A Series of Chiral Polybrominated Biindoles from the Marine Blue-Green Alga Rivularia firma. Application of ¹³C NMR Spin-Lattice Relaxation Data and 13C-¹H Coupling Constants to Structure Elucidation

Six novel polybrominated biindole derivatives have been isolated from the marine blue-green alga Rivularia firma: 2,2′,5,5′-tetrabromo-3,3′-bi-1H-indole (1), (+)-2,3′,5,5′-tetrabromo-7′-methoxy-3,4′-bi-1H-indole (2), (+)-3′,5,5′-tribromo-7′-methoxy-3,4′-bi-1H-indole (3), (+)-2,5,5′-tribromo-7′-methoxy-3,4′-bi-1H-indole (4), (−)-3,3′,5,5′-tetrabromo-7′-methoxy-1,4′-bi-1H-indole (5), and (+)-2,2′,3,4′,5,5′-hexabromo-1,3′-bi-1H-indole (6). Structures were solved by analysis of ¹H and ¹³C chemical shifts, ¹³C-¹H coupling constants, and ¹³C spin-lattice relaxation data (at 15.04 MHz). The contributions of ¹³C-¹H dipolar interactions to the spin-lattice relaxation times (T₁) of quaternary carbons were determined from measured T₁ and nuclear Overhauser enhancement values and then used to estimate the numbers of protons two bonds removed from each quaternary carbon. This information was used in structure determination as well as for resonance assignments. Spin-lattice relaxation data were also used to identify brominated carbons. The biindoles 2–6 are optically active and are therefore new representatives of a very small number of natural products in which chirality is due solely to restricted rotation.