Total synthesis and configurational assignment of the marine natural product haliclamide.

The marine natural product haliclamide has been synthesized based on macrocyclization by ring-closing olefin metathesis. Using either enantiomer of two of the four building blocks that were employed to assemble the diene precursor for the metathesis reaction, three non-natural isomers of haliclamide were also prepared. On the basis of the comparison of the (1)H and (13)C NMR spectra of the individual stereoisomers with literature data for the natural product, the configuration of the previously unassigned stereocenters at C9 and C20 of haliclamide could be determined to be S for both carbons. The absolute configuration of haliclamide thus is 2S, 9S, 14R, 20S. The antiproliferative activity of synthetic haliclamide against several human cancer cell lines was found to be in the high µM range. The compound showed no antifungal or antibiotic activity.

Total synthesis of (-)-zampanolide and structure-activity relationship studies on (-)-dactylolide derivatives.

A new total synthesis of the marine macrolide (-)-zampanolide (1) and the structurally and stereochemically related non-natural levorotatory enantiomer of (+)-dactylolide (2), that is, ent-2, has been developed. The synthesis features a high-yielding, selective intramolecular Horner-Wadsworth-Emmons (HWE) reaction to close the 20-membered macrolactone ring of 1 and ent-2. The ß-keto phosphonate/aldehyde precursor for the ring-closure reaction was obtained by esterification of a ω-diethylphosphono carboxylic acid fragment and a secondary alcohol fragment incorporating the THP ring that is embedded in the macrocyclic core structure of 1 and ent-2. THP ring formation was accomplished through a segment coupling Prins-type cyclization. Employing the same overall strategy, 13-desmethylene-ent-2 as well as the monocyclic desTHP derivatives of 1 and ent-2 were prepared. Synthetic 1 inhibited human cancer cell growth in vitro with nM IC(50) values, while ent-2, which lacks the diene-containing hemiaminal-linked side chain of 1, is 25- to 260-fold less active. 13-Desmethylene-ent-2 as well as the reduced versions of ent-2 and 13-desmethylene-ent-2 all showed similar cellular activity as ent-2 itself. The same activity level was attained by the monocyclic desTHP derivative of 1. Oxidation of the aldehyde functionality of ent-2 gave a carboxylic acid that was converted into the corresponding N-hexyl amide. The latter showed only µM antiproliferative activity, thus being several hundred-fold less potent than 1.