21-Epi-taichunamide D and (±)-versicaline A, three unusual alkaloids from the endophytic Aspergillus versicolor F210

Abstract

Three new alkaloids, 21-epi-taichunamide D (1) and (±)-versicalin A ((±)-6), along with four known analogues (2–5) were isolated from the endophytic Aspergillus versicolor F210, which harbored in the bulbs of Lycoris radiata. Structurally, compound 1 is the third natural indole alkaloid with a rare methylsulfonyl unit, and (±)-6 existing as two pairs of keto-enol tautomers ((+)-6a/(+)-6b and (−)-6a/(−)-6b, respectively), represent the first naturally occurring alkaloids with a rare 2,3-dihydro-1H-isoindol-1-one unit fused to a tautomeric diisopentenyl cyclohexenone unit. The absolute structures of 1 and (±)-6 were characterized by extensive analyses of spectroscopic data, comparison of the electronic circular dichroic (ECD) data, and single-crystal X-ray diffraction analysis, respectively. Compound 6 exhibited moderate cytotoxicity against HL-60 cells with an IC50 value of 5.6 μM.