Unequivocal ¹³C NMR assignment of cyclohexadienyl rings in bromotyrosine-derived metabolites from marine sponges

Bromotyrosine-derived compounds are commonly isolated from Verongida sponges and are a major class of marine natural products. Here we report on the unequivocal ¹³C NMR assignment of the brominated carbons at positions C-2 and C-4 of the cyclohexadiene ring, two carbons whose resonances are often incorrectly assigned. Interpretation of HMBC data acquired for a series of known bromotyrosine analogues, which included ianthesine E (1), aerothionin (2), 11-hydroxyaerothionin (3), and 11,19-dideoxyfistularin-3 (4), allowed us to unequivocally assign the carbons in question, C-2 and C-4, through the observance of unique HMBC correlations from the C-1 hydroxyl proton. Here we present the complete 2D NMR data sets recorded in DMSO-d ₆ for 2-4 that were used to confirm the assignment and establish the working model. Using this model, a survey of the literature revealed that many members of this structure class had been wrongly assigned. This paper serves to reassign those compounds whose ¹³C NMR assignment at positions C-2 and C-4 of the cyclohexadiene ring should be reversed.