Metalation/SRN1 coupling in heterocyclic synthesis. A convenient methodology for ring functionalization

Carbon-Nitrogen Bond Formation Using Sodium Hexamethyldisilazide: Solvent-Dependent Reactivities and Mechanisms

The solvent-dependent reactivity of sodium hexamethyldisilazide (NaHMDS) toward carbon-centered electrophiles reveals reactions that are poorly represented or unrepresented in the literature, including direct aminolysis of aromatic methyl esters to give carboxamides, nitriles, or amidines, depending on the choice of solvent. SNAr substitutions of aryl halides and opening of terminal epoxides are also examined. A combination of 1H and 29Si nuclear magnetic resonance (NMR) spectroscopic studies using [15N]NaHMDS, kinetic studies, and computational studies reveals the complex mechanistic basis of the preferences for simple aryl carboxamides in toluene and dimethylethylamine and arylnitriles or amidines in tetrahydrofuran (THF). A prevalence of dimer- and mixed dimer-based chemistry even starting from the observable NaHMDS monomer in THF solution is notable.

Copper-Catalyzed Trifluoromethylation of Alkoxypyridine Derivatives

The trifluoromethylation of aromatic and heteroaromatic cores has attracted considerable interest in recent years due to its pharmacological relevance. We studied the extension of a simple copper-catalyzed trifluoromethylation protocol to alkoxy-substituted iodopyridines and their benzologs. The trifluoromethylation proceeded smoothly in all cases, and the desired compounds were isolated and characterized. In the trifluoromethylation of 3-iodo-4-methoxyquinoline, we observed a concomitant O-N methyl migration, resulting in the trifluoromethylated quinolone as a product. Overall, the described procedure should facilitate the broader use of copper-catalyzed trifluoromethylation in medicinal chemistry.

Transition-metal-free access to 2-aminopyridine derivatives from 2-fluoropyridine and acetamidine hydrochloride

Under catalyst-free conditions, an efficient method for the synthesis of 2-aminopyridine derivatives through the nucleophilic substitution and hydrolysis of 2-fluoropyridine and acetamidine hydrochloride has been developed. This amination uses inexpensive acetamidine hydrochloride as the ammonia source and has the advantages of a high yield, high chemoselectivity and wide substrate adaptability. The results suggest that other N-heterocycles containing fluorine substituents can also complete the reaction via these reaction conditions and yield the target products.

Synthesis of N-Substituted 3-Amino-4-halopyridines: A Sequential Boc-Removal/Reductive Amination Mediated by Brønsted and Lewis Acids

N-Substituted 3-amino-4-halopyridines are valuable synthetic intermediates, as they readily provide access to imidazopyridines and similar heterocyclic systems. The direct synthesis of N-substituted 3-amino-4-halopyridines is problematic, as reductive aminations and base-promoted alkylations are difficult in these systems. A high yielding deprotection/alkylation protocol mediated by trifluoroacetic acid and trimethylsilyl trifluoromethanesulfonate is described, providing access to a wide scope of N-substituted 3-amino-4-halopyridines. This protocol furnishes many reaction products in high purity without chromatography. Similar reductive amination conditions were also established for deactivated anilines.

Synthesis and trypanocidal activity of a library of 4-substituted 2-(1H-pyrrolo[3,2-c]pyridin-2-yl)propan-2-ols

A library of 16 4-substituted 2-(1H-pyrrolo[3,2-c]pyridin-2-yl)propan-2-ols 17-32 has been synthesized for use in biological testing against Trypanosoma cruzi, the protozoan parasite that causes Chagas disease. The 4-substituted 2-(1H-pyrrolo[3,2-c]pyridin-2-yl)propan-2-ols 17-32 were subjected to biological testing to evaluate their efficacy against intracellular Trypanosoma cruzi (Y strain) amastigotes infecting U2OS human cells, with benznidazole as a reference compound. The assay was performed in duplicate (two independent experiments) and submitted to High Content Analysis (HCA) for determination of trypanocidal activity. Three of the tested compounds presented relatively high trypanocidal activity (19, 22 and 29), however severe host cell toxicity was observed concomitantly. Chemical optimization of the highly active compounds and the synthesis of more compounds for biological testing against Trypanosoma cruzi will be required to improve selectivity and so that a structure-activity relationship can be generated to provide a more insightful analysis of both chemical and biological aspects.

