Geopyxins A–E, ent-Kaurane Diterpenoids from Endolichenic Fungal Strains Geopyxis aff. majalis and Geopyxis sp. AZ0066: Structure–Activity Relationships of Geopyxins and Their Analogues

Four new ent-kaurane diterpenoids, geopyxins A–D (1–4), were isolated from Geopyxis aff. majalis, a fungus occurring in the lichen Pseudevernia intensa, whereas Geopyxis sp. AZ0066 inhabiting the same host afforded two new ent-kaurane diterpenoids, geopyxins E and F (5 and 6), together with 1 and 3. The structures of 1–6 were established on the basis of their spectroscopic data, while the absolute configurations were assigned using modified Mosher’s ester method. Methylation of 1–3, 5, and 6 gave their corresponding methyl esters 7–11. On acetylation, 1 and 7 yielded their corresponding monoacetates 12 and 14 and diacetates 13 and 15. All compounds were evaluated for their cytotoxic and heat-shock induction activities. Compounds 2, 7–10, 12, 14, and 15 showed cytotoxic activity in the low micromolar range against all five cancer cell lines tested, but only compounds 7–9, 14, and 15 were found to activate the heat-shock response at similar concentrations. From a preliminary structure–activity perspective, the electrophilic α,β-unsaturated ketone carbonyl motif present in all compounds except 6 and 11 was found to be necessary but not sufficient for both cytotoxicity and heat-shock activation.