Three new iantherans, iso-iantheran A (1), 8-carboxy-iso-iantheran A (2), and iso-iantheran B (4) were isolated together with two further new brominated tyrosine-derived metabolites 5 and 6 from the polar extract of the Australian marine sponge Ianthella quadrangulata. Structures were elucidated on the basis of extensive spectral analysis. The dimeric benzofuran skeleton including a 2,3-dihydroxy-1,3-butadiene disulfate moiety found in 1, 2, and 4 is a unique feature of iantherans and to date only described for iantherans A and B. Iso-iantheran A (1) and 8-carboxy-iso-iantheran A (2) exhibited agonist activity at P2Y11 receptors with EC50 values of 1.29 muM and 0.48 muM, respectively. Compound 2 showed some selectivity for P2Y11 over P2Y1 and P2Y2 receptors. Compounds 1 and 2 represent a new structural type for the development of P2Y11 receptor agonists.