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

Synthesis of Plectranthons, Diterpenoid Phenanthrene‐1,4‐diones The following phenanthrene‐1,4‐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 ( 1 ), ( RS )‐, ( R )‐, and ( S )‐2‐[3‐hydroxy‐5,7,8‐trimethyl‐1,4‐dioxophenanthren‐2‐yl]‐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‐tri‐methyl‐2‐(prop‐2‐enyl)phenanthrene‐1,4‐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 UV/VIS, IR, 1 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 1 H‐NMR arguments concerning the chemical shifts of HC(9) and HC(10) in 1–3. Extensive 1 H‐NMR investigations have now revealed that the currently stated assignments of the HC(9)/ HC(10) AB system have to be reversed for highly substituted phenanthrene‐1,4‐diones: in the model compounds 2‐methylphenanthrene‐1,4‐dione (41) and 2, HC(10) resonates al lower field as expected (peri‐position), whereas in the highly substituted congeners 1 , 2 , 3 , 31 , and 32 , HC(9) is shifted paramagnetically, a fact which had lead to the erroneous assignment of structure 4 for natural plectranthon D .