Highly Flexible π-Expanded Cyclooctatetraenes: Cyclic Thiazole Tetramers with Head-to-Tail Connection

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Brønsted acid-promoted ring-opening and annulation of thioamides and 2H-azirines to synthesize 2,4,5-trisubstituted thiazoles

In this study, a metal-free synthesis of 2,4,5-trisubstituted thiazoles using 2H-azirines and thioamides is disclosed. Under the catalysis of HClO₄, the protocol was realized through a novel chemical bond breaking of 2H-azirine, which is usually achieved using a metal catalyst. It provides an efficient and green route for the synthesis of substituted thiazoles with a broad substrate scope. Preliminary mechanistic studies show that such a reaction may involve a ring-opening reaction, annulation, and a hydrogen atom rearrangement process.

Pnictogen-Bridged Diphenyl Sulfones as Photoinduced Pnictogen Bond Forming Emission Motifs

In this study, pnictogen (Pn)-bridged diphenyl sulfones were synthesized as motifs for photoinduced dynamic rearrangement. The newly synthesized sulfones exhibited dual fluorescence at 298 K. Density functional theory calculations revealed that the longer-wavelength fluorescence was derived from the geometries after structural relaxation through photo-driven pnictogen bond formation between the O atom lone pair of the sulfonyl moiety and the antibonding orbital of the Pn-C bond. This is the first report on emission dynamics driven by pnictogen bond formation upon photoexcitation.

Tridecacyclene Tetraimide: An Easily Reduced Cyclooctatetraene Derivative

Tridecacyclene tetraimide, TCTI, an electron-deficient non-benzenoid nanocarbon with a C56 N4 polycyclic framework was obtained in a concise synthesis. TCTI has a non-planar structure and forms π-stacked dimers in the solid state. In solution, it undergoes eight single-electron reductions, yielding a range of negatively charged states up to an octaanion. Except for the latter species, which has a remarkably large electronic gap, the anions feature extended near-infrared absorptions, with a particularly strong band at 1692 nm observed for the dianion. A computational analysis of the TCTI anions shows that their stability originates from the combined effects of electron-deficient imide groups and the local aromaticity of reduced acenaphthylene units. The properties of TCTI make it potentially useful in electrochromic and charge storage applications.

Synthesis of octagon-containing molecular nanocarbons

Nanocarbons, such as fullerenes, carbon nanotubes, and graphenes, have long inspired the scientific community. In order to synthesize nanocarbon molecules in an atomically precise fashion, many synthetic reactions have been developed. The ultimate challenge for synthetic chemists in nanocarbon science is the creation of periodic three-dimensional (3D) carbon crystals. In 1991, Mackay and Terrones proposed periodic 3D carbon crystals with negative Gaussian curvatures that consist of six- and eight-membered rings (the so-called Mackay-Terrones crystals). The existence of the eight-membered rings causes a warped nanocarbon structure. The Mackay-Terrones crystals are considered a "dream material", and have been predicted to exhibit extraordinary mechanical, magnetic, and optoelectronic properties (harder than diamond, for example). To turn the dream of having this wonder material into reality, the development of methods enabling the creation of octagon-embedding polycyclic structures (or nanographenes) is of fundamental and practical importance. This review describes the most vibrant synthetic achievements that the scientific community has performed to obtain curved polycyclic nanocarbons with eight-membered rings, building blocks that could potentially give access as templates to larger nanographenes, and eventually to Mackay-Terrones crystals, by structural expansion strategies.

Graphene-induced planarization of cyclooctatetraene derivatives

Stable equilibrium compounds containing a planar antiaromatic cyclooctatetraene (COT) ring are promising candidates for organic electronic devices such as organic semiconductor transistors. The planarization of COT by incorporation into rigid planar π-systems, as well as by oxidation or reduction has attracted considerable attention in recent years. Using dispersion-corrected density functional theory calculations, we explore an alternative approach of planarizing COT derivatives by adsorption onto graphene. We show that strong π-π stacking interactions between graphene and COT derivatives induce a planar structure with an antiaromatic central COT ring. In addition to being reversible, this strategy provides a novel approach for planarizing COT without the need for incorporation into a rigid structure, atomic substitution, oxidation, or reduction.

Synthesis, Structure, and Dynamics of Chiral Eight-Membered Cyclic Molecules with Thienylene and Cyclopropylene Units Alternately Connected

A rhodium(II)-catalyzed asymmetric cyclooligomerization of bifunctional monomers possessing triazolyl and vinyl groups at 2,3- and 3,4-positions on the thiophene ring is studied. Structurally interesting cyclic dimers in which thienylene and cyclopropylene units are alternately connected are obtained as the major components. The eight-membered rings in the center are non-planar and adopt a tub-shaped conformation. We also observe the phenomenon of racemization caused by a tub-to-tub ring-flipping, the activation energy of which is determined as 108 kJ mol -1 by electronic circular dichroism spectra measurement.

