Organic base-promoted enantioselective electrophilic cyanation of β-keto esters by using chiral phase-transfer catalysts

Stereoselective Construction of Less-Accessible Acyclic α,α-Disubstituted β-Ketiminonitriles via Electrophilic Cyanation of β,β-Disubstituted Enesulfinamides

The metalloenamines, formed by NH-deprotonation of β,β-disubstituted enesulfinamides, can undergo reactions with commercially available electrophilic cyanating reagents such as tosyl cyanide (TsCN) or 3-oxo-1,2-benziodoxole-1(3H)-carbonitrile (CBX). Through the utilization of appropriate stereoisomers of enesulfinamides, this method enables the selective synthesis of any of the four stereoisomers of α,α-disubstituted β-sulfinylimino nitriles, which feature acyclic quaternary stereocenters and are typically more challenging to synthesize. These compounds can then undergo stereoselective nucleophilic addition, leading to the creation of valuable enantioenriched multisubstituted β-amino carbonyl surrogates with a high degree of structural diversity.

tert-Butylhydroperoxide mediated radical cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s

A novel sustainable methodology based on one-pot cyanoalkylation/cyanoalkenylation of 2-anilino-1,4-naphthoquinones with vinylarenes/arylalkynes and azobis(alkylcarbonitrile)s involving a three-component radical cascade pathway has been achieved. Here, tert-butylhydroperoxide (TBHP) acts as an efficient oxidant, and it smoothly drives the reaction, producing the three-component products in very good to excellent yields. This cascade reaction eliminates the use of any base, additive, metal and hazardous cyanating agent. Additionally, this protocol exclusively delivers a stereospecific product in the case of arylalkynes. The involvement of radicals is evidenced through various radical trapping experiments.

Metal-free three-component cyanoalkylation of quinoxalin-2(1H)-ones with vinylarenes and azobis(alkylcarbonitrile)s

A K2S2O8-mediated C3 cyanoalkylation of quinoxalin-2(1H)-ones via a three-component radical cascade reaction with vinylarenes and azobis(alkylcarbonitrile)s has been achieved.

N-Cyanation of Primary and Secondary Amines with Cyanobenzio-doxolone (CBX) Reagent

An efficient electrophilic N-cyanation of amines with a stable and less-toxic cyanobenziodoxole reagent towards the synthesis of cyanamides is disclosed. This synthetically practicable strategy allows the construction of a wide variety of cyanamides under very mild and simple conditions with a broad functional group compatibility, and showcases a huge potential in late-stage modification of complex molecules.

Practical Synthesis of 2-Iodosobenzoic Acid (IBA) without Contamination by Hazardous 2-Iodoxybenzoic Acid (IBX) under Mild Conditions

We report a convenient and practical method for the preparation of nonexplosive cyclic hypervalent iodine(III) oxidants as efficient organocatalysts and reagents for various reactions using Oxone® in aqueous solution under mild conditions at room temperature. The thus obtained 2-iodosobenzoic acids (IBAs) could be used as precursors of other cyclic organoiodine(III) derivatives by the solvolytic derivatization of the hydroxy group under mild conditions of 80 °C or lower temperature. These sequential procedures are highly reliable to selectively afford cyclic hypervalent iodine compounds in excellent yields without contamination by hazardous pentavalent iodine(III) compound.

Asymmetric α-Chlorination of β-Keto Esters Using Hypervalent Iodine-Based Cl-Transfer Reagents in Combination with Cinchona Alkaloid Catalysts

We herein report an unprecedented strategy for the asymmetric α-chlorination of β-keto esters with hypervalent iodine-based Cl-transfer reagents using simple Cinchona alkaloid catalysts. Our investigations support an α-chlorination mechanism where the Cinchona species serves as a nucleophilic catalyst by reacting with the chlorinating agent to generate a chiral electrophilic Cl-transfer reagent in situ. Using at least 20 mol-% of the alkaloid catalyst allows for good yields and enantioselectivities for a variety of different β-keto esters under operationally simple conditions.

