Enantioselective Diels-Alder reaction of anthracene by chiral tritylium catalysis

Construction of Chiral Spiro-Bridged Rings with Four Consecutive Stereocenters via Dearomative Diels-Alder Reactions of Anthracenes

A highly diastereo- and enantioselective dearomative Diels-Alder reaction was accomplished by chiral N,N'-dioxide/Mg(II) complex catalyst. Various anthracene derivatives and methyleneindolinones efficiently transformed into the corresponding chiral spiro-bridged cyclic products with four consecutive stereocenters in good yields, excellent dr and er values under mild conditions (46 examples, up to 99% yield, >19:1 dr, >99:1 er). Gram-scale synthesis of chiral products and their further transformations were feasible. On the basis of theoretical calculation, possible working modes were provided to understand the origin of stereoselectivity of this transformation.

Tris(aryl)cyclopropenium Ion as Organic Lewis Acid Catalyst in Carbonyl Activation Reactions

Although, in recent years, cyclopropenium salts have been explored as phase-transfer catalysts, electro-photocatalysts, H-bond donor catalysts, etc., and until now, they have not been utilized directly as Lewis acid catalysts in organic transformations. In this article, we demonstrate a "Proof of Concept" that the tris(aryl)cyclopropenium (TAC) carbocation could be utilized as an organic Lewis acid catalyst in some of the reactions involving carbonyl activation such as 1,2-addition reactions of aldehydes, 1,4-conjugate addition reactions of enones, and 1,6-vinylogous conjugate addition of dienones (p-quinone methides). The mode of activation of carbonyl group by cyclopropenium ion has been studied using NMR titrations and UV kinetics and further supported by computational calculations.

Synthesis and Biochemical Evaluation of Ethanoanthracenes and Related Compounds: Antiproliferative and Pro-Apoptotic Effects in Chronic Lymphocytic Leukemia (CLL)

Chronic lymphocytic leukemia (CLL) is a malignancy of mature B cells, and it is the most frequent form of leukemia diagnosed in Western countries. It is characterized by the proliferation and accumulation of neoplastic B lymphocytes in the blood, lymph nodes, bone marrow and spleen. We report the synthesis and antiproliferative effects of a series of novel ethanoanthracene compounds in CLL cell lines. Structural modifications were achieved via the Diels-Alder reaction of 9-(2-nitrovinyl)anthracene and 3-(anthracen-9-yl)-1-arylprop-2-en-1-ones (anthracene chalcones) with dienophiles, including maleic anhydride and N-substituted maleimides, to afford a series of 9-(E)-(2-nitrovinyl)-9,10-dihydro-9,10-[3,4]epipyrroloanthracene-12,14-diones, 9-(E)-3-oxo-3-phenylprop-1-en-1-yl)-9,10-dihydro-9,10-[3,4]epipyrroloanthracene-12,14-diones and related compounds. Single-crystal X-ray analysis confirmed the structures of the novel ethanoanthracenes 23f, 23h, 24a, 24g, 25f and 27. The products were evaluated in HG-3 and PGA-1 CLL cell lines (representative of poor and good patient prognosis, respectively). The most potent compounds were identified as 20a, 20f, 23a and 25n with IC50 values in the ranges of 0.17-2.69 µM (HG-3) and 0.35-1.97 µM (PGA-1). The pro-apoptotic effects of the potent compounds 20a, 20f, 23a and 25n were demonstrated in CLL cell lines HG-3 (82-95%) and PGA-1 (87-97%) at 10 µM, with low toxicity (12-16%) observed in healthy-donor peripheral blood mononuclear cells (PBMCs) at concentrations representative of the compounds IC50 values for both the HG-3 and PGA-1 CLL cell lines. The antiproliferative effect of the selected compounds, 20a, 20f, 23a and 25n, was mediated through ROS flux with a marked increase in cell viability upon pretreatment with the antioxidant NAC. 25n also demonstrated sub-micromolar activity in the NCI 60 cancer cell line panel, with a mean GI50 value of 0.245 µM. This ethanoanthracene series of compounds offers potential for the further development of lead structures as novel chemotherapeutics to target CLL.

