The Role of N-Substitution in Regio- and Stereoselective Vinylogous Imidonaphthoquinone (VINAquinone) [2 + 2] Photocycloadditions.

Described in this manuscript are intramolecular [2 + 2] photocycloadditions of readily available vinylogous imidonaphthoquinones whose regio- and diastereoselectivity is dependent on the substitution on the vinylogous imide. When exposed to 419 nm light, 2° vinylogous imidonaphthoquinones give novel bridged tetracyclic aza-anthraquinones from a rare crossed [2 + 2] cycloaddition reaction. In contrast, exposure of the corresponding 3° substrates to white light leads to linear adducts. Also outlined here are auxiliary controlled diastereoselective reactions and cyclobutane fragmentations as a means of generating the spirofused γ-lactam moiety present in the ansalactam family of natural product.

A gram-scale synthesis of very-long chain polyunsaturated fatty acids (VLC-PUFAs).

This manuscript describes our third generation, gram-scale synthesis of very long chain-polyunsaturated fatty acids (VLC-PUFAs), a unique and increasingly important class of lipids. Critical to this work and what makes it different from our previous approach to this family was the avoidance of oxidation sequences. Central to accomplishing this involved the use of a Negishi coupling reaction between the acid chloride derived from DHA and a saturated alkyl zinc reaction. Overall, the general approach required 6 synthetic transformations from DHA and was accomplished with an overall yield of 40%.

A Biaryl-Cyclohexenone Photoelectrocyclization/Dearomatization Sequence to Substituted Terpenes.

Described here is the development of sequential cross-coupling, photoelectrocyclizations, and reductive dearomatizations of biaryl cyclohexenones as a means of synthesizing terpene skeletons. This methodology promises to provide insight that will enable us and others to use this approach to generate a variety of biologically active small molecules, including members of the abietane and morphinan skeletons.

Influence of very-long-chain polyunsaturated fatty acids on membrane structure and dynamics.

The unique attributes of very-long-chain polyunsaturated fatty acids (VLC-PUFAs), their long carbon chains (n > 24) and high degree of unsaturation, impart unique chemical and physical properties to this class of fatty acids. The changes imparted by VLC-PUFA 32:6 n-3 on lipid packing and the compression moduli of model membranes were evaluated from π-A isotherms of VLC-PUFA in 1,2-distearoyl-sn-3-glycero-phosphocholine (DSPC) lipid monolayers. To compare the attractive or repulsive forces between VLC-PUFA and DSPC lipid monolayers, the measured mean molecular areas (MMAs) were compared with the calculated MMAs of an ideal mixture of VLC-PUFA and DSPC. The presence of 0.1, 1, and 10 mol % VLC-PUFA shifted the π-A isotherm to higher MMAs of the lipids comprising the membrane and the observed positive deviations from ideal behavior of the mixed VLC-PUFA:DSPC monolayers correspond to repulsive forces between VLC-PUFAs and DSPC. The MMA of the VLC-PUFA component was estimated using the measured MMAs of DSPC of 47.1 ± 0.7 Å2/molecule, to be 15,000, 1100, and 91 Å2/molecule at 0.1, 1, and 10 mol % VLC-PUFA:DSPC mixtures, respectively. The large MMAs of VLC-PUFA suggest that the docosahexaenoic acid tail reinserts into the membrane and adopts a nonlinear structure in the membrane, which is most pronounced at 0.1 mol % VLC-PUFA. The presence of 0.1 mol % VLC-PUFA:DSPC also significantly increased the compression modulus of the membrane by 28 mN/m compared with a pure DSPC membrane. The influence of VLC-PUFA on lipid "flip-flop" was investigated by sum-frequency vibrational spectroscopy. The incorporation of 0.1 mol % VLC-PUFA increased the DSPC flip-flop rate fourfold. The fact that VLC-PUFA promotes lipid translocation is noteworthy as retinal membranes require a high influx of retinoids which may be facilitated by lipid flip-flop.

Biphenyl Cyclobutenone Photoelectrocyclizations.

In this work, we demonstrate that readily available conjugated bis-aryl cyclobutenones undergo photoelectrocyclization reactions to give the corresponding dihydrophenanthrene cyclobutanones when exposed to 350 nm light, TFA, and TMSCl. We have also found that cyclobutenone electrocyclizations and cycloreversions are in equilibrium.

