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.