Thiourea-Catalyzed Addition of Indoles to Aliphatic β,γ-Unsaturated α-Ketoesters

Novel ureido-dihydropyridine scaffolds as theranostic agents

In this work, the synthesis of interesting urea derivatives 5 based on 1,4-dihydropyridines 3 is described for the first time. Considering that both families exhibit potential as drugs to treat various diseases, their activity as anticancer agents has been evaluated in HeLa (cervix), Jurkat (leukaemia) and A549 (lung) cancer cell lines as well as on healthy mice in vivo. In general, whereas 1,4-dihydropyridines show a moderate cytotoxic activity, their urea analogues cause an extraordinary increase in their antiproliferative activity, specially towards HeLa cells. Because of the chiral nature of these compounds, enantiomerically enriched samples were also tested, showing different cytotoxic activity than the racemic mixture. Although the reason is not clear, it could be caused by a complex amalgam of physical and chemical contributions. The studied compounds also exhibit luminescent properties, which allow performing a biodistribution study in cancer cells. They have emission maxima between 420 and 471 nm, being the urea derivatives in general red shifted. Emission quenching was observed for those compounds containing a nitro group (3e,f and 5e,f). Fluorescence microscopy showed that 1,4-dihydropyridines 3a and 3g localised in the lysosomes, in contrast to the urea derivatives 5h that accumulated in the cell membrane. This different distribution could be key to explain the differences found in the cytotoxic activity and in the mechanism of action. Interestingly, a preliminary in vivo study regarding the acute toxicity of some of these compounds on healthy mice has been conducted, using a concentration up to 7200 times higher than the corresponding IC₅₀ value. No downgrade in the welfare of the tested mice was observed, which could support their use in preclinical tumour models.

Simple iodoalkyne-based organocatalysts for the activation of carbonyl compounds

A novel approach for the formation of bisindolylmethane derivatives (BIMs) is described as a proof of concept to evaluate the catalytic capacity of iodoalkynes. The use of these derivatives is reported as an example of simple halogen bond-based organocatalyst. This kind of activation has not been used before for the synthesis of bisindolylmethane derivatives 3. Interestingly, the preparation of 3-(1H-indol-3-yl)-1-phenylbutan-1-one (8) has been also achieved for the first time with an iodoalkyne derivative. We prove the efficiency of this family of new catalysts by developing a simple and easy operational methodology, opening the door to the development of alternative catalysts in the area of halogen bond-based organocatalysts.

Urea Activation by an External Brønsted Acid: Breaking Self-Association and Tuning Catalytic Performance

In this work, we hypothesize that Brønsted acids can activate urea-based catalysts by diminishing its self-assembly tendency. As a proof of concept, we used the asymmetric Friedel–Crafts alkylation of indoles with nitroalkenes as a benchmark reaction. The resulting 3-substituted indole derivatives were obtained with better results due to cooperative effects of the chiral urea and a Brønsted acid additive. Such synergy has been rationalized in terms of disassembly of the supramolecular catalyst aggregates, affording a more acidic and rigid catalytic complex.

Gold(I)-Mediated Thiourea Organocatalyst Activation: A Synergic Effect for Asymmetric Catalysis

Several group 11 metal complexes with chiral thiourea organocatalysts have been prepared and tested as organocatalysts. The promising results on the influence of metal-assisted thiourea organocatalysts in the asymmetric Friedel-Crafts alkylation of indole with nitrostyrene are described. Better results with the metal complexes have been achieved because of the cooperative effects between the chiral thiourea and the metal. The synergic effect between both species is higher than the effect promoted by each one separately, especially for gold(I). These outcomes are attributed to a pioneering gold(I) activation of the thiourea catalysts, affording a more acidic and rigid catalytic complex than that provided by the thiourea alone. Furthermore, the use of the gold-thiourea organocatalyst allows reducing the catalyst loading to 1-3 mol %. This contribution could become an important starting point for further investigations opening a new line of research overlooked so far in the literature.

Organocatalytic Hydrophosphonylation Reaction of Carbonyl Groups

This revision is covering the limited examples reported for a pivotal strategy in the formation of C-P bonds such as the asymmetric organocatalytic hydrophosphonylation of carbonyl groups (Pudovik reaction). The scope and limitations, and the proposed mechanisms for the scarce different possibilities of asymmetric induction are also shown. The recent evolution and future trends of this undeveloped approach are commented.

Chiral Phosphoric Acid Catalyzed Intramolecular Dearomative Michael Addition of Indoles to Enones

An enantioselective intramolecular dearomative Michael addition of indolyl enones is presented. In the presence of catalytic amount of chiral phosphoric acid, various enantioenriched spiro-indolenines bearing a quaternary stereogenic center were obtained with good yields and enantioselectivity (up to 97% ee) under mild reaction conditions.

The aminoindanol core as a key scaffold in bifunctional organocatalysts

The 1,2-aminoindanol scaffold has been found to be very efficient, enhancing the enantioselectivity when present in organocatalysts. This may be explained by its ability to induce a bifunctional activation of the substrates involved in the reaction. Thus, it is easy to find hydrogen-bonding organocatalysts ((thio)ureas, squaramides, quinolinium thioamide, etc.) in the literature containing this favored structural core. They have been successfully employed in reactions such as Friedel-Crafts alkylation, Michael addition, Diels-Alder and aza-Henry reactions. However, the 1,2-aminoindanol core incorporated into proline derivatives has been scarcely explored. Herein, the most representative and illustrative examples are compiled and this review will be mainly focused on the cases where the aminoindanol moiety confers bifunctionality to the organocatalysts.