Dionicio Siegel

Synthesis of (+)-Complanadine A, an Inducer of Neurotrophic Factor Excretion

A total synthesis of the Lycopodium alkaloid (+)-complanadine A is described. Complanadine A has been shown to induce the secretion of neurotrophic factors from 1321N1 cells, promoting the differentiation of PC-12 cells. The use of a simplifying metal mediated [2+2+2] + [2+2+2] sequence using a silyl-substituted diyne and 2 equiv of the corresponding alkyne-nitrile has provided rapid access to the natural product.

Total Synthesis of (±)-Garsubellin A

A concise approach to the laboratory synthesis of garsubellin A is described. Garsubellin A, an effective inducer of choline acetyltransferase (ChAT), has been shown to have potential as a therapeutic agent for the treatment of Alzheimers disease. Starting from 3,5-dimethoxyphenol, the synthesis has provided garsubellin A in an 18-step sequence. Notable transformations include dearomative allylation, diastereoselective vinylogous lactonization, iodocarbocyclization, transannular Wurtz, and bridgehead functionalization reactions.

A Robust Platform for the Synthesis of New Tetracycline Antibiotics

Tetracyclines and tetracycline analogues are prepared by a convergent, single-step Michael-Claisen condensation of AB precursor 1 or 2 with D-ring precursors of wide structural variability, followed by removal of protective groups (typically in two steps). A number of procedural variants of the key C-ring-forming reaction are illustrated in multiple examples. These include stepwise deprotonation of a D-ring precursor followed by addition of 1 or 2, in situ deprotonation of a D-ring precursor in mixture with 1 or 2, and in situ lithium-halogen exchange of a benzylic bromide D-ring precursor in the presence of 1 or 2, followed by warming. The AB plus D strategy for tetracycline synthesis by C-ring construction is shown to be robust across a range of different carbocyclic and heterocyclic D-ring precursors, proceeding reliably and with a high degree of stereochemical control. Evidence suggests that Michael addition of the benzylic anion derived from a given D-ring precursor to enones 1 or 2 is quite rapid at -78 degrees C, while Claisen cyclization of the enolate produced is rate-determining, typically occurring upon warming to 0 degrees C. The AB plus D coupling strategy is also shown to be useful for the construction of tetracycline precursors that are diversifiable by latter-stage transformations, subsequent to cyclization to form the C ring. Results of antibacterial assays and preliminary data obtained from a murine septicemia model show that many of the novel tetracyclines synthesized have potent antibiotic activities, both in bacterial cell culture and in vivo. The platform for tetracycline synthesis described gives access to a broad range of molecules that would be inaccessible by semisynthetic methods (presently the only means of tetracycline production) and provides a powerful engine for the discovery and, perhaps, development of new tetracycline antibiotics.

Caenorhabditis elegans selects distinct crawling and swimming gaits via dopamine and serotonin

Many animals, including humans, select alternate forms of motion (gaits) to move efficiently in different environments. However, it is unclear whether primitive animals, such as nematodes, also use this strategy. We used a multifaceted approach to study how the nematode Caenorhabditis elegans freely moves into and out of water. We demonstrate that C. elegans uses biogenic amines to switch between distinct crawling and swimming gaits. Dopamine is necessary and sufficient to initiate and maintain crawling after swimming. Serotonin is necessary and sufficient to transition from crawling to swimming and to inhibit a set of crawl-specific behaviors. Further study of locomotory switching in C. elegans and its dependence on biogenic amines may provide insight into how gait transitions are performed in other animals.

Metal-free oxidation of aromatic carbon–hydrogen bonds through a reverse-rebound mechanism

Methods for carbon–hydrogen (C–H) bond oxidation have a fundamental role in synthetic organic chemistry, providing functionality that is required in the final target molecule or facilitating subsequent chemical transformations. Several approaches to oxidizing aliphatic C–H bonds have been described, drastically simplifying the synthesis of complex molecules. However, the selective oxidation of aromatic C–H bonds under mild conditions, especially in the context of substituted arenes with diverse functional groups, remains a challenge. The direct hydroxylation of arenes was initially achieved through the use of strong Brønsted or Lewis acids to mediate electrophilic aromatic substitution reactions with super-stoichiometric equivalents of oxidants, significantly limiting the scope of the reaction. Because the products of these reactions are more reactive than the starting materials, over-oxidation is frequently a competitive process. Transition-metal-catalysed C–H oxidation of arenes with or without directing groups has been developed, improving on the acid-mediated process; however, precious metals are required. Here we demonstrate that phthaloyl peroxide functions as a selective oxidant for the transformation of arenes to phenols under mild conditions. Although the reaction proceeds through a radical mechanism, aromatic C–H bonds are selectively oxidized in preference to activated –H bonds. Notably, a wide array of functional groups are compatible with this reaction, and this method is therefore well suited for late-stage transformations of advanced synthetic intermediates. Quantum mechanical calculations indicate that this transformation proceeds through a novel addition–abstraction mechanism, a kind of ‘reverse-rebound’ mechanism as distinct from the common oxygen-rebound mechanism observed for metal–oxo oxidants. These calculations also identify the origins of the experimentally observed aryl selectivity.