One-pot synthesis of 2-chloro-2-thio/selenocyanato ketones from β-keto acids

The chlorothiocyanato difunctionalization reaction has been achieved, and a variety of α-chlorothio/selenocyanato difunctional ketones are synthesized through one-pot strategy from β-keto acids.

Enantioselective Assembly of Cycloenones with a Nitrile-Containing All-Carbon Quaternary Center from Malononitriles Enabled by Ni Catalysis

Chiral nitriles are valuable molecules in modern organic synthesis and drug discovery. Selectively differentiating the two nitrile groups of widely available malononitrile derivatives is a straightforward yet underdeveloped route to construct enantioenriched nitriles. Here we report an enantioselective nickel-catalyzed desymmetrization of malononitriles for the generation of nitrile-containing all-carbon quaternary stereocenters. This protocol involves a nickel-catalyzed addition of aryl boronic acids to alkynes, followed by a selective nitrile insertion, providing unprecedented access to enantioenriched 5-7-membered α-cyano-cycloenones with a fully substituted olefin from a broad range of substrates. The synthetic utility of these nitrile products is demonstrated by gram-scale synthesis and conversion to several useful functional groups.

Oxidative cyanation of 2-oxindoles: formal total synthesis of (±)-gliocladin C

Efficient oxidative direct cyanations of 3-alkyl/aryl 2-oxindoles using Cyano-1,2-BenziodoXol-3(1H)-one (CBX) (2a) have been reported under 'transition metal-free' conditions to synthesize a wide variety of 3-cyano 3-alkyl/aryl 2-oxindoles sharing an all-carbon quaternary center under additive-free conditions. The application of this process is shown by the formal total synthesis of (±)-gliocladin C (11c) in a few steps.

Transition-metal-free direct C-3 cyanation of quinoxalin-2(1H)-ones with ammonium thiocyanate as the “CN” source

A practical protocol for TBHP-mediated oxidative C–H cyanation of quinoxalin-2(1H)-ones utilizing ammonium thiocyanate as the cyanide source has been developed under metal free conditions.

Dibenzofuran-4,6-bis(oxazoline) (DBFOX). A novel trans-chelating bis(oxazoline) ligand for asymmetric reactions

The trans-chelating bis(oxazoline) ligand (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) [(R,R)-DBFOX/Ph] coordinates metal ions to give C2-symmetric complexes which effectively catalyze a variety of asymmetric reactions. (R,R)-DBFOX/Ph·Ni(ClO4)2·3H2O, whose crystal structure is octahedral with three aqua ligands, can be stored under air for several months without loss of catalytic activity and promotes highly enantioselective reactions even in the presence of excess amounts of water, alcohols, amines, and acids. The complex shows remarkable chiral amplification in asymmetric Diels-Alder (DA) reactions. This review focuses on enantioselective reactions catalyzed by (R,R)-DBFOX/Ph·metal complexes.

Non-toxic cyanide sources and cyanating agents

The present review gives an overview over non-toxic cyanation agents and cyanide sources used in the synthesis of structurally diverse products containing the nitrile function. Nucleophilic as well as electrophilic agents/systems that transfer the entire CN-group were taken in consideration. Reactions in which a preexisting carbon functionality is transformed into a nitrile function by addition of nitrogen are however not covered here.

Bioactive Indanes: Proof of Concept Study for Enantioselective Synthetic Routes to PH46A, a New Potential Anti-Inflammatory Agent

PH46A is a single enantiomer and a member of the 1,2-indane dimer family. It has two contiguous stereogenic centers with S,S configurations, one of which being a quaternary center, which has been developed as a clinical candidate for the treatment of inflammatory and autoimmune conditions. The current synthetic route to PH46A involves the generation of an unwanted enantiomer (R,R)-7, thus reducing the final yield significantly. Therefore, we have investigated potential alternatives to improve the efficiency of this synthesis. The first phase of the study has demonstrated proof of principle for a chiral alkylation of ketone 3 using phase-transfer catalysis, providing a key intermediate ketone (S)-4. The parent alkaloids required for the synthesis of PH46A, quinine or cinchonidine, have also been identified. Promising enantiomeric excesses of up to 50% have been achieved to date, and the use of an alternative substrate, unsaturated ketone 9, has also opened up further avenues for optimisation in future studies. The second part of the study involved preliminary screening the effects of a panel of hydrolase enzymes on (rac)-4 in order to identify a potential chemo-enzymatic route to optimise the introduction of chirality into PH46A at early stage of the synthesis. The hydrolase module has also yielded positive results; enzyme AH-46 with MtBE providing a selectivity factor of 8.4 with enantiomeric excess of 77%. Overall, positive results were obtained in this proof of concept study described herein. It is believed that conditions of both chiral PTC alkylation and biocatalytic hydrolysis could be optimised to further enhance the selectivity and improve the overall yield. This work is currently ongoing.