Main Group Catalysis: Cationic Si(II) and Ge(II) Compounds as Catalysts in Organosilicon Chemistry

Cyclopentadienyl (Cp)-coordinated cationic Si(II) (1) and Ge(II) compounds (2) are a new class of catalysts for various transformations in organosilicon chemistry. This review demonstrates that these compounds effectively catalyze technically important reactions, such as the hydrosilylation of carbon-carbon multiple bonds and various types of siloxane-coupling reactions, e.g., the Piers-Rubinsztajn reaction and the oxidative siloxane coupling reaction. Whereas the cationic Si(II) compounds are sensitive to air and moisture, the corresponding cationic Ge(II) compounds are bench stable, thus offering further advantages. The new catalysts contribute to the growing need for the substitution of transition metals and heavier main group metals by their lighter congeners, especially in industrially relevant organosilicon chemistry.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

Chiral Triphenylacetic Acid Esters: Residual Stereoisomerism and Solid-State Variability of Molecular Architectures

We have proven the usability and versatility of chiral triphenylacetic acid esters, compounds of high structural diversity, as chirality-sensing stereodynamic probes and as molecular tectons in crystal engineering. The low energy barrier to stereoisomer interconversion has been exploited to sense the chirality of an alkyl substituent in the esters. The structural information are cascaded from the permanently chiral alcohol (inducer) to the stereodynamic chromophoric probe through cooperative interactions. The ECD spectra of triphenylacetic acid esters are highly sensitive to very small structural differences in the inducer core. The tendencies to maximize the C-H···O hydrogen bonds, van der Waals interactions, and London dispersion forces determine the way of packing molecules in the crystal lattice. The phenyl embraces of trityl groups allowed, to some extent, the control of molecular organization in the crystal. However, the spectrum of possible molecular arrangements is very broad and depends on the type of substituent, the optical purity of the sample, and the presence of a second trityl group in the proximity. Racemates crystallize as the solid solution of enantiomers, where the trityl group acts as a protecting group for the stereogenic center. Therefore, the absolute configuration of the inducer is irrelevant to the packing mode of molecules in the crystal.

Dynamic Induction of Optical Activity in Triarylmethanols and Their Carbocations

A series of artificial triarylmethanols has been synthesized and studied toward the possibility of exhibiting an induced optical activity. The observed chiroptical response of these compounds resulted from the chiral conformation of a triarylmethyl core. The chirality induction from a permanent chirality element to the liable triarylmethyl core proceeds as a cooperative and cascade process. The OH···O(R) and/or (H)O···H_orthoC hydrogen bond formation along with the C–H···π interactions seem to be the most important factors that control efficiency of the chirality induction. The position of chiral and methoxy electron-donating groups within a trityl skeleton affects the amplitude of observed Cotton effects and stability of the trityl carbocations. In the neutral environment, the most intense Cotton effects are observed for ortho-substituted derivatives, which undergo a rapid decomposition associated with the complete decay of ECD signals upon acidification. From all of the in situ generated stable carbocations, only two exhibit intense Cotton effects in the low energy region at around 450 nm. The formation of carbocations is reversible; after alkalization, the ions return to the original neutral forms. Unlike most triarylmethyl derivatives known so far, in the crystal, the triarylmethanol, para-substituted with the chiral moiety, shows a propensity for a solid-state sorting phenomenon.

Tritylium assisted iodine catalysis for the synthesis of unsymmetrical triarylmethanes

The combined Lewis acid catalytic system, generated from molecular iodine and tritylium tetrafluoroborate effectively catalyzed the Friedel-Crafts (FC) arylation of diarylmethyl sulfides providing an efficient access to various unsymmetrical triarylmethanes. The addition of tritylium and iodine created a more active catalytic system to promote the cleavage of sulfidic C-S bonds.