The Synthesis of Conjugated Bis-Aryl Vinyl Substrates and Their Photoelectrocyclization Reactions towards Phenanthrene Derivatives.

The photoelectrocyclization of conjugated vinyl biaryls has proven to be a valuable and efficient strategy for generating phenanthrene derivatives. Contained in this review is an overview of the mechanism for the transformation and a discussion of the reaction scope with a focus on the electrocyclization itself, rearomatization, and the application of the reaction in natural product synthesis.

The synthesis of the very long chain polyunsaturated fatty acid (VLC-PUFA) 32:6 n-3.

This article describes the synthesis of VLC-PUFA 32:6 n-3, D2-labeled 32:6 n-3, and the uptake of 32:6 n-3 into mouse retinal tissue.

Retinal bioavailability and functional effects of a synthetic very-long-chain polyunsaturated fatty acid in mice.

Rare, nondietary very-long-chain polyunsaturated fatty acids (VLC-PUFAs) are uniquely found in the retina and a few other vertebrate tissues. These special fatty acids play a clinically significant role in retinal degeneration and development, but their physiological and interventional research has been hampered because pure VLC-PUFAs are scarce. We hypothesize that if Stargardt-3 or age-related macular degeneration patients were to consume an adequate amount of VLC-PUFAs that could be directly used in the retina, it may be possible to bypass the steps of lipid elongation mediated by the retina's ELOVL4 enzyme and to delay or prevent degeneration. We report the synthesis of a VLC-PUFA (32:6 n-3) in sufficient quantity to study its bioavailability and functional benefits in the mouse retina. We acutely and chronically gavage fed wild-type mice and Elovl4 rod-cone conditional knockout mice this synthetic VLC-PUFA to understand its bioavailability and its role in visual function. VLC-PUFA-fed wild-type and Elovl4 conditional knockout mice show a significant increase in retinal VLC-PUFA levels in comparison to controls. The VLC-PUFA-fed mice also had improvement in the animals' visual acuity and electroretinography measurements. Further studies with synthetic VLC-PUFAs will continue to expand our understanding of the physiological roles of these unique retinal lipids, particularly with respect to their potential utility for the treatment and prevention of retinal degenerative diseases.

Photoelectrocyclization Reactions of Conjugated Cycloalkenones: Scope and Reactivity.

Outlined here are studies exploring the scope of the sequential photoelectrocyclization, [1,5]-hydride shift of conjugated bis-aryl cycloalkenone substrates. We have found not only that the cyclization precursors can be synthesized in a modular fashion but also that the cyclization is efficient and amenable to the presence of a range of cycloalkenones and aromatic systems. Among the interesting discoveries from this work is that the electrocyclization intermediate can be competitively captured with protons and that the nature of the excited state (singlet vs triplet) is dependent on aromatic substitution.

Photoelectrocyclization Reactions of Amidonaphthoquinones.

Readily available acrylamide naphthoquinones can be converted into the corresponding aza-anthraquinones using 6π-photoelectrocyclization reactions. Not only do these reactions not proceed thermally but, as demonstrated here, they can also be used to generate a range of aza-anthraquinone and aza-tetracycline derivatives including the natural products griffithazanone A and marcanine A. Several of the aza-anthraquinones generated in this work showed antibacterial activity.

The one-pot synthesis of amidonapthoquinones from aminonaphthoquinones.

Described here is a one-pot method of synthesizing amidonaphthoquinones from the corresponding aminonaphthoquinones. The scope of amides that can be synthesized using this methodology is relatively broad and the yield of product is higher than the traditional methods of synthesizing these substrates.

Stereodivergent Photoelectrocyclization Reactions of Bis-aryl Cycloalkenones: Intercepting Photoelectrocyclization Intermediates with Acid.

Described here are tandem photoelectrocyclization and [1,5]-hydride shift reactions of heteroaryl-containing bis-aryl cyclohexenone derivatives that give heteroaryl-substituted dihydrophenanthrenes. This Letter demonstrates that electrocyclization intermediates can be trapped with acid when the [1,5]-hydride shift is relatively slow. From a practical perspective, the observation that the acid-mediated reaction gives a divergent stereochemical outcome when compared with the reaction run under neutral conditions makes these transformations powerful.

