Asymmetric Catalytic Access to Piperazin-2-ones and Morpholin-2-ones in a One-Pot Approach: Rapid Synthesis of an Intermediate to Aprepitant.

A one-pot Knoevenagel reaction/asymmetric epoxidation/domino ring-opening cyclization (DROC) has been developed from commercial aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide, 1,2-ethylendiamines, and 1,2-ethanol amines to provide 3-aryl/alkyl piperazin-2-ones and morpholin-2-ones in yields of 38 to 90% and up to 99% ee. Two out of the three steps are stereoselectively catalyzed by a quinine derived urea. The sequence has been applied for a short enantioselective entry to a key intermediate, in both absolute configurations, involved in the synthesis of the potent antiemetic drug Aprepitant.

2-Substituted 1,5-benzothiazepine-based HDAC inhibitors exert anticancer activities on human solid and acute myeloid leukemia cell lines.

Herein, we report the development of a new series of histone deacetylase inhibitors (HDACi) containing a 2-substituted 1,5-benzothiazepine scaffold. First, a virtual combinatorial library (∼1.6 × 103 items) was built according to a convenient synthetic route, and then it was submitted to molecular docking experiments on seven HDACs isoforms belonging to classes I and II. Integrated computational filters were used to select the most promising ones that were synthesized through an optimized approach, also amenable to generating both racemic and enantioenriched benzothiazepine-based derivatives. The obtained compounds showed potent HDAC inhibitory activity, especially those containing the sulphone moiety, endowed with IC50 in the nanomolar range. In addition, in vitro outcomes of our synthesized compounds demonstrated a cytotoxic effect on U937 and HCT116 cell lines and an arrest in the G2/M phase (13 ≤ IC50 ≤ 18 µM). Finally, Western blot analyses outlined the modulation of the histone acetyl markers such as H3K9/14, acetyl-tubulin, and the apoptotic indicator p21 in both cancer cell lines, disclosing a good HDAC inhibitor activity exerted by the designed items. Given the key role of HDACs in many cellular pathways, which makes these enzymes appealing and "hot" drug targets, our findings highlighted the importance of these 2-substituted 1,5-benzothiazepine-based compounds (both in the reduced and oxidized version) for the development of novel epidrugs.

Diastereoselective Synthesis of Functionalized 5-Amino-3,4-Dihydro-2H-Pyrrole-2-Carboxylic Acid Esters: One-Pot Approach Using Commercially Available Compounds and Benign Solvents.

A novel three-step four-transformation approach to highly functionalized 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylic acid esters, starting from commercially available phenylsulfonylacetonitrile, aldehydes, and N-(diphenylmethylene)glycine tert-butyl ester, was developed. The one-pot strategy delivered this class of amidines bearing, for the first time, three contiguous stereocenters, in good to high yield and diastereoselectivity. The entire sequence was carried out using diethyl carbonate and 2-methyl tetrahydrofuran as benign solvents, operating under metal-free conditions. The process could be conveniently scaled-up, and the synthetic utility of the products was demonstrated.

Continuous Flow Synthesis of α-Trifluoromethylthiolated Esters and Amides from Carboxylic Acids: a Telescoped Approach.

A continuous flow approach to access α-trifluoromethylthiolated esters and amides using commercially available arylacetic acids and N-(trifluoromethylthio)phthalimide as the electrophilic reagent is described. The experimental protocol involves the in-flow conversion of the carboxylic acid into N-acylpyrazole followed by the α-trifluoromethylthiolation in a PTFE coil reactor and final reaction with primary or secondary amines, or alcohols, to afford in a telescoped process α-substituted SCF3 amides and esters, respectively, in good overall yield and short reaction times.

Catalytic Enantioselective Access to Dihydroquinoxalinones via Formal α-Halo Acyl Halide Synthon in One Pot.

An enantioselective one-pot catalytic strategy to dihydroquinoxalinones, featuring novel 1-phenylsulfonyl-1-cyano enantioenriched epoxides as masked α-halo acyl halide synthons, followed by a domino ring-opening cyclization (DROC), is documented. A popular quinine-derived urea served as the catalyst in two out of the three steps performed in the same solvent using commercially available aldehydes, (phenylsulfonyl)acetonitrile, cumyl hydroperoxide and 1,2-phenylendiamines. Medicinally relevant 3-aryl/alkyl-substituted heterocycles are isolated in generally good to high overall yield and high enantioselectivity (up to 99 % ee). A rare example of excellent reusability of an organocatalyst at higher scale, subjected to oxidative conditions, is demonstrated. Mechanistically, labile α-ketosulfone has been detected as the intermediate involved in the DROC process. Theoretical calculations on the key epoxidation step rationalize the observed stereocontrol, highlighting the important role played by the sulfone group.

Synergistic Catalysis: Highly Enantioselective Cascade Reaction for the Synthesis of Dihydroacridines.

The first stereoselective synthesis of dihydroacridines through synergistic catalysis, achieving the final target compounds with good to excellent yields and good to excellent enantioselectivities and diastereoselectivities, is reported. The synergistic approach consists in the activation of substituted quinolines with a Lewis acid catalyst that react in a cascade fashion with activated enals in the iminium form. Mechanistic calculations support a consecutive Michael-aldol reaction, followed by dehydration.