Double C-S bond formation via multiple Csp3-H bond cleavage: synthesis of 4-hydroxythiazoles from amides and elemental sulfur under metal-free conditions

A novel and efficient approach for the synthesis of 4-hydroxythiazoles from amides and elemental sulfur has been developed. In the presence of P₂O₅, DMSO and HMPA, this metal-free protocol proceeds smoothly and tolerates a spectrum of functional groups. Furthermore, this strategy involves the process of double Csp³-S bond formation through the cleavage of multiple Csp³-H bonds for the first time.

NH4I-promoted oxidative formation of benzothiazoles and thiazoles from arylacetic acids and phenylalanines with elemental sulfur

A NH4I/K3PO4-based catalytic system has been established to enable oxidative formation of thiazole compounds from arylacetic acids and phenylalanines with elemental sulfur. While the three-component reaction of anilines or β-naphthylamines with arylacetic acids and elemental sulfur affords benzo[2,1-d]thiazoles and naphtho[2,1-d]thiazoles, the annulation of phenylalanines with elemental sulfur produces 2-benzyl and 2-benzoylthiazoles. This work well complements previous three-component annulations of benzothiazoles from other coupling partners.

π-π Catalysis in Carbon Flatland-Flipping [8]Annulene on Graphene

Noncovalent interactions are an integral part of the modern catalysis toolbox. Although stronger noncovalent interactions such as hydrogen bonding are commonly the main driving force of catalysis, π-π interactions typically provide smaller additional stabilizations, for example, to afford selectivity enhancements. Here, it is shown computationally that pristine graphene flakes may efficiently catalyze the skeletal inversions of various benzannulated cyclooctatetraene derivatives, providing an example of a catalytic process driven solely by π-π stacking interactions. Hereby, the catalytic effect results from disproportionate shape complementarity between catalyst and transition structure compared with catalyst and reactant. An energy decomposition analysis reveals electrostatic and, especially with increasing system size, to a larger extent, dispersion interactions as the origin of stabilization.

Uses of Cyclohexan-1,3-dione for the Synthesis of Thiazole, Pyrazole, Thiophene, Isoxazole and Pyran Derivatives with Antitumor Activities

Background:

A wide range of thiazole, pyrazole and pyran derivatives gained special attention due to pharmacological activities especially therapeutic activities. Many pharmacological drugs containing the thiazole and pyrazole nuclei are known in the market.

Methods:

The 2-arylidencyclohexan-1,3-dione 3a-c were the key starting compounds for many heterocyclic reactions to produce substituted heterocyclic derivatives.

Results:

Antiproliferative activities of the produced compounds against six cancer cell lines A549, HT-29, MKN-45, U87MG, SMMC-7721 and H460 were measured in which the compounds showed high inhibition. The most promising compounds were tested against tyrosine kinases (c-Kit, Flt-3, VEGFR-2, EGFR, and PDGFR). Structure-Activity Relationship (SAR) was rationalized by assessing the varying structural features of the molecules. In addition, the most active compounds were selected for Pim-1 inhibition.

Conclusion:

Thirty compounds were synthesized. Ten of them (3a, 3c, 5a, 5c, 7a, 10f, 11a, 13c, 16a and 16c) were the most active compounds for selected cancer cell lines. Compounds 3c, 5c, 7a, 10f, 13c and 16c showed high inhibition toward the tyrosine kinases while compounds 3c, 5c and 10f were the most potent to inhibit Pim-1.

Synthesis and Biological Evaluation of Novel 4,5,6,7-Tetrahydrobenzo[D]-Thiazol-2- Yl Derivatives Derived from Dimedone with Anti-Tumor, C-Met, Tyrosine Kinase and Pim-1 Inhibitions

BACKGROUND

Dimedone and thiazole moieties are privileged scaffolds (acting as primary pharmacophores) in many compounds that are useful to treat several diseases, mainly tropical infectious diseases. Thiazole derivatives are a very important class of compounds due to their wide range of pharmaceutical and therapeutic activities. On the other hand, dimedone is used to synthesize many therapeutically active compounds. Therefore, the combination of both moieties through a single molecule to produce heterocyclic compounds will produce excellent anticancer agents.

OBJECTIVE

The present work reports the synthesis of 47 new substances belonging to two classes of compounds: Dimedone and thiazoles, with the purpose of developing new drugs that present high specificity for tumor cells and low toxicity to the organism. To achieve this goal, our strategy was to synthesize a series of 4,5,6,7-tetrahydrobenzo[d]-thiazol-2-yl derivatives using the reaction of the 2-bromodimedone with cyanothioacetamide.

METHODS

The reaction of 2-bromodimedone with cyanothioacetamide gave the 4,5,6,7-tetrahydrobenzo[d]- thiazol-2-yl derivative 4. The reactivity of compound 4 towards some chemical reagents was observed to produce different heterocyclic derivatives.

