Stereochemical Assignment of the 36-Membered Macrolide Ring Portion of Poecillastrin C.

Absolute configuration at 12 stereocenters in the 36-membered macrocyclic ring portion of poecillastrin C (1) was disclosed by chemical degradation and NMR analyses of 1, chemical synthesis, and molecular modeling techniques.

Synthesis of a fully protected long-chain polyamine subunit of aculeine B using the photoremovable NPEC group.

Polyamines are ubiquitously found in nature. In this paper, we disclose our iterative coupling strategy for the synthesis of a structurally defined polymer of 1,3-propanediamine, and the polymer can be used for the synthesis of both the initially proposed structure and the revised structure of protoaculeine B isolated from a marine sponge. We first attempted the synthesis of polyamines using "the Ns strategy" but found that a polyamine with eleven Ns groups has solubility problems. We then examined the versatility of the photoremovable NPEC protecting group in polyamine synthesis. Finally, the synthesis of a suitably protected 12-mer polyamine was achieved employing the NPEC group for the temporary protection of a terminal amino group.

Oxa-Michael-based divergent synthesis of artificial glutamate analogs.

Herein we report stereoselective generation of two skeletons, 1,3-dioxane and tetrahydropyranol, by oxa-Michael reaction as the key reaction from δ-hydroxyenone. The construction of the 1,3-dioxane skeleton, achieved through hemiacetal formation followed by oxa-Michael reaction from δ-hydroxyenone, was exploited to access structurally diverse heterotricyclic artificial glutamate analogs. On the other hand, formation of a novel tetrahydro-2H-pyranol skeleton was accomplished by the inverse reaction order: oxa-Michael reaction followed by hemiacetal formation. Thus, this study succeeded in showing that structural diversity in a compound collection can be acquired by interchanging the order of just two reactions. Among the skeletally diverse, heterotricyclic artificial glutamate analogs synthesized in this study, a neuronally active compound named TKM-50 was discovered in the mice in vivo assay.

Structure Revision of Protoaculeine B, a Post-translationally Modified N-Terminal Residue in the Peptide Toxin Aculeine B.

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-ß-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.

Menthyl esterification allows chiral resolution for the synthesis of artificial glutamate analogs.

Herein, we report the enantiospecific synthesis of two artificial glutamate analogs designed based on IKM-159, an antagonist selective to the AMPA-type ionotropic glutamate receptor. The synthesis features the chiral resolution of the carboxylic acid intermediate by the esterification with ʟ-menthol, followed by a configurational analysis by NMR, conformational calculation, and X-ray crystallography. A mice in vivo assay showed that (2R)-MC-27, with a six-membered oxacycle, is neuroactive, whereas the (2S)-counterpart is inactive. It was also found that TKM-38, with an eight-membered azacycle, is neuronally inactive, showing that the activity is controlled by the ring C.

Brevisulcenals-A1 and A2, Sulfate Esters of Brevisulcenals, Isolated from the Red Tide Dinoflagellate Karenia brevisulcata.

Two different types of polycyclic ether toxins, namely brevisulcenals (KBTs) and brevisulcatic acids (BSXs), produced by the red tide dinoflagellate Karenia brevisulcata, were the cause of a toxic incident that occurred in New Zealand in 1998. Four major components, KBT-F, -G, -H, and -I, shown to be cytotoxic and lethal in mice, were isolated from cultured K. brevisulcata cells, and their structures were elucidated by spectroscopic analyses. New analogues, brevisulcenal-A1 (KBT-A1) and brevisulcenal-A2 (KBT-A2), toxins of higher polarity than that of known KBTs, were isolated from neutral lipophilic extracts of bulk dinoflagellate culture extracts. The structures of KBT-A1 and KBT-A2 were elucidated as sulfated analogues of KBT-F and KBT-G, respectively, by NMR and matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI TOF/TOF), and by comparison with the spectra of KBT-F and KBT-G. The cytotoxicities of the sulfate analogues were lower than those of KBT-F and KBT-G.

Total Synthesis of the Proposed Structure for Protoaculeine B, a Polycationic Marine Sponge Metabolite, with a Homogeneous Long-Chain Polyamine.

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.

Poecillastrin H, a Chondropsin-Type Macrolide with a Conjugated Pentaene Moiety, from a Characella sp. Marine Sponge.

Poecillastrin H (1), a chondropsin-type macrolide with a conjugated pentaene moiety, was isolated from the Characella sp. marine sponge. The planar structure of 1 was elucidated by analysis of spectroscopic data. The absolute configuration of the ß-hydroxyaspartic acid residue (ß-OHAsp) was determined to be d- threo by Marfey's analysis, and the mode of lactone ring formation through the OHAsp residue was determined by chemical degradation. Poecillastrin H was extremely sensitive toward light and showed potent cytotoxic activity against 3Y1 cells with an IC50 value of 4.1 nM.

Resolution of the Confusion in the Assignments of Configuration for the Ciliatamides, Acylated Dipeptides from Marine Sponges.

Direct comparison of authentic ciliatamide A with four synthetic isomers (1-4) by means of NMR and chiral-phase HPLC revealed that ciliatamide A possesses the 12R (d-N-MePhe residue) and 22S (l-Lys residue) configurations, which were not identical with either our previous assignment or those proposed by others through total synthesis. The absolute configuration of the methionine sulfoxide residue in ciliatamide D was also revised to be d.

Structure Revision of Poecillastrin C and the Absolute Configuration of the ß-Hydroxyaspartic Acid Residue.

The planar structure of poecillastrin C (1) was revised through selective reduction of the ester carbon. The absolute configuration of the ß-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 ß-carboxyl group in poecillastrin C was esterified. The structures of poecillastrins B-D and 73-deoxychondropsin A were also revised.

Brevisulcatic acids, marine ladder-frame polyethers from the red tide dinoflagellate Karenia brevisulcata in New Zealand.

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 ß-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.

Interaction analysis of a ladder-shaped polycyclic ether and model transmembrane peptides in lipid bilayers by using Förster resonance energy transfer and polarized attenuated total reflection infrared spectroscopy.

Ladder-shaped polycyclic ethers (LSPs) are predicted to interact with membrane proteins; however, the underlying mechanism has not been satisfactorily elucidated. It has been hypothesized that LSPs possess non-specific affinity to α-helical segments of transmembrane proteins. To verify this hypothesis, we constructed a model LSP interaction system in a lipid bilayer. We prepared 5 types of α-helical peptides and reconstituted them in liposomes. The reconstitution and orientation of these peptides in the liposomes were examined using polarized attenuated total reflection infrared (ATR-IR) spectroscopy and gel filtration. The results revealed that 4 peptides were retained in liposomes, and 3 of them formed stable transmembrane structures. The interaction between the LSP and the peptides was investigated using Förster resonance energy transfer (FRET). In the lipid bilayer, the LSP strongly recognized the peptides that possessed aligned hydrogen donating groups with leucine caps. We propose that this leucine-capped 16-amino acid sequence is a potential LPS binding motif.