SYNTHESIS OF PLECTRANTHONS, DITERPENOID PHENANTHRENE-1,4-DIONES

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, 1H‐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 1H‐NMR arguments concerning the chemical shifts of HC(9) and HC(10) in 1–3. Extensive 1H‐NMR investigations have now revealed that the currently stated assignments of the HC(9)/ HC(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, HC(10) resonates al 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. Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim