By
establishing the procedures for sequential deprotections, reaction
monitoring, purification, and handling, for the first time, we achieved
the total synthesis of the proposed structure for protoaculeine B
(2), which is a highly hydrophilic and polycationic amino
acid. The NMR and mass spectra and chemical reactivity of the synthetic
sample differed from those of natural protoaculeine B, which indicates
the necessity for revision of the originally reported structure.
The structure of protoaculeine B,
the N-terminal residue of the
marine peptide toxin aculeine B, is revised to the cis-1,3-disubstituted tetrahydro-β-carboline framework. We prepared
two truncated model compounds that lack a long-chain polyamine using
the one-step Pictet–Spengler reaction of tryptophan and compared
their NMR, mass spectra, and chemical reactivity with those of the
natural protoaculeine B. The synthetic models reproduced the profiles
of the natural product well, which confirmed the appropriateness of
the structure revision.
The planar structure of poecillastrin C (1) was revised through selective reduction of the ester carbon. The absolute configuration of the β-hydroxyaspartic acid (OHAsp) residue was determined to be d-threo by Marfey's analysis. The acid hydrolysate of the reduction product of 1 liberated (2R,3R)-2-amino-3,4-dihydroxybutanoic acid, demonstrating that the β-carboxyl group in poecillastrin C was esterified. The structures of poecillastrins B-D and 73-deoxychondropsin A were also revised.
The isolation and structural determination of new marine ladder-frame polyethers, brevisulcatic acids-1 (1) and -4 (2) are reported. Brevisulcatic acids were isolated from the dinoflagellate Karenia brevisulcata, which was identified as the causative species of a major red tide event in New Zealand in 1998. The ether ring composition and a β-hydroxy, γ-methylene valeric acid side chain of 1 and 2 are common, but 2 has a γ-lactone as the 5-membered A-ring while 1 is the seco acid analogue. Compound 2 has structural and bioactivity similarities to brevetoxin A.
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