Asymmetric Isothiourea-Catalysed Formal [3+2] Cycloadditions of Ammonium Enolates with Oxaziridines

Catalytic Atroposelective Synthesis of N-N Axially Chiral Indolylamides

The catalytic atroposelective synthesis of N-N axially chiral indolylamides was established via dynamic kinetic resolution, which makes use of chiral Lewis base-catalyzed asymmetric acylation of N-acylaminoindoles as a new type of platform molecule with anhydrides. By this strategy, a series of N-N axially chiral indolylamides were synthesized in overall good yields (up to 98%) with excellent enantioselectivities (up to 99% ee). Moreover, some of these N-N axially chiral indolylamides display some extent of anticancer activity, which demonstrates their potential application in medicinal chemistry. Therefore, this work has not only provided a new strategy for the synthesis of N-N axially chiral monoaryl indoles but also offered a new member of N-N axially chiral monoaryl indoles with configurational stability and promising application, thereby solving the challenges in atroposelective synthesis and application of N-N axially chiral monoaryl indoles.

Enantioselective α-heterofunctionalization reactions of catalytically generated C1-Lewis base enolates

Chiral Lewis base (LB) organocatalysis has emerged as a powerful covalent catalysis concept which allows for highly selective asymmetric C-C and C-heteroatom bond formations. Considering significant recent progress in the development of strategies to access α-heterofunctionalized carboxylic acid derivatives under chiral LB catalysis, we wish to summarize the most significant concepts and advances in this field within this mini review now.

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts

The regio- and stereoselective addition of C(1)-ammonium enolates - generated in situ from aryl esters and the isothiourea catalyst (R)-BTM - to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Enantiopure isothiourea@carbon-based support: stacking interactions for recycling a lewis base in asymmetric catalysis

An enantiopure isothiourea (hyperBTM) was functionalized by a pyrene moiety via click chemistry; immobilized on reduced Graphene Oxide, this recyclable chiral organocatalyst promotes formal [3+2] cycloaddition of ammonium enolates with oxaziridines.

Sustainable, three-component, one-pot procedure to obtain active anti-flavivirus agents

The mosquito-borne viruses belonging to the genus Flavivirus such as Dengue virus (DENV) and Zika virus (ZIKV) cause human infections ranging from mild flu-like symptoms to hemorrhagic fevers, hepatitis, and neuropathies. To date, there are vaccines only for few flaviviruses while no effective treatments are available. Pyridobenzothiazole (PBTZ) derivatives are a class of compounds endowed with a promising broad-spectrum anti-flavivirus activity and most of them have been reported as potent inhibitors of the flaviviral NS5 polymerase. However, synthesis of PBTZ analogues entails a high number of purification steps, the use of hazardous reagents and environmentally unsustainable generation of waste. Considering the promising antiviral activity of PBTZ analogues which require further exploration, in this work, we report the development of a new and sustainable three-component reaction (3CR) that can be combined with a basic hydrolysis in a one-pot procedure to obtain the PBTZ scaffold, thus reducing the number of synthetic steps, improving yields and saving time. 3CR was significantly explored in order to demonstrate its wide scope by using different starting materials. In addition, taking advantage of these procedures, we next designed and synthesized a new set of PBTZ analogues that were tested as anti-DENV-2 and anti-ZIKV agents. Compound 22 inhibited DENV-2 NS5 polymerase with an IC₅₀ of 10.4 μM and represented the best anti-flavivirus compound of the new series by inhibiting DENV-2- and ZIKV-infected cells with EC₅₀ values of 1.2 and 5.0 μM, respectively, that translates into attractive selectivity indexes (SI - 83 and 20, respectively). These results strongly reaffirm PBTZ derivatives as promising anti-flavivirus agents that now can be synthesized through a convenient and sustainable 3CR in order to obtain more potent compounds for further pre-clinical development studies.