Practical and economic lithiations of functionalized arenes and heteroarenes using Cy2NLi in the presence of Mg, Zn or La halides in a continuous flow

The economic amide base lithium dicyclohexylamide (Cy2NLi) allows fast and convenient (40 s, 0 °C) in situ trapping flow metalations of a broad range of functionalized arenes, heteroarenes and acrylate derivatives in the presence of various metal salts (ZnCl2·2LiCl, MgCl2, LaCl3·2LiCl). The resulting Zn-, Mg- or La-organometallic intermediates are trapped with various electrophiles in high yields. These flow metalations are easily scaled-up without further optimization.

Selective C-H fluorination of pyridines and diazines inspired by a classic amination reaction

Fluorinated heterocycles are prevalent in pharmaceuticals, agrochemicals, and materials. However, reactions that incorporate fluorine into heteroarenes are limited in scope and can be hazardous. We present a broadly applicable and safe method for the site-selective fluorination of a single carbon-hydrogen bond in pyridines and diazines using commercially available silver(II) fluoride. The reactions occur at ambient temperature within 1 hour with exclusive selectivity for fluorination adjacent to nitrogen. The mild conditions allow access to fluorinated derivatives of medicinally important compounds, as well as a range of 2-substituted pyridines prepared by subsequent nucleophilic displacement of fluoride. Mechanistic studies demonstrate that the pathway of a classic pyridine amination can be adapted for selective fluorination of a broad range of nitrogen heterocycles.

Light-driven coordination-induced spin-state switching: rational design of photodissociable ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin-state switching to isolated molecules at room temperature (light-driven coordination-induced spin-state switching, or LD-CISSS). Whereas bistability and hysteresis in conventional spin-crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD-CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel-porphyrin as the transition-metal complex and 3-phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni-porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72% paramagnetic Ni-porphyrin after irradiation at 365 nm, 32% paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD-CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light-controlled magnetic levitation are conceivable applications.

Design and synthesis of fluorinated iron chelators for metabolic study and brain uptake

A range of fluorinated 3-hydroxypyridin-4-ones has been synthesized where fluorine or fluorinated substituent was attached at 2- or 5- position of the pyridine ring in order to improve chemical and biological properties of 3-hydroxypyridin-4-ones. The synthetic route is different from conventional counterparts where a functional group is introduced to a preformed 3-hydroxypyridin-4-one ring. Herein, we introduce a novel method which starts with a fluorine containing precursor and the two hydroxyl groups at 3- and 4- positions of the pyridine ring are introduced at a later stage. The pK(a) values of the free ligands and the affinity constants of their iron complexes demonstrate that the presence of fluorine dramatically alters the values. The distribution coefficient values of the free ligands and corresponding iron(III) complexes between 1-octanol and MOPS buffer (pH 7.4) are also influenced. Glucuronidation and oxidation studies of selected fluorinated 3-hydroxypyridin-4-ones demonstrate that some such fluorinated compounds have clear advantage over deferiprone in that they are metabolized more slowly. Blood-brain barrier permeability studies indicated that although lipophilicity influences the permeability it is not the only factor. Two of the selected seven fluorinated 3-hydroxypyridin-4-ones have improved brain distribution when compared with deferiprone.

A general modular method of azaindole and thienopyrrole synthesis via Pd-catalyzed tandem couplings of gem-dichloroolefins

A palladium-catalyzed reaction of gem-dichloroolefins and a boronic acid via a tandem intramolecular C-N and intermolecular Suzuki coupling process gave corresponding substituted azaindoles or thienopyrroles. This method is a very modular protocol to synthesize all four isomers of azaindole and two isomers of thienopyrroles in good to excellent yield.

Pyridine elaboration through organometallic intermediates: regiochemical control and completeness

Pyridines carrying heterosubstituents (such as carboxy, amido, amino, alkoxy or trifluoromethyl groups or solely individual halogen atoms) can be readily and site selectively metalated. Subsequent reaction with a suitable electrophile opens rational access to a wealth of new building blocks for the synthesis of biologically active compounds. This approach relies on organometallic methods, which are both efficacious and extremely flexible as far as the substitution site and the product structure are concerned (86 references).