Solvent-Free Enantioselective Michael Reactions Catalyzed by a Calixarene-Based Primary Amine Thiourea.

An upper-rim functionalized calix[4]arene-based thiourea installed onto the ( R, R)-1,2-cyclohexanediamine scaffold was synthesized with a view to investigate its catalytic ability in enantioselective Michael additions. The reactions were found to conveniently proceed under solvent-free conditions, observing good to high enantioselectivities. From this preliminary study, the calix[4]arene unit is likely to play a role in affecting the conversion and to a lesser extent to the stereochemical outcome of the reactions through van der Waals contacts and C-H···π interactions with the substrates.

Catalytic enantioselective one-pot approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines.

The first enantioselective catalytic approach to cis- and trans-2,3-diaryl substituted 1,5-benzothiazepines has been conveniently developed in a one-pot fashion, starting from α,ß-unsaturated acyl pyrazoles and 2-aminothiophenol. The organocatalytic two-step sulfa-Michael/lactamization sequence is promoted by a readily available bifunctional thiourea and p-toluenesulfonic acid, respectively. The protocol enables access to both N-unprotected cis- and trans-diastereoisomers in moderate to satisfactory overall yields (up to 84%) and good to excellent ee values (up to 99%). Mechanistic investigations helped to shed light on the regio- and stereoselective outcome of the process.

Catalytic Enantioselective Synthesis of Protecting-Group-Free 1,5-Benzothiazepines.

A one-pot enantioselective route to N-unprotected 2,3-dihydro-1,5-benzothiazepinones, by an organocatalyzed sulfa-Michael reaction of readily available α,ß-unsaturated N-acyl pyrazoles with 2-aminothiophenols followed by silica-gel-catalyzed lactamization, has been developed. The method proceeds under mild conditions at room temperature and it requires only 1 mol % catalyst loading, to give 2-aryl/alkyl-substituted 1,5-benzothiazepines in generally good to excellent yields and enantioselectivities. The process, used for a short synthesis of antidepressant drug (R)-(-)-thiazesim, represents the first method to access enantioenriched unprotected 1,5-benzothiazepines, which are useful for rapid derivatization in drug discovery.

Organocatalytic Asymmetric Reactions of Epoxides: Recent Progress.

In this Minireview recent advances in the asymmetric reactions of meso and racemic epoxides promoted by organocatalysts is reviewed. Organic promoters, such as chiral phosphoric acids, amino- and peptidyl thioureas, and sulfinamides, have been successfully used for a variety of enantioselective transformations of epoxides under catalytic conditions, involving direct nucleophilic attack at the oxirane ring, base-catalysed ß-eliminations and Brønsted acid catalysed 1,2-rearrangements. Accordingly, highly valuable enantioenriched 1,2-functionalised alcohols, carbonyl compounds and nitroepoxides are attainable. Dual activation of the reagents, provided by the organocatalysts, appears to be the most recurrent strategy, potentially suitable to face other unmet challenges in asymmetric ring-opening reactions of epoxides.

Asymmetric Aldol Reaction with Formaldehyde: a Challenging Process.

The asymmetric aldol reaction with formaldehyde is a fundamental carbon-carbon bond-forming reaction in organic synthesis, as well as in the quest of the origin of life, as it is thought to have been the first "molecular brick" involved in the synthetic path to complex sugars. Products of aldol reactions, i.e., the ß-hydroxy carbonyl compounds, are versatile building blocks used to access a great variety of functionalised molecules. The employment of formaldehyde, as a C1 symmetric electrophile, in aldol reactions can be likely considered the most challenging, yet simplest, process to introduce a hydroxymethyl group in an asymmetric fashion. In this account, an overview of the progress achieved in the asymmetric metal- and organocatalysed aldol reaction, using readily available formalin or paraformaldehyde sources, is illustrated. Our recent contribution to this area, with the application of asymmetric hydroxymethylation in cascade processes for the synthesis of γ-butyrolactones, is also shown.

Absolute Configuration Assignment of a Paraconic Acid Derivative via Vibrational Circular Dichroism Spectroscopy and Density Functional Theory Calculation.

Density functional theory calculation of the vibrational circular dichroism spectrum was used to assign the absolute configuration of an all-carbon quaternary ß-stereocenter of a γ-butyrolactone recently synthesized through an asymmetric organocatalytic tandem aldol/lactonization sequence. Comparison with the experimental spectrum is satisfactory, on account of the fact that spectroscopic features are weak due to the presence of multiple conformers. As a result, the (R) absolute configuration was assigned to the (+) optical isomer.

Stereoselective amine-thiourea-catalysed sulfa-Michael/nitroaldol cascade approach to 3,4,5-substituted tetrahydrothiophenes bearing a quaternary stereocenter.

An investigation on the stereoselective cascade sulfa-Michael/aldol reaction of nitroalkenes and commercially available 1,4-dithiane-2,5-diol to 3,4,5-substituted tetrahydrothiophenes, bearing a quaternary stereocenter, is presented. A secondary amine thiourea derived from (R,R)-1,2-diphenylethylamine was found to be the most effective catalyst when using trans-ß-methyl-ß-nitrostyrenes affording the heterocyclic products in good yields and moderate stereoselectivities.