Insights into highly selective ring expansion of oxaziridines under Lewis base catalysis: a DFT study

The possible mechanism and stereoselectivity of the NHC-catalyzed ring expansion reaction of oxaziridines have been theoretically studied for the first time.

Recent Advances in the Catalytic Asymmetric Reactions of Oxaziridines

Oxaziridines have emerged as powerful and elegant oxygen- and nitrogen-transfer agents for a broad array of nucleophiles, due to the remarkably high and tunable reactivities. However, the asymmetric catalysis involving oxaziridines is still in its infancy. Herein, this review aims to examine recent advances in the catalytic asymmetric transformations of oxaziridines, including oxidation, amination, cycloaddition and deracemization.

β-Functionalization of Indolin-2-one-Derived Aliphatic Acids for the Divergent Synthesis of Spirooxindole γ-Butyrolactones

β-Functionalization of indolin-2-one-derived aliphatic acids has been applied in formal [3 + 2] annualtions for catalyst-free and divergent synthesis of two series of structurally interesting 3,3'-spirooxindole γ-butyrolactones that may be attractive for potential drug discovery. These findings also pave the way for further diversity-oriented synthesis of spirooxindoles starting from indolin-2-one-derived aliphatic acids or their derivatives.

[3 + 2] Cycloaddition of aza-oxyallyl cations with aldehydes

We report here the [3 + 2] cycloaddition reactions of in situ-generated aza-oxyallyl cations with commercially available aldehydes.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.

Enantioselective isothiourea-catalysed trans-dihydropyridinone synthesis using saccharin-derived ketimines: scope and limitations

The catalytic enantioselective synthesis of a range of trans-dihydropyridinones from aryl-, heteroaryl- and alkenylacetic acids and saccharin-derived ketimines with good to excellent stereocontrol (15 examples, up to >95 : 5 dr, up to >99 : 1 er) is reported. After extensive optimisation, HyperBTM proved the optimal isothiourea catalyst for this transformation at -78 °C, giving trans-dihydropyridones with generally excellent levels of diastereo- and enantioselectivity.

Non-bonding 1,5-S···O interactions govern chemo- and enantioselectivity in isothiourea-catalyzed annulations of benzazoles

Isothiourea-catalyzed annulations between 2-acyl benzazoles and α,β-unsaturated acyl ammonium intermediates are selectively tuned to form either lactam or lactone heterocycles in good yields (up to 95%) and high ee (up to 99%) using benzothiazole or benzoxazole derivatives, respectively. Computation gives insight into the significant role of two 1,5-S···O interactions in controlling the structural preorganization and chemoselectivity observed within the lactam synthesis with benzothiazoles as nucleophiles. When using benzazoles the absence of a second stabilizing non-bonding 1,5-S···O interaction leads to a dominant C-H···O interaction in determining structural preorganization and lactone formation.

Enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofurans using a Brønsted base/thiourea bifunctional catalyst

The enantioselective synthesis of 2,3-disubstituted trans-2,3-dihydrobenzofuran derivatives via intramolecular Michael addition has been developed using a bifunctional tertiary amine–thiourea catalyst.

Enantioselective Synthesis of 3,5,6-Substituted Dihydropyranones and Dihydropyridinones using Isothiourea-Mediated Catalysis

The scope of dihydropyranone and dihydropyridinone products accessible by isothiourea-catalyzed processes has been expanded and explored through the use of 2-N-tosyliminoacrylates and 2-aroylacrylates in a Michael addition-lactonization/lactamization cascade reaction. Notably, to ensure reproducibility it is essential to use homoanhydrides as ammonium enolate precursors with 2-aroyl acrylates, while carboxylic acids can be used with 2-N-tosyliminoacrylates, delivering a range of 3,5,6-substituted dihydropyranones and dihydropyridinones with high enantioselectivity (typically >90 % ee). The derivatization of the heterocyclic core of a 3,5,6-substituted dihydropyranone through hydrogenation is also reported.