Didemnidines A and B, Indole Spermidine Alkaloids from the New Zealand Ascidian Didemnum sp.

Two new indole spermidine alkaloids, didemnidines A (1) and B (2), have been isolated from the New Zealand ascidian Didemnum sp. The structures of the metabolites, determined by analysis of 2D NMR spectra and confirmed via synthesis, embody an indole-3-glyoxylamide moiety linked to the N¹ position of spermidine, the latter motif being particularly rare among marine natural products. Didemnidine B and a synthetic precursor exhibited mild in vitro growth inhibition of Plasmodium falciparum with IC₅₀'s of 15 and 8.4 μM, respectively.