Towards an Asymmetric Organocatalytic α-Azidation of β-Ketoesters

Detailed investigations concerning the organocatalytic (asymmetric) α-azidation of prochiral β-ketoesters were carried out. It was shown that the racemic version of such a reaction can either be carried out under oxidative conditions using TMSN₃ as the azide-source with quaternary ammonium iodides as the catalysts, or by using hypervalent iodine-based electrophilic azide-transfer reagents with different organocatalysts. In addition, the latter strategy could also be carried out with modest enantioselectivities when using simple cinchona alkaloid catalysts, albeit with relatively low yields.

Nitro-enabled catalytic enantioselective formal umpolung alkenylation of β-ketoesters

A formal umpolung strategy is presented for the enantioselective installation of an alkenyl group with a terminal double bond at a tertiary center. This one-pot two-step sequence relies on the unique features of the nitro group, which after inverting the polarity of the alkenylating agent toward the desired bond formation, itself serves as a leaving group. The application of this protocol to cyclic β-ketoesters results in densely functionalized products, bearing an all-carbon quaternary stereocenter including an alkenyl substituent with a terminal double bond, in high yields with excellent enantioselectivities.

Room temperature decarboxylative cyanation of carboxylic acids using photoredox catalysis and cyanobenziodoxolones: a divergent mechanism compared to alkynylation

The one-step conversion of aliphatic carboxylic acids to the corresponding nitriles has been accomplished via the merger of visible light mediated photoredox and cyanobenziodoxolones (CBX) reagents. The reaction proceeded in high yields with natural and non-natural α-amino and α-oxy acids, affording a broad scope of nitriles with excellent tolerance of the substituents in the α position. The direct cyanation of dipeptides and drug precursors was also achieved. The mechanism of the decarboxylative cyanation was investigated both computationally and experimentally and compared with the previously developed alkynylation reaction. Alkynylation was found to favor direct radical addition, whereas further oxidation by CBX to a carbocation and cyanide addition appeared more favorable for cyanation. A concerted mechanism is proposed for the reaction of radicals with EBX reagents, in contrast to the usually assumed addition elimination process.

Enantioselective electrophilic cyanation of β-keto amides catalysed by a cinchona organocatalyst

An operationally simple protocol for the enantioselective electrophilic α-cyanation of β-keto amides catalyzed by cinchona-derived catalysts has been demonstrated.

Intramolecular aminocyanation of alkenes promoted by hypervalent iodine

Hypervalent iodine-promoted aminocyanation of unactivated alkenes has been developed and the orientation of cyanation on the double bond can be controlled.

Enantioselective Organocatalyzed Transformations of β-Ketoesters

The β-ketoester structural motif continues to intrigue chemists with its electrophilic and nucleophilic sites. Proven to be a valuable tool within organic synthesis, natural product, and medicinal chemistry, reports on chiral β-ketoester molecular skeletons display a steady increase. With the reignition of organocatalysis in the past decade, asymmetric methods available for the synthesis of this structural unit has significantly expanded, making it one of the most exploited substrates for organocatalytic transformations. This review provides comprehensive information on the plethora of organocatalysts used in stereoselective organocatalyzed construction of β-ketoester-containing compounds.

New strategies and applications using electrophilic cyanide-transfer reagents under transition metal-free conditions

The use of electrophilic cyanide-transfer reagents has become a versatile strategy to access important structural motives in a complementary way compared to other methods.