Ethylidene-Tethered Chromene-Pyrone Hybrids as Potential Plant-Growth Regulators from an Endolichenic Phaeosphaeria Species.

Phaeosphaeria sp., a lichen-associated fungus, produced six skeletally new dimeric spiciferones ( 1 - 6 ) and four known metabolites ( 7 - 10 ). The new structures were elucidated by spectroscopic analysis, and their absolute configurations were determined by electronic circular dichroism calculations. Compounds 1 and 3 - 6 represent the first examples of ethylidene-bridged dimers from the building blocks 4 H -chromene-4,7(8 H )-dione and α-pyrone, and 2 is a unique homodimer of spiciferone. Compounds 1 , 2 , and 5 - 9 significantly inhibited the growth of weed-like dicot Arabidopsis thaliana at 100.0 μM. Notably, 8 showed the strongest inhibitory activity against the fresh weight and root elongation of A. thaliana with the IC 50 values of 32.04 and 26.78 μM, respectively, whereas 1 , 8 , and 9 stimulated the growth of A. thaliana at lower concentrations. Meanwhile, compounds 2 and 6 exhibited weak inhibitory effects on the root elongation of monocot rice, while 1 and 8 exhibited growth-promoting effects on the shoot and root elongation of rice in a roughly dose-dependent manner.