Computationally assisted structure elucidation of new 2-guanidinoethanesulfonyl sesquiterpenoid alkaloids: Agelasidines G–I from the marine sponge Agelas nakamurai

Three new 2-guanidinoethanesulfonyl sesquiterpene analogues of (−)-agelasidine A, agelasidines G–I (1–3), were isolated from a marine sponge Agelas nakamurai collected in Orchid Island. The absolute configurations of agelasidines G–I (1–3) were established by utilizing the computational NMR data, a statistical DP4+ method, and comparing their experimental optical rotations with the B3LYP calculated data. Moreover, an optical pure bromopyrrole alkaloid, (−)-mukanadin C (4), was discovered for the first time and its absolute configuration was confirmed by a single crystal X-ray diffraction analysis. These marine natural products were evaluated cytotoxic, antimicrobial, and anti-inflammatory activities.