Quinolizinium as a new fluorescent lysosomotropic probe.

We have synthesized a collection of quinolizinium fluorescent dyes for the purpose of cell imaging. Preliminary biological studies in human U2OS osteosarcoma cancer cells have shown that different functional groups appended to the cationic quinolizinium scaffold efficiently modulate photophysical properties but also cellular distribution. While quinolizinium probes are known nuclear staining reagents, we have identified a particular quinolizinium derivative salt that targets the lysosomal compartment. This finding raises the question of predictability of specific organelle targeting from structural features of small molecules.

Recent Advances in the Synthesis of Azonia Aromatic Heterocycles.

Azonia aromatic heterocycles are an important subclass of aza-heterocyles as they are the structural motif of relevant cationic alkaloids, and they have a wide range of potential applications such as bioactive compounds and organic materials. In this Synopsis, recent and novel approaches to their synthesis are surveyed, with particular emphasis on ring-closing metathesis reactions and annulation reactions based on C-H activation.

Targeting DNA with small molecules: a comparative study of a library of azonia aromatic chromophores.

A library of azonia aromatic cations has been studied in order to gain insights into the effect of the size, shape and charge distribution on the fluorescence, DNA interactions and DNA sequence selectivity properties. Fluorescence-based thermal denaturation experiments, spectrofluorimetric titrations, circular dichroism measurements and theoretical simulations have shown that some of the studied chromophores have interesting fluorescence properties and two of them also show a consistent DNA-binding ability by intercalation, with a potential preference for AT-rich sequences.

Nonlinear emission of quinolizinium-based dyes with application in fluorescence lifetime imaging.

Charged molecules based on the quinolizinum cation have potential applications as labels in fluorescence imaging in biological media under nonlinear excitation. A systematic study of the linear and nonlinear photophysics of derivatives of the quinolizinum cation substituted by either dimethylaniline or methoxyphenyl electron donors is performed. The effects of donor strength, conjugation length, and symmetry in the two-photon emission efficiency are analyzed in detail. The best performing nonlinear fluorophore, with two-photon absorption cross sections of 1140 GM and an emission quantum yield of 0.22, is characterized by a symmetric D-π-A(+)-π-D architecture based on the methoxyphenyl substituent. Application of this molecule as a fluorescent marker in optical microscopy of living cells revealed that, under favorable conditions, the fluorophore can be localized in the cytoplasmatic compartment of the cell, staining vesicular shape organelles. At higher dye concentrations and longer staining times, the fluorophore can also penetrate into the nucleus. The nonlinearly excited fluorescence lifetime imaging shows that the fluorophore lifetime is sensitive to its location in the different cell compartments. Using fluorescence lifetime microscopy, a multicolor map of the cell is drafted with a single dye.

Synthesis of charged bis-heteroaryl donor-acceptor (D-A+) NLO-phores coupling (π-deficient-π-excessive) heteroaromatic rings.

Charged chromophores based on heteroaromatic cations were prepared by reaction of alkylazinium salts with N-heteroarylstannanes under Stille conditions. This approach provides easy access to potential single donor D-A(+) chromophores in which the acceptor moiety A(+) is the pyridinium cation and the donors are different π-excessive N-heterocycles. The ß hyperpolarizabilities were measured in hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis that combines a variety of computational procedures, including density functional theory and correlated Hartree-Fock-based methods. In some chromophores, the absence of a bridge between donor and acceptor fragments increases the NLO properties.

Donor-(π-bridge)-azinium as D-π-A+ one-dimensional and D-π-A(+)-π-D multidimensional V-shaped chromophores.

Heteroaromatic cations reacted with N-heteroarylacetylenes under Sonogashira conditions to allow easy access to potential single donor D-π-A(+) and V-shaped D-π-A(+)-π-D chromophores, where the acceptor moiety A is the π-deficient pyridinium cation and the donor moiety is represented by different π-excessive N-heterocycles. The ß hyperpolarizabilities were measured using hyper-Rayleigh scattering experiments and the experimental data are supported by a theoretical analysis that combines a variety of computational procedures, including Density Functional Theory (DFT) and correlated Hartree-Fock-based methods (RCIS(D)).

Two-photon activated precision molecular photosensitizer targeting mitochondria.

Mitochondria metabolism is an emergent target for the development of novel anticancer agents. It is amply recognized that strategies that allow for modulation of mitochondrial function in specific cell populations need to be developed for the therapeutic potential of mitochondria-targeting agents to become a reality in the clinic. In this work, we report dipolar and quadrupolar quinolizinium and benzimidazolium cations that show mitochondria targeting ability and localized light-induced mitochondria damage in live animal cells. Some of the dyes induce a very efficient disruption of mitochondrial potential and subsequent cell death under two-photon excitation in the Near-infrared (NIR) opening up possible applications of azonia/azolium aromatic heterocycles as precision photosensitizers. The dipolar compounds could be excited in the NIR due to a high two-photon brightness while exhibiting emission in the red part of the visible spectra (600-700 nm). Interaction with the mitochondria leads to an unexpected blue-shift of the emission of the far-red emitting compounds, which we assign to emission from the locally excited state. Interaction and possibly aggregation at the mitochondria prevents access to the intramolecular charge transfer state responsible for far-red emission.