Gambierol and n-alkanols inhibit Shaker Kv channel via distinct binding sites outside the K(+) pore.

The marine polycyclic-ether toxin gambierol and 1-butanol (n-alkanol) inhibit Shaker-type Kv channels by interfering with the gating machinery. Competition experiments indicated that both compounds do not share an overlapping binding site but gambierol is able to affect 1-butanol affinity for Shaker through an allosteric effect. Furthermore, the Shaker-P475A mutant, which inverses 1-butanol effect, is inhibited by gambierol with nM affinity. Thus, gambierol and 1-butanol inhibit Shaker-type Kv channels via distinct parts of the gating machinery.

Voltage-sensor conformation shapes the intra-membrane drug binding site that determines gambierol affinity in Kv channels.

Marine ladder-shaped polyether toxins are implicated in neurological symptoms of fish-borne food poisonings. The toxin gambierol, produced by the marine dinoflagellate Gambierdiscus toxicus, belongs to the group of ladder-shaped polyether toxins and inhibits Kv3.1 channels with nanomolar affinity through a mechanism of gating modification. Binding determinants for gambierol localize at the lipid-exposed interface of the pore forming S5 and S6 segments, suggesting that gambierol binds outside of the permeation pathway. To explore a possible involvement of the voltage-sensing domain (VSD), we made different chimeric channels between Kv3.1 and Kv2.1, exchanging distinct parts of the gating machinery. Our results showed that neither the electro-mechanical coupling nor the S1-S3a region of the VSD affect gambierol sensitivity. In contrast, the S3b-S4 part of the VSD (paddle motif) decreased gambierol sensitivity in Kv3.1 more than 100-fold. Structure determination by homology modeling indicated that the position of the S3b-S4 paddle and its primary structure defines the shape and∖or the accessibility of the binding site for gambierol, explaining the observed differences in gambierol affinity between the channel chimeras. Furthermore, these findings explain the observed difference in gambierol affinity for the closed and open channel configurations of Kv3.1, opening new possibilities for exploring the VSDs as selectivity determinants in drug design.

Synthesis of the ABCDEF and FGHI ring system of yessotoxin and adriatoxin.

Yessotoxin and adriatoxin are members of the polycyclic ether family of marine natural products. Outlined in this article is our synthetic approach to two subunits of these targets. Central to our strategy is a coupling sequence that employs an olefinic-ester cyclization reaction. As outlined, this sequence was used in two coupling sequences. First, it was used to merge the A, B- and E, F-bicyclic precursors and in the process generate the C- and D-rings. Second, it was used to couple the F- and I-rings while building the eight-membered G-ring and subsequently the H-ring pyran.

Fluorescent kapakahines serve as non-toxic probes for live cell Golgi imaging.

AIMS: There is an ongoing need for fluorescent probes that specifically-target select organelles within mammalian cells. This study describes the development of probes for the selective labeling of the Golgi apparatus and offers applications for live cell and fixed cell imaging. MAIN METHODS: The kapakahines, characterized by a common C(3)-N(1') dimeric tryptophan linkage, comprise a unique family of bioactive marine depsipeptide natural products. We describe the uptake and subcellular localization of fluorescently-labeled analogs of kapakahine E. Using confocal microscopy, we identify a rapid and selective localization within the Golgi apparatus. Comparison with commercial Golgi stains indicates a unique localization pattern, which differs from currently available materials, therein offering a new tool to monitor the Golgi in live cells without toxic side effects. KEY FINDINGS: This study identifies a fluorescent analog of kapakahine E that is rapidly uptaken in cells and localizes within the Golgi apparatus. SIGNIFICANCE: The advance of microscopic methods is reliant on the parallel discovery of next generation molecular probes. This study describes the advance of stable and viable probe for staining the Golgi apparatus.

Reactivity of vinyl phosphonate containing diazoesters: formation, reactivity, and utility.

Treatment of diazo vinyl phosphonate with alcohols, amines, and thiols in the presence of Rh(II) results in the chemo- and stereoselective generation of enol ethers, enamines and vinyl sulfides via an X-H insertion process. The utility of the products from these reactions was demonstrated through their conversion into quaternary substituted heterocycles including furans and oxetanes as highlighted by the generation of a bicyclic phosphonate analogue of neodysiherbaine.

