Synthesis studies on the lomaiviticin A aglycone core: development of a divergent, two-directional strategy

The enantiomer of the bicyclic lomaiviticin aglycone A core was prepared via a two-directional, divergent approach featuring (1) a double Ireland Claisen rearrangement to establish key core bonds with correct relative stereochemistry and (2) a double olefin metathesis reaction to deliver both cyclohexene rings of the target.

Synthesis studies on the Melodinus alkaloid meloscine

The pentacyclic Melodinus alkaloid (±)-meloscine was synthesized in 19 chemical steps from 2-bromobenzaldehyde through a route featuring an allenyl azide cyclization cascade to deliver the core azabicyclo[3.3.0]octane substructure. Peripheral functionalization of this core included a Tollens-type aldol condensation to set the quaternary center at C(20) and a diastereoselective ring closing metathesis to forge the tetrahydropyridine ring.

Total Synthesis of (±)-Dragmacidin E: Problems Solved and Lessons Learned

(±)-Dragmacidin E was synthesized in 25 steps from commercially available 7-(benzyloxy)indole. Key transformations in this sponge metabolite's preparation include (a) a Witkop cyclization to establish the bridging indole core, (b) cyclo-dehydrative pyrazinone formation to unite the two indole-bearing components, and (c) late-stage guanidine installation via chemoselective carbonyl activation.

Palau'amine and related oroidin alkaloids dibromophakellin and dibromophakellstatin inhibit the human 20S proteasome

We report herein that the oroidin-derived alkaloids palau'amine (1), dibromophakellin (2), and dibromophakellstatin (3) inhibit the proteolytic activity of the human 20S proteasome as well as the (i)20S immunoproteasome catalytic core. Palau'amine is found to prevent the degradation of ubiquitinylated proteins, including IκBα, in cell culture, which may be indicative of the potential mechanism by which these agents exhibit their exciting cytotoxic and immunosuppressive properties.

Allenyl azide cycloaddition chemistry: application to the total synthesis of (±)-meloscine

The pentacyclic alkaloid (±)-meloscine was prepared in 19 steps through a reaction sequence that features a putative azatrimethylenemethane intermediate, generated through cascade cyclization of an allenyl azide substrate, to deliver the core azabicyclo[3.3.0]octadiene substructure. Subsequent manipulation of the peripheral functionality then delivered (±)-meloscine.

Total synthesis of (±)-dragmacidin E

The bis indole sponge alkaloid dragmacidin E was synthesized in racemic form over 25 steps starting from 7-benzhydroxyindole. Key steps include (a) a Witkop cyclization to facilitate construction of the indole-spanning seven-membered ring and (b) a cyclodehydrative pyrazinone synthesis that unites the two indole-containing sectors.

Extending Pummerer reaction chemistry: studies in the palau'amine synthesis area

Exploratory oxidative cyclization studies on cyclopentanelated and cyclohexenelated oroidin derivatives utilized Pummerer chemistry to generate pentacyclic structures related to the palau'amine family of sponge metabolites. Stereochemical issues were paramount, and appropriate choice of annelated ring size led to formation of the pentacyclic framework with complete diastereoselectivity for all of the core bonds.

Dragmacidin E synthesis studies. Preparation of a model heptacyclic core structure

The conversion of a cycloheptannelated indole platform into the heptacyclic core structure of dragmacidin E proceeded over nine steps. Key sequences include a cyclocondensation to form an intermediate dihydropyrazinone ring and the conversion of a cyclic urea into the cyclic guanidine of the target.

Extending Pummerer reaction chemistry: application to the assembly of the pentacyclic core of dibromopalau'amine

A pentacyclic model system featuring the trans azabicyclo[3.3.0]octane unit of dibromopalau'amine was prepared with complete diastereoselectivity in the polycyclic core from a tricyclic precursor. The key transformations of this sequence include (a) a Pummerer reaction-mediated oxidative bicyclization, and (b) a Wolff rearrangement-based ring contraction to deliver the strained azabicyclo[3.3.0]octane core.

EXPLORATION OF BRAVERMAN REACTION CHEMISTRY. SYNTHESIS OF TRICYCLIC DIHYDROTHIOPHENE DIOXIDE DERIVATIVES FROM BISPROPARGYL SULFONES

The base-mediated bicyclization of unsymmetrical bispropargyl sulfones furnishes varying yields of dihydroisobenzothiophene dioxides through a presumed diradical intermediate. Attempts to trap a putative thiophene dioxide intermediate via Diels-Alder reaction with a pendant alkyne were not successful.

