Computationally driven reassignment of the structures of aldingenins A and B

Earlier four brominated sesquiterpenes, aldingenins A–D, were isolated from the red algae Laurencia aldingensis, and their structure elucidated by spectroscopic methods including NMR. Total syntheses of the proposed structures of aldingenin B (Org. Lett.2012, 14, 2168) and aldingenin C (J. Org. Chem.2014, 79, 9373) have demonstrated that the structures are misassigned. Koshino has proposed aldingenins C and D to be caespitol and 5-(S)-acetoxycaespitol. Computational evidence presented in this Letter and based primarily on the computed proton spin–spin coupling constants (but also including 13C NMR chemical shifts) leads to the conclusion that the remaining two aldingenins A and B are also halogenated sesquiterpenes of the same caespitol family. Aldingenin A is assigned the structure of 5-(S)-hydroxycaespitol 1, and aldingenin B—hemiacetal 2 of a related 8-oxo compound.

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