RESULTS

A cytotoxic screening was performed to evaluate the performance of the new derivatives in six tumor cell lines. Thirteen compounds were shown to be promising toward the tumor cell lines which were further evaluated toward five tyrosine kinases.

CONCLUSION

The results of antitumor screening showed that many of the tested compounds were of high inhibition towards the tested cell lines. Compounds 6c, 8c, 11b, 11d, 13b, 14b, 15c, 15g, 21b, 21c, 20d and 21d were the most potent compounds toward c-Met kinase and PC-3 cell line. The most promising compounds 6c, 8c, 11b, 11d, 13b, 14b, 15c, 15g, 20c, 20d, 21b, 21c and 21d were further investigated against tyrosine kinase (c-Kit, Flt-3, VEGFR-2, EGFR, and PDGFR). Compounds 6c, 11b, 11d, 14b, 15c, and 20d were selected to examine their Pim-1 kinase inhibition activity the results revealed that compounds 11b, 11d and 15c had high activities.

Non-aromatic annulene-based aggregation-induced emission system via aromaticity reversal process

Aggregation-induced emission (AIE) is a photophysical phenomenon correlated closely with the excited-state intramolecular motions. Although AIE has attracted increasing attention due to the significant applications in biomedical and optoelectronics, an in-depth understanding of the excited-state intramolecular motion has yet to be fully developed. Here we found the non-aromatic annulene derivative of cyclooctatetrathiophene shows typical AIE phenomenon in spite of its rotor-free structure. The underlying mechanism is investigated through photoluminescence spectra, time-resolved absorption spectra, theoretical calculations, circular dichroism as well as by pressure-dependent fluorescent spectra etc., which indicate that the aromaticity reversal from ground state to the excited state serves as a driving force for inducing the excited-state intramolecular vibration, leading to the AIE phenomenon. Therefore, aromaticity reversal is demonstrated as a reliable strategy to develop vibrational AIE systems. This work also provides a new viewpoint to understand the excited-state intramolecular motion behavior of lumiongens.

Three-component bis-heterocycliation for synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles

A cooperative base system has been developed for the novel three-component synthesis of 2-aminobenzo[4,5]thieno[3,2-d]thiazoles via bis-heterocyclization of methylketoxime acetates.

Tuning the Conformation and Color of Conjugated Polyheterocyclic Skeletons by Installing ortho-Methyl Groups

ortho-Methyl effects are exploited to tune steric hindrance between side-chain N,N'-diaryls and polycyclic dihydrodibenzo[a,c]phenazine, and in turn control the conformations of N,N'-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) and its ortho-methyl derivatives Mx-My (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho-positiond of N,N'-diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V-shaped dihydrodibenzo[a,c]phenazine skeleton is gradually tuned from a bent (DPAC) to planar (M2-M2) structure in the ground state. As a result, the relaxation of the excited-state structure of DPAC and its numerous analogues could be mimicked by model structures Mx-My, demonstrating for the first time the the conformation change from bent-to-planar and hence a large range of energy-gap tuning of polycyclic conjugated structures controlled by the steric hindrance.

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Synthesis and ring opening reaction of octaoctyl substituted [2.2.2.2](2,7)-fluorenophanetetraene by photooxidation

Octaoctyl substituted fluorenophanetetraene has been synthesized and the photooxidation of the fluorenophanetetraene gives all trans linear fluorenevinylene with aldehyde end groups.

Cu(i)-Catalyzed oxidative homo-coupling of thiazoline-4-carboxylates: synthesis of 4,4′-bithiazoline derivatives

Cu(i)-Catalyzed oxidative homo-coupling of thiazoline-4-carboxylates with good functional group tolerance has been developed.

Bent π-conjugated systems composed of three-dimensional benzoannulenes

This article describes bent π-conjugated systems composed of alternating o-phenylene and Z-vinylene units. all-Z-[n]Benzo[4n]annulenes are higher homologues of dibenzocyclooctatetraene (DBCOT) with a concave π system, and attempts were made to convert [20]- and [24]annulenes having partial belt structures of fullerenes and carbon nanotubes into [10]- and [12]phenacenes. A bent π-conjugated system composed of two DBCOT units showed dynamic syn-anti equilibrium in solution and behaved as dynamic molecular tweezers (DMTs). The syn isomers of the DMTs formed blue charge-transfer complexes with DDQ in solution, and this complexation and color change were applied to thermochromism. Furthermore, DMTs having two CN groups on each COT ring exhibited crystalline-state emission in the presence of solvent molecules, although almost no emission was observed in solution or the solid state. Based on this crystalline-state emission, a vapochromic system was established. The multifunctional properties of DMTs caused by the flexibility of the nonplanar π system are summarized.