Synthesis and evaluation of photophysical properties of Series of π-conjugated oxazole dyes

Triple-Consecutive Isocyanide Insertions with Aldehydes: Synthesis of 4-Cyanooxazoles

An efficient TMSOTf-promoted selective triple consecutive insertions of tert-butyl isocyanide into aldehydes has been developed, affording pharmacological interesting 4-cyanooxazoles in high yields in a one pot manner. The given method encompasses a wide range of substrates with tert-butyl isocyanide serving as sources of critical "CN" and "C-N═C" moieties. The versatile transformations of the resulting 4-cyanooxazoles were demonstrated. The key reaction intermediates for plausible mechanisms were determined.

Synthesis, photophysical and nonlinear optical properties of push-pull tetrazoles

A 2,5-disubstituted tetrazole with p-nitrophenyl and 3-pyridyl units as acceptors (1a), and three push–pull tetrazoles with p-nitrophenyl as an acceptor and phenyl (1b), 2-(dibenzo[b,d]furan-4-yl) (1c), and 4-(N,N-diphenylamino)phenyl (1d) as donor groups, were synthesized by copper-catalyzed aerobic C–N coupling of p-nitrophenyl tetrazole with appropriately substituted aryl boronic acids. The absorption and emission spectra of 1a–c showed minimal dependence on the polarity of the solvent; however, in the case of 1d a blue shift was noted in the longest absorption band (λ₁) as the polarity increased. The fluorescence intensity of the title compounds was found to be solvent-dependent; however, no apparent correlation to solvent polarity could be established. The absorption and emission characteristics of 1a–d were also influenced by the nature of the substituent as 1d, bearing a strong electron donating 4-(N,N-diphenylamino)phenyl group, displayed a significant red shifted absorption (λ₁) as well as emission (λ_em) bands compared to other compounds. Time dependent density functional calculations (CAM-B3LYP/6-311++G**) revealed that the longest wavelength band (λ₁) is associated with an intramolecular charge transfer (ICT) from HOMO/HOMO-1/HOMO-2 → LUMO/LUMO+1 in these molecules. The first hyperpolarizability values, β_HRS, of 1a–d were measured using the solution-based hyper-Rayleigh scattering technique using a femtosecond Ti:Sapphire laser and the highest NLO activity was measured for 1d with the greatest push–pull characteristics. A strong correlation was observed between the calculated hyperpolarizability (β_tot) and experimentally measured values (β_HRS).

A 2,5-disubstituted tetrazole with a p-nitrophenyl unit as an acceptor and a 4-(N,N-diphenylamino) phenyl group as a donor exhibits strong push–pull characteristics and displays high NLO activity.

Exploring the effect of nitrile substituent position on fluorescence quantum yield of ESIPT-based oxazoline derivatives: A TDDFT investigation

We explore the mechanism specifically on quantum yields difference of 2-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (1-CN) and 4-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (3-CN) by density functional theory and time-dependent density functional theory within the Tamm-Dancoff approximation. The structures optimization and the potential energy curves scanning of singlet excited state directly prove that the excited state intramolecular proton transfer (ESIPT) can take place in 1-CN and 3-CN molecules. The calculated spectra show that the fluorescence peaks of two molecules come from the emission of keto* configuration. The non-covalent interaction and the atomic dipole moment corrected Hirshfeld charge are also analyzed. Through the comparison of emission oscillator strength between 1-CN and 3-CN molecules suggests that the radiative transition process is not the main reason for the difference on quantum yields. Internal conversion process is also excluded on account of the large energy gap between S₀ and S₁. Considering the interaction between singlet and triplet states, both molecules can undergo intersystem crossing. The prominent difference is that, compared with 3-CN, the larger spin-orbit coupling constant and smaller energy level difference promote the intersystem crossing process of 1-CN. This provides direct evidence for the fluorescence quantum yield of 1-CN is lower than that of 3-CN. We envision that the present work can provide help for the synthesis and application of ESIPT compounds with high quantum yields.

Palladium-Catalyzed Dehydrogenative C-2 Alkenylation of 5-Arylimidazoles and Related Azoles with Styrenes

The construction of carbon–carbon bonds by direct involvement of two unactivated carbon–hydrogen bonds, without any directing group, ensures a high atom economy of the entire process. Here, we describe a simple protocol for the Pd(II)/Cu(II)-promoted intermolecular cross-dehydrogenative coupling (CDC) of 5-arylimidazoles, benzimidazoles, benzoxazole and 4,5-diphenylimidazole at their C-2 position with functionalized styrenes. This specific CDC, known as the Fujiwara–Moritani reaction or oxidative Heck coupling, also allowed the C-4 alkenylation of the imidazole nucleus when both 2 and 5 positions were occupied.

Facile synthesis of 2-alkynyl oxazoles via a Ce(OTf)3-catalyzed cascade reaction of alkynyl carboxylic acids with tert-butyl isocyanide

We developed an efficient and novel protocol to synthesize 2-alkynyl oxazoles from tert-butyl isocyanide and alkynyl carboxylic acids. This method allowed the synthesis of diversely functionalized oxazoles under mild reaction conditions, coupled with operational simplicity and these functionalized oxazoles showed a certain degree of biological activity. Moreover, compounds 2b, 2h, 2k, 2n, 2p and 2t exhibited good anticancer activities in human gastric cancer cells (MGC803) and human bladder tumor cells (T24), with IC₅₀ below 20.0 μM.