Quinols As Novel Therapeutic Agents. 7. Synthesis of Antitumor 4-[1-(Arylsulfonyl-1H-indol-2-yl)]-4-hydroxycyclohexa-2,5-dien-1-ones by Sonogashira Reactions

Interaction of 2-iodoaniline or 5-fluoro-2-iodoaniline with a range of arylsulfonyl chlorides affords sulfonamides that undergo Sonogashira couplings under thermal or microwave conditions with the alkyne 4-ethynyl-4-hydroxycyclohexa-2,5-dien-1-one followed by cyclization to 4-[1-(arylsulfonyl-1H-indol-2-yl)]-4-hydroxycyclo-hexa-2,5-dien-1-ones. This method allows for incorporation of a range of substituents into the arylsulfonyl moiety, and compounds showed selective in vitro inhibition of cancer cell lines of colon and renal origin, a feature of compounds bearing the quinol pharmacophore.

A novel one-step synthesis of 2-substituted 6-azaindoles from 3-amino-4-picoline and carboxylic esters

Dilithiation of 3-amino-4-picoline (1) was achieved with sec-BuLi at room temperature. Condensation of the resulting dianion (2) with carboxylic esters afforded a wide range of 2-substituted 6-azaindoles in good yields.

Synthesis of 2H-1,2-Benzothiazine 1,1-Dioxides via Heteroannulation Reactions of 2-Iodobenzenesulfonamide with Ketone Enolates under SRN1 Conditions

[Reaction: see text]. 2-iodobenzenesulfonamide (1a) underwent photostimulated S(RN)1 reactions in liquid NH3 with the potassium enolates derived from acetone, pinacolone, butanone, and 3-methyl-2-butanone to give fair to good yields of 2H-1,2-benzothiazine 1,1-dioxides 2. Reductive dehalogenation of 1a was found to predominate in photoinduced reactions of 1a with 3-pentanone, 2-methyl-3-pentanone, and 2,4-dimethyl-3-pentanone, the extent of reduction being proportional to the number of beta-hydrogen atoms present in the ketone enolate. Isotopic labeling studies with 2,4-dimethyl-3-pentanone-d14 (24) confirmed the major role of the beta-hydrogens in the reduction process. Reactions of 1a with cyclopentanone, cyclohexanone, and cyclooctanone enolates afforded new tricyclic benzothiazine derivatives 26-29. Attempts to extend the heteroannulation reaction to the preparation of 2H-1,2-benzothiazin-3(4H)-one 1,1-dioxides 3 (R = H, Ph) through reactions of 1a with tert-butyl acetate and ethyl phenylacetate enolates resulted only in hydrodehalogenation of 1a. However, oxazoline anion 30, a synthetic equivalent of ethyl phenylacetate, was successfully employed in an alternative S(RN)1-based synthesis of benzothiazine 3 (R = Ph).

Novel and efficient access to phenylamino-pyrimidine type protein kinase C inhibitors utilizing a Negishi cross-coupling strategy

A novel, short, and efficient synthetic pathway to 3-{4-[2-(3-chlorophenylamino)-pyrimidin-4-yl]-pyridin-2-ylamino}-propanol (CGP 60474) and a series of analogues was developed. The synthetic sequence consisted of a Negishi-type cross-coupling reaction in the key step followed by two subsequent nucleophilic substitution reactions. This strategy represents a versatile and robust protocol to access diverse analogues of the title compound for subsequent SAR studies as potential phenylamino-pyrimidine type protein kinase C inhibitors.

Selective Functionalization of 2-Fluoropyridine, 2,3-Difluoropyridine, and 2,5-Difluoropyridine at Each Vacant Position

The concept of "regioexhaustive substitution" has been successfully applied to 2-fluoro-, 2,3-difluoro-, and 2,5-difluoropyridine. All vacant positions were amenable to regioselective metalation and subsequent carboxylation by employing either chlorine as a neighboring site activating protective group or trimethylsilyl as a neighboring site screening protective group. In this way, approximately half a dozen fluorinated pyridinecarboxylic acids were derived from each starting material.