Selective Inactivation of a Human Neuronal Silencing Phosphatase by a Small Molecule Inhibitor

The unstructured C-terminal domain (CTD) of eukaryotic RNA polymerase II dynamically regulates the process of transcription by recruiting different factors to nascent mRNA through its multiple phosphorylation patterns. A newly discovered class of phosphatases, the human small C-terminal domain phosphatases (Scps), specifically dephosphorylates phosphorylated Ser(5) (phospho.Ser5) of the tandem heptad repeats of the CTD of RNA polymerase II. Scps also function as transcription regulators that epigenetically silence the expression of specific neuronal genes, whose inactivation leads to neuronal stem cell differentiation. Small molecule inhibitors of Scps will be valuable for elucidating their mechanism in nervous system development and can possibly offer new strategies to treat diseases related to neurodegeneration. Despite the difficulty in developing selective inhibitors of protein phosphatases, we have recognized a characteristic hydrophobic binding pocket adjacent to the active site in Scps that may facilitate selective inhibition. In the present study, we successfully identified the first selective lead compound, rabeprazole, for the Scp/TFIIF-interacting CTD phosphatase (Fcp) family. The high-resolution crystal structure of rabeprazole-bound Scp1 showed that the compound indeed binds to the hydrophobic binding pocket. We further confirmed that rabeprazole only targets Scps but not its close family members Fcp1 and Dullard or bacteriophage λ Ser/Thr phosphatase. Such specificity may prove important for In Vivo studies since accidental inhibition of Fcp1 or Dullard would result in cell malfunctions and even cell death.

Biomimetic syntheses of the neurotrophic natural products caryolanemagnolol and clovanemagnolol.

Separate short and modular syntheses of the isomeric natural products caryolanemagnolol and clovanemagnolol have been achieved starting from commercially available (-)-caryophyllene. The postulated biosynthetic pathways guided the syntheses of the neuroregenerative small molecules allowing their assembly in as few as two steps.

Syntheses of (+)-complanadine A and lycodine derivatives by regioselective [2 + 2 + 2] cycloadditions.

The dimeric alkaloid complanadine A has shown promise in regenerative science, promoting neuronal growth by inducing the secretion of growth factors from glial cells. Through the use of tandem, cobalt-mediated [2 + 2 + 2] cycloaddition reactions, two synthetic routes have been developed with different sequences for the formation of the unsymmetric bipyridyl core. The regioselective formation of each of the pyridines was achieved based on the inherent selectivity of the molecules or by reversing the regioselectivity through the addition of Lewis bases. This strategy has been successfully employed to provide laboratory access to complanadine A as well as structurally related compounds possessing the lycodine core.

Hydroxyl-Directed Cyclizations of 1,6-Enynes

The palladium-catalyzed, hydroxyl-directed cyclization reactions of 1,6-enynes provide a highly diastereoselective process for the syntheses of stereochemically defined cyclopentanes. Consistently high levels of cis-selectivity are possible using homopropargyl alcohols in contrast to the corresponding propargyl alcohols. Hydroborylative enyne cyclizations coupled with this directing group effect provide a useful method for the syntheses of multifaceted compounds bearing all carbon quaternary centers.

Semaphorin3a Promotes Advanced Diabetic Nephropathy

The onset of diabetic nephropathy (DN) is highlighted by glomerular filtration barrier abnormalities. Identifying pathogenic factors and targetable pathways driving DN is crucial to developing novel therapies and improving the disease outcome. Semaphorin3a (sema3a) is a guidance protein secreted by podocytes. Excess sema3a disrupts the glomerular filtration barrier. Here, using immunohistochemistry, we show increased podocyte SEMA3A in renal biopsies from patients with advanced DN. Using inducible, podocyte-specific Sema3a gain-of-function (Sema3a+) mice made diabetic with streptozotocin, we demonstrate that sema3a is pathogenic in DN. Diabetic Sema3a+ mice develop massive proteinuria, renal insufficiency, and extensive nodular glomerulosclerosis, mimicking advanced DN in humans. In diabetic mice, Sema3a+ exacerbates laminin and collagen IV accumulation in Kimmelstiel-Wilson-like glomerular nodules and causes diffuse podocyte foot process effacement and F-actin collapse via nephrin, αvβ3 integrin, and MICAL1 interactions with plexinA1. MICAL1 knockdown and sema3a inhibition render podocytes not susceptible to sema3a-induced shape changes, indicating that MICAL1 mediates sema3a-induced podocyte F-actin collapse. Moreover, sema3a binding inhibition or podocyte-specific plexinA1 deletion markedly ameliorates albuminuria and abrogates renal insufficiency and the diabetic nodular glomerulosclerosis phenotype of diabetic Sema3a+ mice. Collectively, these findings indicate that excess sema3a promotes severe diabetic nephropathy and identifies novel potential therapeutic targets for DN.