Enantioselective α-hydroxylation of ß-ketoamides.

The first enantioselective α-hydroxylation reaction of α-substituted ß-ketoamides has been developed by using the commercially available hydroquinine/TBHP system. The tertiary alcohols are obtained in good to high yield and up to 83% ee, which can be improved by a single crystallization.

Single-Flask Enantioselective Synthesis of α-Amino Acid Esters by Organocatalysis.

An operationally simple Knoevenagel condensation/asymmetric epoxidation/domino ring-opening esterification (DROE) approach has been disclosed to successfully access a good variety of (R)- and (S)-α-arylglycine esters from commercially available aldehydes, phenylsulfonyl acetonitrile, cumyl hydroperoxide, anilines, and readily available Cinchona alkaloid-based catalysts using a single solvent and reaction vessel. DFT calculations performed on the key asymmetric epoxidation showed the importance of cooperative H-bonding interactions in affecting the stereocontrol.

Epoxides: Small Rings to Play with under Asymmetric Organocatalysis.

Optically pure epoxides are recognized as highly valuable products and key intermediates, useful in different areas from pharmaceutical and agrochemical industries to natural product synthesis and materials science. The predictable fate of the ring-opening process, in terms of stereoselectivity and often of regioselectivity, enables useful functional groups to be installed at vicinal carbon atoms in a desired manner. In this way, products of widespread utility either for synthetic applications or as final products can be obtained. The advent of asymmetric organocatalysis provided a new convenient tool, not only for their preparation but also for the elaboration of this class of heterocycles. In this review, we focus on recent developments of stereoselective organocatalytic ring-opening reactions of meso-epoxides, kinetic resolution of racemic epoxides, and Meinwald-type rearrangement. Examples of asymmetric organocatalytic processes toward specific synthetic targets, which include ring opening of an epoxide intermediate, are also illustrated.

Catalytic Enantioselective Entry to Triflones Featuring a Quaternary Stereocenter.

A highly enantioselective one-pot synthesis of functionalized triflones, bearing a quaternary stereocenter, has been developed, exploiting the Michael reaction of α-(trifluoromethylsulfonyl) aryl acetic acid esters with N-acryloyl-1H-pyrazole catalyzed by commercially available Takemoto's catalyst, followed by nucleophilic acyl substitution with alcohols. Preliminary investigations highlighted the attractive potential of the triflinate anion as the leaving group for stereocontrolled postfunctionalizations.

Valorization of Waste: Sustainable Organocatalysts from Renewable Resources.

One of the greatest challenges facing our society is to reconcile our need to develop efficient and sophisticated chemical processes with the limited resources of our planet and its restricted ability to adsorb pollution. Organocatalysis has allowed many issues to be addressed in the development of sophisticated, but less polluting, processes. However, minimizing waste also means an efficient utilization of raw and renewable materials. Waste biomass represents an alternative to conventional petroleum-based chemical manufacturing and is a highly attractive renewable resource for the production of chemicals and high-value-added organocatalysts. Recent achievements in the use of renewable biomass feedstocks for the synthesis of organocatalysts are presented. Their application in synthetic methodologies, including multicomponent reactions, which are performed under solvent-free conditions or in eco-friendly reaction media, as well as recycling and reusing the organocatalysts, is illustrated. A few pioneering examples that demonstrate the potential of these promoters in asymmetric synthesis have also been documented. In particular, this review covers examples on the use of hetero- and homogeneous organocatalysts derived from 1) waste biopolymers, such as chitosan, alginic acid, and cellulose; ii) renewable platform molecules, such as levoglucosenone, isosorbide, mannose, d-glucosamine, and lecithin; 3) terpenes and rosin, such as pinane, isosteviol, and abietic acid; and iv) natural proteins (gelatin, bovine tendons, silk fibroin proteins).

Formal α-trifluoromethylthiolation of carboxylic acid derivatives via N-acyl pyrazoles.

Herein we disclosed a one-pot two-step protocol for the first direct, base-catalyzed α-trifluoromethylthiolation of carboxylic acid derivatives by using readily available N-acyl pyrazoles, N-(trifluoromethylthio)phthalimide and a nucleophile such as amines, alcohols and water. Straightforward elaboration of the products to alcohols and triflones expands further the synthetic utility of the process.

Direct α-Imination of N-Acyl Pyrazoles with Nitrosoarenes.

Unprecedented α-imino N-acyl pyrazoles were efficiently and selectively prepared through the 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed reaction of nitrosoarenes with N-acyl pyrazoles via an N-nitroso aldol reaction/dehydration sequence. The α-imino acyl pyrazoles were demonstrated to be new versatile intermediates for practical one-pot syntheses of α-imino amides, dipeptide precursors, esters, and ß-amino alcohols. The synthetic method competes with known protocols in terms of ready availability of the reagents and catalyst, mild and catalytic reaction conditions, gram-scale applicability, and scope of the α-imino acid derivatives achievable.