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 13 C 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. [ABSTRACT FROM AUTHOR]