Two New Neo-debromoaplysiatoxins-A Pair of Stereoisomers Exhibiting Potent Kv1.5 Ion Channel Inhibition Activities

A pair of stereoisomers possessing novel structures with 6/6/5 fused-ring systems, neo-debromoaplysiatoxin E ( ) and neo-debromoaplysiatoxin F ( ), were isolated from the marine cyanobacterium sp. Their structures were elucidated using various spectroscopic techniques including high resolution electrospray ionization mass spectroscopy (HRESIMS) and nuclear magnetic resonance (NMR). The absolute stereochemistry was determined by calculated electronic circular dichroism (ECD) and gauge-independent atomic orbital (GIAO) NMR shift calculation followed by DP4+ analysis. Significantly, this is the first report on aplysiatoxin derivatives with different absolute configurations at C9-C12 ( : 9 , 10 , 11 , 12 ; : 9 , 10 , 11 , 12 ). Compounds and exhibited potent blocking activities against Kv1.5 with IC values of 1.22 ± 0.22 μM and 2.85 ± 0.29 μM, respectively.