Xylapyrrosides A and B, two rare sugar-morpholine spiroketal pyrrole-derived alkaloids from Xylaria nigripes: isolation, complete structure elucidation, and total syntheses

Two new [named xylapyrrosides A (1) and B (2)] along with two known [pollenopyrrosides A (3) and B (=acortatarin A, 4)] naturally occurring spirocyclic pyrrole alkaloids were isolated and identified as minor components from the EtOH extract of the dried mycelia of the edible medicinal fungus Xylaria nigripes. Their structures were established by a combination of interpretation of spectroscopic data and single-crystal X-ray diffraction analyses. The isolates possess a unique tricyclic skeleton comprising a common bicyclic 2-formyl-pyrrole-fused morpholine, with a variable ketohexoside ring. This class of alkaloids is quite rare from natural sources. In this study, the total syntheses of compounds 1, 2 and 4 were successfully achieved by two alternative strategies, and three new analogs [named xylapyrrosides A1 (1a), A2 (1b) and B1 (2a)] were also produced. Notably, the total synthesis of such spiroketal alkaloids with a pyranose ring (e.g.,1) was accomplished for the first time. The absolute configurations of the new isolates can be thereafter unequivocally secured by the total syntheses. The above isolated and synthesized spiro-alkaloids were found to show moderate antioxidant effects by preventing the oxidative stress-induced cytotoxicity of A7r5 rat vascular smooth muscle cells (VSMCs).