Allenyl azide cycloaddition chemistry. 2,3-cyclopentennelated indole synthesis through indolidene intermediates

The thermal, photochemical, and photochemical/CuI-mediated cascade cyclizations of a range of substituted 1-(2-azidophenyl)-3-alkenylallenes are described. These reactions provide both 1,2- and 2,3-cyclopentennelated indole products in varying ratios. In most cases, high regioselectivity for the 2,3-annelated isomer can be achieved under the hnu/CuI conditions. Computational studies of this multistep reaction support the intermediacy of indolidene intermediates whose electrocyclizations (with or without copper present) define the regioselectivity branch point in the sequence.

Design and evaluation of consensus PCR assays for henipaviruses

Henipaviruses were first discovered in the 1990s, and their potential threat to public health is of increasing concern with increasing knowledge. Old-world fruit bats are the reservoir hosts for these viruses, and spill-over events cause lethal infections in a wide range of mammalian species, including humans. In anticipation of these spill-over events, and to investigate further the geographical range of these genetically diverse viruses, assays for detection of known and potentially novel strains of henipaviruses are required. The development of multiple consensus PCR assays for the detection of henipaviruses, including both SYBR Green and TaqMan real-time PCRs and a conventional heminested PCR is described. The assays are highly sensitive and have defined specificity. In addition to being useful tools for detection of known and novel henipaviruses, evaluation of assay efficiency and sensitivity across both biological and synthetic templates has provided valuable insight into consensus PCR design and use.

Extending pummerer reaction chemistry. Application to the total synthesis of (+/-)-dibromoagelaspongin

The sponge metabolite dibromoagelaspongin was synthesized in 16 steps from imidazole. The route features two successive oxidative cyclizations with complete control of regiochemistry to deliver the unusual triaminomethane core of the target. These oxidative cyclizations likely resulted from Pummerer-like processes on the imidazole-2-sulfoxide (sulfide) precursors.

Allenyl azide cycloaddition chemistry: exploration of the scope and mechanism of cyclopentennelated dihydropyrrole synthesis through azatrimethylenemethane intermediates

Detailed studies of the thermal conversion of 1-azidohepta-3,4,6-trienes into cyclopentennelated dihydropyrroles are presented. High levels of diastereoselectivity and regioselectivity are documented. A mechanistic proposal that accounts for all of the diverse results is developed through the use of density functional calculations. These calculations provide support for the intervention of unexpected mechanistic subtleties, such as the planarity of an azatrimethylenemethane diyl intermediate and an apparent Woodward-Hoffmann-type electrocyclization of a five-atom diyl array.

Allenyl azide cycloaddition chemistry. photochemical initiation and CuI mediation leads to improved regioselectivity

Irradiation of 2-(3-alkenyl)allenylphenyl azides in the presence of excess CuI furnished functionalized 2,3-cyclopentenylindoles in good yield with only trace amounts of competitive C-N-bonded regioisomeric products. These results represent a significant departure from the modest-to-nonexistent regioselectivity that attended thermal cyclization of these allenyl azide substrates.

Synthesis of the pentacyclic core of lihouidine

The pentacyclic base of the sponge-derived alkaloid lihouidine has been assembled from two quinoline fragments. The key step is a nitration-promoted cyclization to form the C-C bond between the two quinoline units.

Extending Pummerer reaction chemistry. Synthesis studies in the phakellin alkaloid area

The syntheses of (+/-)-dibromophakellstatin and, from this species, (+/-)-dibromophakellin are described. Oxidative cyclization of a phenylthiolated dihydrooroidin derivative triggered by a Pummerer reaction constitutes the key step in this biomimetic approach to this family of marine alkaloids.

A relationship between amide hydrogen bond strength and quinone reduction potential: implications for photosystem I and bacterial reaction center quinone function

A series of 11 simple phylloquinone derivatives, each lacking the extended phytyl side chain but featuring H-bond donor amides at one or both peri positions, were prepared and some salient physical properties were measured. A correlation between both IR frequency and NMR peak position, as indicators of internal H-bond strength, and the quinone half-wave reduction potential, was observed. These data are consistent with the prevailing hypothesis that quinone carbonyl H-bonding in general, and stronger H-bonds in particular, favorably bias the endogenous quinone's electrochemical potential toward easier reduction.

Cyclization cascade of allenyl azides: a dual mechanism

A density functional theory based computational approach to describing the mechanistic course of the allene azide cycloaddition cascade sequence has been developed. The results of these calculations permit characterization of key reactive intermediates (diradicals and/or indolidenes) and explain the different behaviors observed in the experimental studies between conjugated and nonconjugated species. Furthermore, computational analysis of certain intermediates offer insight into issues of regioselectivity and stereoselectivity in cases where different reaction channels are in competition, suggesting suitable substitutions to achieve a single regioisomer in the indole synthesis via azide-allene cyclization.