Palladium-catalyzed direct C2-arylation of azoles with aromatic triazenes

A highly efficient palladium-catalyzed arylation of azoles at the C2-position using 1-aryltriazenes as aryl reagents was developed. Azoles including oxazoles, thiazoles, imidazoles, 1,3,4-oxadiazoles, and oxazolines could react with 1-aryltriazenes smoothly to generate the corresponding products in good to excellent yields, and various substitution patterns were tolerated toward the reaction.

First molecular electronic hyperpolarizability of series of π-conjugated oxazole dyes in solution: an experimental and theoretical study

In this work, we report the experimental and theoretical first molecular electronic hyperpolarizability (β HRS) of eleven π-conjugated oxazoles compounds in toluene medium. The Hyper-Rayleigh Scattering (HRS) technique allowed the determination of the experimental dynamic β HRS values, by exciting the compounds with a picosecond pulse trains from a Q-switched and mode-locked Nd:YAG laser tuned at 1064 nm. Theoretical predictions based on time-dependent density functional theory level using the Gaussian 09 program package were performed with three different functionals (B3LYP, CAM-B3LYP, and M06-2X), to calculate both static and dynamic theoretical β HRS values. Good accordance was found between the experimental and theoretical values, in particular for the CAM-B3LYP and M06-2X functionals.

Structure-Activity Relationships of New Natural Product-Based Diaryloxazoles with Selective Activity against Androgen Receptor-Positive Breast Cancer Cells

Targeted therapies for ER+/PR+ and HER2-amplified breast cancers have improved patient survival, but there are no therapies for triple negative breast cancers (TNBC) that lack expression of estrogen and progesterone receptors (ER/PR), or amplification or overexpression of HER2. Gene expression profiling of TNBC has identified molecular subtypes and representative cell lines. An extract of the Texas native plant Amyris texana was found to have selective activity against MDA-MB-453 cells, a model of the luminal androgen receptor (LAR) subtype of TNBC. Bioassay-guided fractionation identified two oxazole natural products with selective activity against this cell line. Conducted analog synthesis and structure-activity relationship studies provided analogs with more potent and selective activity against two LAR subtype cell line models, culminating in the discovery of compound 30 (CIDD-0067106). Lead compounds discovered have potent and selective antiproliferative activities, and mechanisms of action studies show they inhibit the activity of the mTORC1 pathway.

Copper-catalyzed Direct 2-Arylation of Benzoxazoles and Benzoimidazoles with Aryl Bromides and Cytotoxicity of Products

An efficient copper-catalyzed direct 2-arylation of benzoxazoles and benzoimidazoles with aryl bromides is presented. The CuI/PPh₃-based catalyst promotes the installation of various aryl and heteroaryl groups through a C-H activation process in good to excellent yields. The cytotoxicity of obtained 2-aryl benzoxazoles (benzoimidazoles) was also evaluated and 1-methyl-2-(naphthalen-1-yl)benzoimidazole showed potential cytotoxicity.

Synthesis of extended oxazoles II: Reaction manifold of 2-(halomethyl)-4,5-diaryloxazoles

2-(Halomethyl)-4,5-diphenyloxazoles are effective, reactive scaffolds which can be utilized for synthetic elaboration at the 2-position. Through substitution reactions, the chloromethyl analogue is used to prepare a number of 2-alkylamino-, 2-alkylthio- and 2-alkoxy-(methyl) oxazoles. The 2-bromomethyl analogue offers a more reactive alternative to the chloromethyl compounds and is useful in the C-alkylation of a stabilized (malonate) carbanion as exemplified by a concise synthesis of Oxaprozin.

Synthesis of 4-substituted oxazolo[4,5-c]quinolines by direct reaction at the C-4 position of oxazoles

Cu(TFA)₂ catalysed synthesis of 4-arylsubstituted oxazolo[4,5-c]quinolines/[1,8] naphthyridines has been described via a modified Pictet–Spengler method, without prefunctionalization of the unreactive 4^th position of oxazoles.

Synthesis, photophysics and electrochemical properties of 1,1′-(2,2′-bithiophene-5,5′-diyl)bis(cycloalkeno[c]pyridine) as a result of the Diels–Alder reaction of 3-(2-thienyl)-1,2,4-triazine

Fused pyridine 2,2′-bithiophenes were prepared via [4+2] cycloaddition based on 3-(2-thienyl)-1,2,4-triazine, which was obtained by the Stille coupling reaction.

Iodine catalysed intramolecular C(sp3)–H functionalization: synthesis of 2,5-disubstituted oxazoles from N-arylethylamides

Iodine catalyzed synthesis of 2,5-substituted oxazoles from N-arylethylamides through intramolecular C(sp³)–H functionalization under metal-free conditions is described.

Ligand promoted Pd-catalyzed dehydrogenative alkenylation of hetereoarenes

An efficient Pd-catalyzed cross-dehydrogenative coupling of heteroarenes with styrenes and other olefinic substrates has been developed. This alkenylation paradigm encompasses a wide range of substrates and provides a straightforward approach toward C2-E-alkenylated azole motifs.

Room-temperature palladium-catalyzed direct 2-arylation of benzoxazoles with aryl and heteroaryl bromides

An efficient room-temperature palladium-catalyzed direct 2-arylation of benzoxazoles with aryl bromides is presented. The Pd(OAc)2/NiXantphos-based catalyst enables the introduction of various aryl and heteroaryl groups, via a deprotonative cross-coupling process (DCCP) in good to excellent yields (60-99%).

C-H alkenylation of azoles with enols and esters by nickel catalysis

Rather u(Ni)que: Two new C-H alkenylation reactions, that is C-H/C-O alkenylation and decarbonylative C-H alkenylation, of azoles are uniquely catalyzed by Ni/dcype. These azole alkenylation reactions are successfully applied to the convergent formal synthesis of siphonazole B.