A pair of unprecedented spiro-trisindole enantiomers fused through a five-member ring from Laurencia similis

Similisines A (1a) and B (1b), a pair of unprecedented polybrominated spiro-trisindole enantiomers fused through a five-member ring, along with seven known biologically related indole alkaloids, were isolated from Laurencia similis. The structures and absolute configurations of the enantiomers were elucidated by comprehensive spectroscopic analysis, computer modeling, and comparison of experimental and theoretically calculated ECD. The proposed biogenetic pathway of 1a and 1b and cytotoxic activity of all isolates were also discussed.