Selective mono- and difluorination of enolates

Radical Decarboxylation-Initiated SH2' Reaction of β,β-Difluoroenol Sulfonates: Access to α,α-Difluoroketones

Reported herein is a novel radical decarboxylation-initiated SH2' reaction of β,β-difluoroenol sulfonates. This transformation is characterized by mild reaction conditions, a broad substrate scope, and late-stage modification of drug molecules, providing general and mechanistically distinct access to bioactive and synthetically versatile α,α-difluoroketones. Preliminary mechanistic studies demonstrate that this reaction proceeds through a succession of silver-mediated decarboxylative radical generation and radical-addition-induced β-elimination of the sulfonyl radical.

Synthesis of 2,2-difluoro-1,3-diketone and 2,2-difluoro-1,3-ketoester derivatives using fluorine gas

Solutions of 1,3-diketones and 1,3-ketoester derivatives react with fluorine to give the corresponding 2,2-difluoro-1,3-dicarbonyl derivatives in the presence of quinuclidine. Quinuclidine reacts with fluorine in situ to generate a fluoride ion that facilitates limiting enolization processes, and an electrophilic N-F fluorinating agent that is reactive towards neutral enol species.

An Expedient Method for the Umpolung Coupling of Enols with Heteronucleophiles**

In this paper, we present an unprecedented and general umpolung protocol that allows the functionalization of silyl enol ethers and of 1,3‐dicarbonyl compounds with a large range of heteroatom nucleophiles, including carboxylic acids, alcohols, primary and secondary amines, azide, thiols, and also anionic carbamates derived from CO₂. The scope of the reaction also extends to carbon‐based nucleophiles. The reaction relies on the use of 1‐bromo‐3,3‐dimethyl‐1,3‐dihydro‐1λ³[d][1,2]iodaoxole, which provides a key α‐brominated carbonyl intermediate. The reaction mechanism has been studied experimentally and by DFT, and we propose formation of an unusual enolonium intermediate with a halogen‐bonded bromide.

Bench-Stable Electrophilic Fluorinating Reagents for Highly Selective Mono- and Difluorination of Silyl Enol Ethers

Efficient methods for the synthesis of fluorinated compounds have been intensively studied, recently. Development of practical fluorinating reagents is indispensable for this purpose. Herein, bench-stable electrophilic fluorinating reagents were synthesized as N-fluorobenzenesulfonimide (NFSI) substitutes. Reagents obtained by replacing one of the NFSI sulfonyl groups with an acyl group led to the highly selective monofluorination of silyl enol ethers with suppression of undesired overreaction, that is, difluorination. On the other hand, reagents bearing electron-withdrawing substituents at NFSI benzenesulfonyl groups efficiently facilitated the difluorination of silyl enol ethers under base-free conditions. Thus, both mono- and difluorinated target materials were prepared from the same substrate.

Design, Synthesis, and Biophysical Evaluation of Mechanism-Based Probes for Condensation Domains of Nonribosomal Peptide Synthetases

Nonribosomal peptide synthetases (NRPSs) are remarkable modular enzymes that synthesize peptide natural products. The condensation (C) domain catalyzes the key amide bond-forming reaction, but structural characterization with bound donor and acceptor substrates has proven elusive. We describe the chemoenzymatic synthesis of condensation domain probes C1 and C2 designed to cross-link the donor and acceptor substrates within the condensation domain active site. These pantetheine probes contain nonhydrolyzable ketone and α,α-difluoroketone isosteres of the native thioester linkage. Using the bimodular NRPS responsible for synthesis of the siderophore enterobactin as a model system, probe C2 was shown by surface plasmon resonance (SPR) to stabilize an intermolecular interaction between the peptidyl carrier protein (PCP) and C domains in EntB and EntF, respectively, with a dissociation constant of 1-2 nM, whereas the unmodified holo-EntB showed no interaction with EntF. The described condensation domain chemical probes provide powerful tools to study dynamic multifunctional NRPS systems.

Exploiting the unusual effects of fluorine in methodology

Fluorination of organic molecules significantly impacts the basic physicochemical properties of small and large biologically active molecules, agrichemicals, and materials. Thus, the development of synthetic reactions to access these substructures is important for many applied fields of chemistry. However, these fluorine-induced perturbations of chemical properties can inhibit standard chemical transformations, which provides unique challenges for synthetic organic chemists. In addition, the physicochemical properties imparted by fluorinated substituents can enable distinct reactivity patterns relative to non-fluorinated substrates, thus making synthetic organofluorine chemistry a fertile ground for developing new, exciting transformations. In this feature article, we detail our experiences in methodology, wherein fluorinated substrates have enabled unique reactivity patterns relative to non-fluorous substrates. Specifically, we highlight the non-standard chemo- and regio-selectivities imparted by fluorinated substrates on Pd-catalyzed coupling reactions, nucleophilic addition reactions of olefins, and Cu-catalyzed decarboxylative fluoroalkylation reactions.

Palladium-Catalyzed Cross-Coupling of Ethyl Bromodifluoroacetate with Aryl Bromides or Triflates and Cross-Coupling of Ethyl Bromofluoroacetate with Aryl Iodides

A palladium-catalyzed Negishi cross-coupling reaction of ethyl bromodifluoroacetate with aryl bromides or aryl triflates to construct C(sp²)-CF₂ bonds is described. The reaction was conducted under mild reaction conditions, and no preparation of organozinc reagents is required. This is the first report encompassing the conversion of aryl triflates into products containing C-CF₂ bonds. In addition, the construction of C(sp²)-CHF bonds was achieved under mild conditions via a cross-coupling of aryl iodides with ethyl bromofluoroacetate.

