DFT supported structural elucidations of seiridiasteriscane A, unique 15-nor-asteriscane and novel pestalotiopsin Congeners from Seiridium sp. KT3957

Seiridiasteriscane A (1), pestalotiopsins L-N (2–4), and 2-O-deacetylpestalotiopsin A (5) were isolated along with known pestalotiopsins A (7), B (6), and fuscoatrol A (8) from Seiridium sp. KT3957. Their relative structures were elucidated by extensive NMR analysis and verified using DFT (density functional theory) chemical shift calculations. The absolute configurations were determined by reproducing their experimental ECD spectra based on DFT calculations. Although 3 and 4 existed as conformational mixtures in CDCl₃, DFT-based ¹³C chemical shift calculations were useful for evaluating each conformer. Simultaneous isolation of seiridiasteriscane A (1) and pestalotiopsins 2–8 suggested a plausible route for the biosynthesis of 1 involving the expansion of the cyclobutane moiety into a cyclopentane ring.