Palmyrolide A, an Unusually Stabilized Neuroactive Macrolide from Palmyra Atoll Cyanobacteria

Abstract

Palmyrolide A (1) is a new neuroactive macrolide isolated from a marine cyanobacterial assemblage composed of Leptolyngbya cf. and Oscillatoria spp. collected from Palmyra Atoll. It features a rare N-methyl enamide and an intriguing t-butyl branch; the latter renders the adjacent lactone ester bond resistant to hydrolysis. Consistent with its significant suppression of calcium influx in cerebrocortical neurons (IC 50 =3.70 µM), palmyrolide A (1) showed relatively potent sodium channel blocking activity in neuro-2a cells (IC 50 =5.2 µM), without appreciable cytotoxicity.Suppression and/or activation of spontaneous Ca 2+ oscillations of murine cerebrocortical neurons1 has proven to be an extremely sensitive screening method for the discovery of new neurotoxins, including the recently reported cyanobacterial metabolites hoiamide A,2a alotamide A,2b and palmyramide A.2c In the case of hoiamide A, further pharmacological characterization found it to be a partial agonist at neurotoxin site 2 of voltage-gated sodium channels (VGSCs).2aPrimary cultures of cerebrocortical neurons allow the detection of two distinct actions when cells are loaded with the Ca 2+ sensitive fluorescent dye, fluo-3.1 First, those metabolites that trigger Ca 2+ influx can be easily revealed by monitoring for this ion using a Fluorometric Imaging Plate Reader (FLIPR). Secondly, because these cultures display spontaneous Ca 2+ oscillations, they provide a robust screening system for the discovery of small molecule ion wgerwick@ucsd.edu. Supporting Information Available: Experimental, full NMR data of 1, 2 and 4 (all stereoisomers), bioassay data, and taxonomic characterization. This material is available free of charge via the Internet at http://pubs.acs.org. (Table 1 and SI) was an intense singlet at δ0.86 (nine protons), which could be attributed to three isochronous methyl groups comprising a t-butyl moiety.8 Also present were the methyl doublets at δ0.90 and δ1.21, as well as the N-methyl singlet at δ3.04. The 1 H NMR of 1 was completed by a deshielded methine proton at δ4.88 and a terminal 1,2-disubstituted vinylic system represented by protons at δ5.27 (dt) and δ6.47 (d). The presence of a t-butyl moiety was supported in the 13 C NMR spectrum of 1 by a very intense resonance at δ26.1, as well as a quaternary carbon at δ35.2. Additionally, the deshielded carbons at δ117.3 and δ130.6 were in agreement with a 1,2-disubstituted double bond, whereas the carbonyls at δ172.9 and δ175.3 indicated a total of two ester or/and amide functionalities. As detailed below, extensive analysis of these 1 H and 13 C NMR resonances using HSQC, HMBC, COSY and NOESY led us to deduce the planar structure of 1. NIH Public AccessThe intense singlet at δ0.86 (H9, H10, H11) showed HMBC correlations with the quaternary carbon C8 (δ35.2), oxymethine C7 (δ76.9), and methylene carbon C6 (δ35.6) ( Figure 1A). This last carbon was found by HSQC to bear the diastereotopic proton resonances H6a (δ1.38) and H6b (δ1.66), which according to the COSY spectrum, participa...

Total Synthesis of Laingolide A Diastereomers

“…However, in our hands, the addition of ammonia to 12 under a variety of conditions failed to give the desired amide 15, probably because of the steric hindrance provided by the contiguous tert-butyl substituent at C(9), as observed by Gerwick et al 4 in their attempts to hydrolyze the lactone of (-)-palmyrolide A. We therefore prepared amide 15 in three steps: reduction of lactone 12 with lithium aluminum hydride to give the corresponding diol 13, chemoselective oxidation of the primary alcohol group of 13 with a combination of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) and [bis(acetyloxy)iodo]benzene (BAIB), 13 and coupling of the resulting carboxylic acid 14 with aqueous ammonia in the presence of N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide (EDCI) and 1H-1,2,3-benzotriazol-1-ol (HOBt).…”

“…species and Oscillatoria species showed promising biological properties, significantly inhibiting calcium(2+) ion oscillations in murine cerebrocortical neurons with an IC 50 value of 3.70 μM. 4 Several total syntheses of this compound have recently appeared in the literature. 5 The first of those, reported by Maio and co-workers, 5a,b permitted the determination of the absolute stereochemistry of palmyrolide A and a revision of the initially suggested relative stereochemistry between the C(5) methyl and the C(7) tert-butyl stereocenters.…”

Bastimolide A, a Potent Antimalarial Polyhydroxy Macrolide from the Marine Cyanobacterium Okeania hirsuta

“…16 The occurrence of a sterically bulky and hydrophobic tert-butyl group adjacent to the lactone ester in bastimolide may protect it from hydrolysis, as was found for palmyrolide A. 17 A major focus of the Panama ICBG program has been the discovery of new classes of agents for the treatment of neglected tropical diseases such as malaria, leishmaniasis, and Chagas disease. 18,19 Thus, bastimolide A (1) was evaluated against a panel of these various human parasites and, remarkably, showed highly potent and selective antimalarial activity against four resistant strains of Plasmodium falciparum [TM90-C2A (chloroquine, mefloquine, and pyrimethamine resistant), TM90-C2B (chloroquine, mefloquine, pyrimethamine, and atovaquone resistant), W2 (chloroquine and pyrimethamine resistant), and TM91-C235 (chloroquine, mefloquine, and pyrimethamine resistant)] with IC 50 values of 80, 90, 140, and 270 nM, respectively.…”

Biological targets and mechanisms of action of natural products from marine cyanobacteria

“…171 Additional studies on kalkitoxin, as well as other neuroactive compounds purified from marine cyanobacteria (Fig. 12) including the macrolide palmyrolide, 172 modified peptide alotamide 173 and the lipopeptide jamaicamides A–C, 174 are needed to establish the detailed MOA.…”