Gambierol inhibition of voltage-gated potassium channels augments spontaneous Ca2+ oscillations in cerebrocortical neurons.

Gambierol is a marine polycyclic ether toxin produced by the marine dinoflagellate Gambierdiscus toxicus and is a member of the ciguatoxin toxin family. Gambierol has been demonstrated to be either a low-efficacy partial agonist/antagonist of voltage-gated sodium channels or a potent blocker of voltage-gated potassium channels (Kvs). Here we examined the influence of gambierol on intact cerebrocortical neurons. We found that gambierol produced both a concentration-dependent augmentation of spontaneous Ca(2+) oscillations, and an inhibition of Kv channel function with similar potencies. In addition, an array of selective as well as universal Kv channel inhibitors mimicked gambierol in augmenting spontaneous Ca(2+) oscillations in cerebrocortical neurons. These data are consistent with a gambierol blockade of Kv channels underlying the observed increase in spontaneous Ca(2+) oscillation frequency. We also found that gambierol produced a robust stimulation of phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2). Gambierol-stimulated ERK1/2 activation was dependent on both inotropic [N-methyl-d-aspartate (NMDA)] and type I metabotropic glutamate receptors (mGluRs) inasmuch as MK-801 [NMDA receptor inhibitor; (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate], S-(4)-CGP [S-(4)-carboxyphenylglycine], and MTEP [type I mGluR inhibitors; 3-((2-methyl-4-thiazolyl)ethynyl) pyridine] attenuated the response. In addition, 2-aminoethoxydiphenylborane, an inositol 1,4,5-trisphosphate receptor inhibitor, and U73122 (1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), a phospholipase C inhibitor, both suppressed gambierol-induced ERK1/2 activation, further confirming the role of type I mGluR-mediated signaling in the observed ERK1/2 activation. Finally, we found that gambierol produced a concentration-dependent stimulation of neurite outgrowth that was mimicked by 4-aminopyridine, a universal potassium channel inhibitor. Considered together, these data demonstrate that gambierol alters both Ca(2+) signaling and neurite outgrowth in cerebrocortical neurons as a consequence of blockade of Kv channels.

The synthesis of indoline and benzofuran scaffolds using a Suzuki-Miyaura coupling/oxidative cyclization strategy.

The generation of indolines and benzofurans from the combination of Suzuki-Miyaura coupling reactions with oxidative cyclizations is described.

The ladder-shaped polyether toxin gambierol anchors the gating machinery of Kv3.1 channels in the resting state.

Voltage-gated potassium (Kv) and sodium (Nav) channels are key determinants of cellular excitability and serve as targets of neurotoxins. Most marine ciguatoxins potentiate Nav channels and cause ciguatera seafood poisoning. Several ciguatoxins have also been shown to affect Kv channels, and we showed previously that the ladder-shaped polyether toxin gambierol is a potent Kv channel inhibitor. Most likely, gambierol acts via a lipid-exposed binding site, located outside the K(+) permeation pathway. However, the mechanism by which gambierol inhibits Kv channels remained unknown. Using gating and ionic current analysis to investigate how gambierol affected S6 gate opening and voltage-sensing domain (VSD) movements, we show that the resting (closed) channel conformation forms the high-affinity state for gambierol. The voltage dependence of activation was shifted by >120 mV in the depolarizing direction, precluding channel opening in the physiological voltage range. The (early) transitions between the resting and the open state were monitored with gating currents, and provided evidence that strong depolarizations allowed VSD movement up to the activated-not-open state. However, for transition to the fully open (ion-conducting) state, the toxin first needed to dissociate. These dissociation kinetics were markedly accelerated in the activated-not-open state, presumably because this state displayed a much lower affinity for gambierol. A tetrameric concatemer with only one high-affinity binding site still displayed high toxin sensitivity, suggesting that interaction with a single binding site prevented the concerted step required for channel opening. We propose a mechanism whereby gambierol anchors the channel's gating machinery in the resting state, requiring more work from the VSD to open the channel. This mechanism is quite different from the action of classical gating modifier peptides (e.g., hanatoxin). Therefore, polyether toxins open new opportunities in structure-function relationship studies in Kv channels and in drug design to modulate channel function.