Ni(ii)-Catalyzed enantioselective Mukaiyama–Mannich reaction between silyl enol ethers and cyclic N-sulfonyl α-ketiminoesters

The first Ni(ii)-catalyzed Mukaiyama–Mannich reaction of cyclic N-sulfonyl α-ketiminoesters with silyl enol ethers was realized and provided enantioenriched benzofused sultams containing a quaternary α-amino ester.

(1S)-(−)-N-Trifluoromethylthio-2,10-camphorsultam and its derivatives: easily available, optically pure reagents for asymmetric trifluoromethylthiolation

Optically pure reagents that are able to deliver a trifluoromethylthio group with good to excellent enantioselectivities.

Organocatalytic enantioselective Mukaiyama–Mannich reaction of fluorinated enol silyl ethers and cyclic N-sulfonyl ketimines

The first catalytic asymmetric Mukaiyama–Mannich reaction of fluorinated silyl enol ethers and ketimines is developed, allowing highly enantioselective synthesis of benzosultam based β-fluorinated C^α-tetrasubstituted α-amino acid derivatives.

Halogenative difluorohomologation of ketones

A method for the difluorohomologation of ketones accompanied by halogenation of a C-H bond is described. The reaction involves silylation, difluorocarbene addition using Me3SiCF2Br activated by a bromide ion, and halogenation of intermediate cyclopropanes with N-bromo- or N-iodosuccinimide. The whole process is performed without isolation of intermediates. The resulting α,α-difluoro-β-halo-substituted ketones can be readily converted into fluorine containing pyrazole derivatives and oxetanes.

Nickel-catalyzed monofluoromethylation of aryl boronic acids

Aryl boronic acids can be monofluoromethylated under nickel catalysis. The utility of this method is demonstrated by the monofluoromethylation of a borylated and acyl-protected derivative of the statin drug ezetimibe. Mechanistic investigations indicate that a fluoromethyl radical is involved in the Ni(I)/Ni(III) catalytic cycle.

Practical Synthesis of Sultams via Sulfonamide Dianion Alkylation: Application to the Synthesis of Chiral Sultams

[reaction: see text]. A practical synthesis of sultams was developed via intramolecular sulfonamide dianion alkylation. This method has been applied toward the synthesis of chiral sultams, which are synthetically valuable as chiral auxiliaries.

Electrophilic fluorination of pyroglutamic acid derivatives: application of substrate-dependent reactivity and diastereoselectivity to the synthesis of optically active 4-fluoroglutamic acids

Electrophilic fluorination of enantiomerically pure 2-pyrrolidinones (4) derived from (L)-glutamic acid has been investigated as a method for the synthesis of single stereoisomers of 4-fluorinated glutamic acids. Reaction of the lactam enolate derived from 9 with NFSi results in a completely diastereoselective monofluorination reaction to yield the monocyclic trans-substituted alpha-fluoro lactam product 21. Unfortunately, a decreased kinetic acidity in 21 and other structurally related monofluorinated products renders them resistant to a second fluorination. In contrast, the bicyclic lactam 12 is readily difluorinated under the standard conditions described to yield the alpha,alpha-difluoro lactam 24. The difference in reactivity between the two types of related lactams is attributed mainly to the presence or lack of a steric interaction between the base used for deprotonation and the protecting group present in the pyrrolidinone substrates. This conclusion was reached based on analysis of the X-ray crystal structure of 21, molecular modeling, and experimental evidence. The key intermediates 21 and 24 are converted to (2S,4R)-4-fluoroglutamic acid and (2S)-4,4-difluoroglutamic acid, respectively.

Direct Synthesis of Previously Inaccessible Bridgehead Azabicyclics by Intramolecular Cyclization of α-Sulfonamido and α-Sulfonimido Radicals

Syntheses of the first bridgehead sultams and the only known bridgehead disulfonimide are described. Both approaches capitalize on the electrophilicity of alpha-sulfonyl radicals and their propensity to undergo intramolecular ring closure. Where double bonds are concerned, 5-exo and 6-exo pathways operate preferentially as long as structural strain is not excessive. When the reaction center is a carbon-carbon triple bond, the first cyclization gives rise to vinyl radicals that hold sufficient reactivity to capture solvent benzene. In the case of 45, this sequential reaction leads importantly to the introduction of a styrene functionality sufficiently activated to allow a second ring closure to be kinetically feasible. The solid-state structural features of 12 and 17 have been elucidated by X-ray crystallographic methods. Despite key differences from the norm in the alignment of the nitrogen lone pair relative to the adjacent sulfonyl groups, these compounds exhibit good hydrolytic stability. For 13, generation of the alpha-sulfonamide carbanion is possible and regiospecific oxidation with chromyl acetate has been achieved.

Novel phosphate mimetics for the design of non-peptidyl inhibitors of protein tyrosine phosphatases

Benzylic alpha,alpha-difluorosulfonates, alpha,alpha-difluorotetrazoles, and alpha,alpha-difluorocarboxylates of type 5 and 6 were synthesized and examined as potential phosphate biosteres for PTP1B inhibition. The alpha,alpha-difluorosulfonates and alpha,alpha-difluorotetrazoles were found to be more effective inhibitors than the analogous compounds bearing the fluoromalonyl group, a phosphate biostere currently being used for PTP inhibition.