Ring-closing metathesis reactions on azinium salts: straightforward access to quinolizinium cations and their dihydro derivatives.

The ring-closing metathesis reaction of 1-butenyl-2-vinylpyridinium salts and 2-butenyl-1-vinylpyridinium salts using Grubbs second generation and Hoveyda-Grubbs catalysts proved to be an efficient approach to 3,4-dihydro- and 1,2-dihydroquinolizinium salts and the corresponding quinolizinium derivatives by an improved thermal oxidation in the presence of Pd/C without solvent. A comparative study showed that the quinolizinium system was obtained in better yields through the 3,4-dihydroquinolizinium route, thus allowing the synthesis of quinolizinium derivatives or improvements in the yields of some examples reported previously.

A new approach to polycyclic azonia cations by ring-closing metathesis.

The ring-closing metathesis (RCM) reaction of N-vinyl-alpha-(2-styryl)azinium salts, using the Hoveyda-Grubbs catalyst, leads to different tricyclic and tetracyclic azonia cations with moderate to good yields. This is the first time that a highly electron-deficient alkene such as an N-vinylpyridinium has been involved in an RCM process.

First synthesis of biquinoliziniun salts: novel example of a chiral azonia dication.

[Structure: see text] The 1,1'-, 2,2'-, and 3,3'-biquinolizinium dications are described for the first time and were prepared using palladium-catalyzed homocoupling reactions of the corresponding isomeric bromoquinolizinium bromides. Theoretical calculations show 1,1'- and 4,4'-biquinolizinium dications to be chiral molecules, the latter of which has a high energy of formation, a factor that probably precludes its formation.

Enyne ring-closing metathesis on heteroaromatic cations.

Cationic heteroaromatic enynes have been employed as substrates in enyne ring-closing metathesis, under an atmosphere of ethylene and using the Hoveyda-Grubbs catalyst, for the first time; the reaction affords new 1-vinyl- and 2-vinyl-substituted 3,4-dihydroquinolizinium salts, useful precursors for biologically relevant cations based on the quinolinizium system.

A unified approach to quinolizinium cations and related systems by ring-closing metathesis.

[reaction: see text] The first example of an olefin ring-closing metathesis reaction on cationic heteroaromatic systems is described. Dihydroquinolizinium cations and a variety of related cationic systems are synthesized in an efficient approach from N-alkenyl alpha-vinyl azinium salts using Grubbs' catalysts.

Sonogashira reaction on quinolizium cations.

The four isomeric bromoquinolizium bromides reacted with aryl- and heteroarylacetylenes under Sonogashira conditions. The reactions proceed with moderate-to-high yields to afford aryl- and heteroarylethynyl quinolizium cations. This is the first reported example of the Sonogashira reaction on heteroaromatic cations, and it allowed easy access to potential pi-donor pi-acceptor systems bearing cationic units.

Efficient synthesis of an indoloquinolizinium alkaloid selective DNA-binder by ring-closing metathesis.

Two total syntheses of the indolo[2,3-a]quinolizinium cation have been accomplished through the application of two ring-closing metathesis reactions to form the pyridinium ring. One of these approaches provides the tetracyclic cation in only five steps from commercially available harmane. Fluorescence-based thermal denaturation experiments, as well as spectrofluorimetric titration, circular dichroism measurements, and theoretical simulations, showed a consistent DNA-binding capacity by intercalation with a marked preference for AT-rich sequences.

A V-shaped cationic dye for nonlinear optical bioimaging.

A symmetric cationic molecule with D-π-A(+)-π-D architecture was synthesized with high two-photon absorption cross-section (σ(2) ≈ 1140 GM). Application as a marker in fluorescence microscopy of living cells revealed its presence inside the cell staining vesicular shape organelles in the cytoplasm. Fluorescence lifetime imaging microscopy shows that it is also able to penetrate within the nucleus.

Palladium-mediated functionalization of heteroaromatic cations: comparative study on quinolizinium cations.

An efficient palladium-catalyzed cross-coupling reaction on heteroaromatic cations is described. A comparative study of the Stille and Suzuki reactions shows that only the Stille reaction is able to produce an efficient C-C bond formation between any of the four isomeric bromoquinolizinium bromides and a variety of stannanes. In the presence of the catalysts Pd(PPh3)4 or Pd2(dba)3P(o-Tol)3, vinyl, ethynyl, aryl, and heteroaryl groups are successfully incorporated into the quinolizinium system in satisfactory yields under mild reaction conditions. This procedure represents a marked improvement on the functionalization of this class of heteroaromatic cation.