Allenyl azide cycloaddition chemistry. Synthesis of annelated indoles from 2-(allenyl)phenyl azide substrates

[reaction: see text] Thermolysis of 2-(allenyl)phenyl azides leads to a cascade cyclization sequence furnishing both C(2)-C(3) and N-C(2) cyclopentannelated indoles.

Studies on the mechanism of action of prekinamycin, a member of the diazoparaquinone family of natural products: evidence for both sp2 radical and orthoquinonemethide intermediates

The putative reductive activation chemistry of the diazoparaquinone antibiotics was modeled with Bu(3)Sn-H and prekinamycin dimethyl ether along with prekinamycin itself. Reaction in various combinations of aromatic solvents, with and without the nucleophile benzylmercaptan present, led to isolation of both radical-trapping arene adducts and nucleophilic capture benzyl thioether products. On the basis of these product distribution studies, the intermediacies of, first, a cyclopentenyl radical and, next, an orthoquinonemethide electrophile are postulated.

Extending Pummerer Reaction Chemistry. Asymmetric Synthesis of Spirocyclic Oxindoles via Chiral Indole-2-sulfoxides

The conversion of (Ss)-3-(omega-allylsilane and silyl enol ether)indole-2-sulfoxides to spirocyclic indolenines and then to oxindoles proceeds, in favorable cases, with moderate levels of chirality transfer from sulfur to C3 of the indole core. A mechanistic model, which features either an Sn2'-like additive Pummerer sequence or a tight ion pair generated by an Sn1-like vinylogous Pummerer transform, is proposed to rationalize the sense of asymmetric induction.

A proposal for the mechanism-of-action of diazoparaquinone natural products

Treatment of the representative diazoparaquinone prekinamycin dimethyl ether with Bu3SnH/AIBN in aromatic solvents furnishes moderate-to-good yields of formal aryl adducts wherein a molecule of solvent is attached to the carbon (C(11)) previously bearing the diazo function. Substituent studies provide evidence in support of a radical aromatic substitution mechanism, in which radical addition to the diazoparaquinone function generates an intermediate C(11) vinylic radical.

Extending Pummerer reaction chemistry. Synthesis of (+/-)-dibromophakellstatin by oxidative cyclization of an imidazole derivative

The diastereoselective oxidative cyclization of a dihydrooroidin derivative is reported. The thioimidate product formed by application of a new variant of the Pummerer reaction serves as a precursor to dibromophakellstatin. [reaction: see text]

Dragmacidin E Synthesis Studies. Preparation of a Model Cycloheptannelated Indole Fragment

[reaction: see text] The conversion of N-2,2-dichloropropionyl indole methyl ester into a tetracyclic cycloheptannelated indole model compound for the synthesis of dragmacidin E was accomplished in 10 steps. Key reactions include a Witkop cyclization to fashion a C-C bond at C(4) of the indole nucleus and a subsequent Dieckmann cyclization to deliver the desired cycloheptanoid ring.

Extending Pummerer reaction chemistry. development of a strategy for the regio- and stereoselective oxidative cyclization of 3-(omega-nucleophile)-tethered indoles

The brominative cyclization of diastereomeric beta-silyloxy tryptophan derivatives proceeded with divergent regiochemistry (C(2) or C(3) addition), depending on the relative stereochemistry of the silyloxy substituent. This lack of C(2) vs C(3) regiochemical predictability led to the development of a new approach, which featured Pummerer-type chemistry on an indole C(2) sulfoxide or sulfide substrate, for steering nucleophilic addition to C(3) of the indole. Extension of this transformation from carboxylate nucleophiles to carbon analogues such as allylsilane, silyl enol ether, and silyl ketene iminal bearing substrates led to the formation of spirocyclic oxindole derivatives in good yields with complete regioselectivity for C(3) cyclization and with good diastereoselectivity where relevant.

Recent progress in ellagitannin chemistry

Continuing studies on the total synthesis of ellagitannin plant metabolites have led to the preparation of the dimeric antitumor compound, coriariin A, as well as designed structural analogues. In related investigations, the synthesis of a 2,4-hexahydroxydiphenoyl (HHDP)-bearing glucopyranose structure has been achieved. This species is related to the geraniin family of ellagitannins, and its subsequent chemistry is suggestive of a mechanistic rationale for the observation that the HHDP units within (3,6-bridged)2,4-HHDP-containing ellagitannins invariably are oxidized further in vivo. Companion studies designed to assay the immunomodulatory properties of coriariin A and analogues have led to the thesis that tumor necrosis factor alpha (TNFalpha) serves as a mediator of this ellagitannin's tumor remissive activity. Furthermore, certain tannins and tannin analogues appear to act in an immunosuppressive capacity with peripheral blood monocytes that were exposed to the bacterially derived septic shock inducing agent lipid A.