5-substituted derivatives of 6-halogeno-3-((2-(S)-azetidinyl)methoxy)pyridine and 6-halogeno-3-((2-(S)-pyrrolidinyl)methoxy)pyridine with low picomolar affinity for alpha4beta2 nicotinic acetylcholine receptor and wide range of lipophilicity: potential probes for imaging with positron emission tomography

Potential positron emission tomography (PET) ligands with low picomolar affinity at the nicotinic acetylcholine receptor (nAChR) and with lipophilicity (log D) ranging from -1.6 to +1.5 have been synthesized. Most members of the series, which are derivatives of 5-substituted-6-halogeno-A-85380, exhibited a higher binding affinity at alpha4beta2-nAChRs than epibatidine. An analysis, by molecular modeling, revealed an important role of the orientation of the additional heterocyclic ring on the binding affinity of the ligands with nAChRs. The existing nicotinic pharmacophore models do not accommodate this finding. Two compounds of the series, 6-[(18)F]fluoro-5-(pyridin-3-yl)-A-85380 ([(18)F]31) and 6-chloro-3-((2-(S)-azetidinyl)methoxy)-5-(2-[(18)F]fluoropyridin-5-yl)pyridine) ([(18)F]35), were radiolabeled with (18)F. Comparison of PET data for [(18)F]31 and 2-[(18)F]FA shows the influence of lipophilicity on the binding potential. Our recent PET studies with [(18)F]35 demonstrated that its binding potential values in Rhesus monkey brain were ca. 2.5 times those of 2-[(18)F]FA. Therefore, [(18)F]35 and several other members of the series, when radiolabeled, will be suitable for quantitative imaging of extrathalamic nAChRs.

Synthesis of novel heteroaromatics structurally related to ellipticine alkaloids via thermolysis of pyridannulated enyne-carbodiimides

New synthetic pathways to the 5H-pyrido[3',4':4,5]pyrrolo[2,3-b]quinolines 6, the 6H-pyrido[3',4':4,5]pyrrolo[2,3-b][1,6]naphthyridines 7, and the 11H-pyrido[4',3':4,5]pyrrolo[2,3-b]quinolines 8 via thermolysis of the pyridannulated enyne-carbodiimides 14, 19, and 23 were established. These novel heteroaromatic systems are structurally related to ellipticine alkaloids and could serve as DNA-intercalating agents.

Sequential photostimulated reactions of trimethylstannyl anions with aromatic compounds followed by palladium-catalyzed cross-coupling processes

The photostimulated reactions of several mono-, di-, and trichloroarenes and aryltrimethylammonium salts with Me(3)Sn(-) ions in liquid ammonia gave good yields of stannanes by the S(RN)1 mechanism. If the chloroarenes are not soluble in liquid ammonia, diglyme is another solvent to perform these reactions. The stannanes thus obtained can be arylated by further reaction with haloarenes through palladium-catalyzed reactions. If the palladium-catalyzed reaction is performed with a chloroiodoarene as substrate, the stannane reacts faster by the C-I bond via chemoselective cross-coupling reaction to give a chloroarene as product, which can be further arylated by a consecutive S(RN)1-Stille reaction or react with other substrates by another palladium-catalyzed reaction. These sequential reactions can also be performed with substrates with two leaving groups to give products in high yields.

Photocycloaddition of four-carbon-tethered pyridones. Intramolecular hydrogen bonding and facilitated amide hydrolysis by a proximal secondary alcohol

Four-carbon-tethered pyridones undergo photocycloaddition to give exclusively trans-[4 + 4] products. The presence of a tether alcohol engenders a solvent-dependent diastereoselectivity for the cycloaddition by intramolecular hydrogen bonding to the adjacent pyridone. Following cycloaddition, the alcohol can deliver a carbonyl group to the proximal, hindered amide nitrogen, leading to a very facile amide hydrolysis.

Synthesis and sar of 2- and 3-substituted 7-azaindoles as potential dopamine D4 ligands

7-azaindole compounds bearing a cyclic amine moiety linked by a one or two carbon chain attached at the 2- or 3-position were synthesised and evaluated as potential dopamine D4 ligands. Highest affinity and selectivity for the D4 receptor resided in the 3-aminomethyl-7-azaindole series.

A dipyrido [2,3-b:3',2'-f]azepine analog of the HIV-1 reverse transcriptase inhibitor nevirapine

The syntheses of 11-ethyl and 11-cyclopropyldipyrido[2,3-b:3',2'-f]azepines, analogs of the HIV-1 reverse transcriptase inhibitor nevirapine 1, are described. These compounds exhibit potency equivalent to nevirapine in the inhibition of wild-type and some mutant RT enzymes.