Racemosins A and B, two novel bisindole alkaloids from the green alga Caulerpa racemosa.

Racemosin A (1), a structurally unique bisindole alkaloid possessing the seco-indolo[3,2-a]carbazole skeleton with two uncommon indolinenone units both conjugated with a methyl propenoate moiety, and its unusual cyclized derivative, racemosin B (2), were isolated from the green alga Caulerpa racemosa, together with the most commonly encountered pigment in the genus Caulerpa, caulerpin (3). Their structures were elucidated by extensive spectroscopic analysis and by comparison with data for related known compounds. A plausible biosynthetic pathway of 1 and 2 was proposed. In a neuro-protective assay, compound 1 significantly attenuated the Aβ2(5-35)-induced SH-SY5Y cell damage with a 14.6% increase in cell viability at the concentration of 10μM when compared to epigallocatechin gallate (EGCG, 16.57% increase at 10 μM) as the positive control.