Allenyl Azide Cycloaddition Chemistry. Synthesis of Pyrrolidine-Containing Bicycles and Tricycles via the Possible Intermediacy of Azatrimethylenemethane Species

Thermolysis of 5-azidoallenes bearing a C(1) methyl group and either an aryl ring or an alkene on C(1) furnishes tricyclic (from the aryl substrates) or bicyclic (from the alkenyl substrates) pyrrolidine products following formal H-CN addition across an intermediate imine. High levels of diastereoselectivity are observed in all cases studied. This reaction cascade presumably passes through unobserved triazoline and azatrimethylenemethane diyl intermediates en route to product.

Alkynyliodonium salts in organic synthesis. Application to the preparation of the tricyclic core of (+/-)-halichlorine

The tricyclic core of halichlorine has been synthesized through the use of an alkynyliodonium salt/alkylidenecarbene/1,5 C-H insertion sequence that sets the pivotal quaternary center in the target.

Extending Pummerer Reaction Chemistry. Application to the Oxidative Cyclization of Tryptophan Derivatives

The diastereoselective oxidative cyclization of a beta-oxygenated tryptophan derivative is reported. This process, which utilizes a new variant of the Pummerer reaction, may have application in TMC-95A synthesis.

Extending Pummerer Reaction Chemistry. Application to the Oxidative Cyclization of Indole Derivatives

Treatment of 2-(phenylsulfinyl)indoles bearing a pendant nucleophile at C(3) with Tf(2)O/lutidine triggers a Pummerer-like cyclization to furnish 3,3-spirocyclic-2-(phenylthio)indolenine products, which can, in turn, be hydrolyzed to 3,3-spirocyclic oxindoles. [reaction: see text]

Ellagitannin chemistry. Studies on the stability and reactivity of 2,4-HHDP-containing glucopyranose systems

The synthesis of a 2,4-HHDP containing glucopyranose ellagitannin model system has been achieved. Attempts to prepare a related target bearing a 1,6 bridge led instead to the discovery of a likely strain-driven tautomerization to cyclohexadienone intermediates.

Total synthesis of the tropoloisoquinoline alkaloid pareitropone via alkynyliodonium salt chemistry and related studies

The first chemical synthesis of pareitropone, by a route featuring application of alkynyliodonium salt chemistry, is described. The key transform initiates with addition of an alkylidenecarbene, derived by intramolecular nucleophile addition to the alkynyliodonium moiety, to a proximal aromatic ring. This addition delivers a highly strained norcaradiene substructure that rapidly reorganizes to furnish the pareitropone skeleton.

Alkynyliodonium salts in organic synthesis. Development of a unified strategy for the syntheses of (-)-agelastatin A and (-)-agelastatin B

The total syntheses of natural agelastatin A and agelastatin B were accomplished via a strategy that utilized an alkynyliodonium salt --> alkylidenecarbene --> cyclopentene transformation to convert a relatively simple amino alcohol derivative to the functionalized core of the agelastatin system. Subsequent manipulations delivered debromoagelastatin, which served as a precursor to both agelastatin A and agelastatin B. Alkylidenecarbene insertion chemoselectivity issues were explored en route to the final targets.

Diazonamide synthesis studies: use of Negishi coupling to fashion diazonamide-related biaryls with defined axial chirality

The syntheses of a bis indole and an indole salicylate with the required axial chirality for diazonamide A are reported. Atropselectivity in these biaryl systems is enforced by an sp(3) stereogenic center in a lactone tether in both cases. [structure: see text]

Alkynyliodonium Salts in Organic Synthesis. Application to the Total Synthesis of the Tropoloisoquinoline Alkaloid Pareitropone

The synthesis of the tropoloisoquinoline alkaloid pareitropone has been accomplished in 14 steps from 2,3,4-trimethoxybenzoic acid. The key transformations include the generation of an alkylidenecarbene intermediate through intramolecular addition of a tosylamide anion to an alkynyliodonium salt, and the cycloaddition of that carbene to a peri positioned aromatic ring to afford a cycloheptatrienylidene product featuring the intact pareitropone skeleton.

Alkynyliodonium salts in organic synthesis. Application to the total synthesis of (-)-agelastatin A and (-)-agelastatin B

The asymmetric total syntheses of (-)-agelastatin A and (-)-agelastatin B were accomplished in 14 steps each from (R)-epichlorohydrin. The pivotal transformation in both sequences was a sulfinate-promoted cyclization of an alkynyliodonium salt to furnish a key functionalized cyclopentene intermediate. Selective bromination in the final step led to either agelastatin A or agelastatin B, depending upon conditions.