Xylaridines C and D, Unusual Thiopyranodipyridine Alkaloids from the Fungus Xylaria longipes

Structurally unique thiopyranodipyridine alkaloids xylaridines C ( 1 ) and D ( 2 ) were isolated from the fungus Xylaria longipes . Their structures were established by comprehensive spectroscopic analysis combined with single-crystal X-ray diffraction and electron-capture detection (ECD) calculations. Compound 1 possesses two piperidine moieties fused with a centered thiopyran ring, while compound 2 is a dimer of 1 . Compound 1 was resoluted into optical pure enatiomers (+)- 1 and (−)- 1 by chiral HPLC. Moreover, compound 2 was resoluted into an optically pure compound (+)- 2 and a mixture of (−)- 2 and meso- 2 . Plausible biosynthetic pathways of compounds 1 and 2 are proposed.