantiospecific ODPM transformation of bridged 0,7-unsaturated ketones has paved the way to a ready access to enantiomeric target compounds.480 C. Azadl-r-methane Rearrangements Horspool and colleagues have studied in great detail the effect of substituents with respect to the formation of cyclopropylimines from a,/3-unsaturated imines or azadi-ir-methane (ADPM) systems.54 It was found that electron-withdrawing groups located on the nitrogen atom have a beneficial influence on the ADPM rearrangement. Thus, the acetoxy-substituted imine 168 afforded compound 169 in the presence of suitable sensitizers.…”

Corrosion News

“…Other simple phenanthrene natural products that were made by means of a [6π] cyclization include the plectranthones,29 aristolochic acid,30 (±)‐tylophorine,31 the phenanthrenes TaIV and TaVIII,32 and the diterpenoid quinone (±)‐danshexinkun A 33. The [6π] cyclization can even be used effectively in the final stages of the synthesis of complex phenanthrenes.…”