Asymmetric organocatalytic monofluorovinylations

Base-Mediated Regioselective [3 + 3] Annulation of Alkylidene Malononitriles with Trifluoromethyl Alkenes via Dual C-F Bond Cleavage

A base-mediated regioselective [3 + 3] annulation of alkylidene malononitriles with trifluoromethyl alkenes was described. The reaction proceeds through sequential intermolecular SN2' and intramolecular SNV-type cyclization by cleaving dual C-F bonds in a trifluoromethyl group, which discriminate multiple carbon-nucleophilic sites using a single base. Various bicycles bearing a monofluorocyclohexene motif were assembled from readily available starting materials under mild conditions via a one-pot cascade approach.

Latest Developments of the Julia-Kocienski Olefination Reaction: Mechanistic Considerations

Since its discovery, the Julia-Kocienski olefination reaction has over past 30 years become one of the key C-C connective methods that is used in late-stage natural product synthesis. The reaction proceeds under mild reaction conditions, with a wide substrate scope and functional group tolerance range and with high (E) selectivity. In this focused review, we discuss the reaction from a mechanistic point of view and disclose key features that play an important role in reaction selectivity. Finally, the mechanistic aspects of the newly developed modification of the Julia-Kocienski reaction, which allows the formation of both (E) and (Z) olefins from the same reaction partners, are discussed.

Photocatalytic Defluorinative Three-Component Reaction of α-Trifluoromethyl Alkenes, Alkenes, and Sodium Sulfinates: Synthesis of Monofluorocyclopentenes

A photocatalytic three-component reaction of α-trifluoromethyl alkenes, electron-rich alkenes, and sodium sulfinates for the synthesis of gem-difluoroalkenes in a radical/polar crossover manner was developed. Due to the strong electron-withdrawing nature of the sulfonyl group, the resultant gem-difluoroalkenes can be converted into various monofluorocyclopentenes via intramolecular base-mediated S_NV reactions in one pot.

Highly Stereoselective Intramolecular Carbofluorination of Internal α,β-Ynones Promoted by Selectfluor

Herein, we report a metal-free intramolecular carbofluorination protocol for the synthesis of tetrasubstituted monofluoroalkenes from internal α,β-ynones and Selectfluor with both high stereoselectivity and broad functional group tolerance. The chelation between tetrafluoroborate anion and the oxygen present in the aldehyde group rendered the reaction highly stereoselective, with the tetrafluoroborate serving as the direct fluorine source. Therefore, with addition of sodium tetrafluoroborate, Selectfluor could be reused several times without sacrificing reactivity.

High Chemo-/Stereoselectivity for Synthesis of Polysubstituted Monofluorinated Pyrimidyl Enol Ether Derivatives

A novel intramolecular Smiles rearrangement of α-fluoro-β-keto-pyrimidylsulfones (usually used as a carbon nucleophile) was developed, providing a versatile avenue for synthesis of tri/tetra-substituted monofluorinated pyrimidyl enol ethers. Among these, diverse (Z)-monofluorovinylsulfones and sulfinates were efficiently assembled by adding extra electrophile and fine-tuning reaction conditions. The process is triggered by a keto-enol tautomerism from enol oxyanion to pyrimidine 2-carbon, completely different from the classical carbon nucleophilic addition reaction approach.

Catalytic asymmetric synthesis of monofluoroalkenes and gem-difluoroalkenes: advances and perspectives

The latest achievements in the catalytic asymmetric synthesis of both monofluoro- and gem-difluoroalkenes are discussed.

Intermolecular fluoroamination of allenes towards substituted vinyl fluorides

Allene difunctionalization: Selectfluor mediated intermolecular aminofluorination of allenes with triazole and tetrazole nucleophiles towards vinyl fluorides.

Highly stereoselective nickel-catalyzed difluoroalkylation of aryl ketones to tetrasubstituted monofluoroalkenes and quaternary alkyl difluorides

A nickel-catalyzed difluoroalkylation of α-C-H bonds of aryl ketones to furnish highly stereo-defined tetrasubstituted monofluoroalkenes or quaternary alkyl difluorides from secondary or tertiary ketones, respectively, has been established. Mechanistic investigations indicated that these C-H fluoroalkylations proceed via a Ni(i)/Ni(iii) catalytic cycle. An obvious fluorine effect was observed in the reaction, and this reaction has demonstrated high stereoselectivity, mild conditions, and broad substrate scopes, thus enabling the late-stage fluoroalkylation of bioactive molecules. This method offers a solution for expedient construction of monofluoroalkenes from readily available materials, and provides an efficient approach for the synthesis of bioactive fluorinated compounds for the discovery of lead compounds in medicinal chemistry.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Stereodivergent synthesis of alkenes by controllable syn-/anti-fragmentation of β-hydroxysulfonyl intermediates

Complementary fragmentation mechanisms of Hawkins and one-pot Julia olefinations were employed for the stereodivergent synthesis of alkenes.

Synthesis and Deployment of an Elusive Fluorovinyl Cation Equivalent: Access to Quaternary α-(1'-Fluoro)vinyl Amino Acids as Potential PLP Enzyme Inactivators

Developing specific chemical functionalities to deploy in biological environments for targeted enzyme inactivation lies at the heart of mechanism-based inhibitor development but also is central to other protein-tagging methods in modern chemical biology including activity-based protein profiling and proteolysis-targeting chimeras. We describe here a previously unknown class of potential PLP enzyme inactivators; namely, a family of quaternary, α-(1'-fluoro)vinyl amino acids, bearing the side chains of the cognate amino acids. These are obtained by the capture of suitably protected amino acid enolates with β,β-difluorovinyl phenyl sulfone, a new (1'-fluoro)vinyl cation equivalent, and an electrophile that previously eluded synthesis, capture and characterization. A significant variety of biologically relevant AA side chains are tolerated including those for alanine, valine, leucine, methionine, lysine, phenylalanine, tyrosine, and tryptophan. Following addition/elimination, the resulting transoid α-(1'-fluoro)-β-(phenylsulfonyl)vinyl AA-esters undergo smooth sulfone-stannane interchange to stereoselectively give the corresponding transoid α-(1'-fluoro)-β-(tributylstannyl)vinyl AA-esters. Protodestannylation and global deprotection then yield these sterically encumbered and densely functionalized quaternary amino acids. The α-(1'-fluoro)vinyl trigger, a potential allene-generating functionality originally proposed by Abeles, is now available in a quaternary AA context for the first time. In an initial test of this new inhibitor class, α-(1'-fluoro)vinyllysine is seen to act as a time-dependent, irreversible inactivator of lysine decarboxylase from Hafnia alvei. The enantiomers of the inhibitor could be resolved, and each is seen to give time-dependent inactivation with this enzyme. Kitz-Wilson analysis reveals similar inactivation parameters for the two antipodes, L-α-(1'-fluoro)vinyllysine (Ki = 630 ± 20 μM; t1/2 = 2.8 min) and D-α-(1'-fluoro)vinyllysine (Ki = 470 ± 30 μM; t1/2 = 3.6 min). The stage is now set for exploration of the efficacy of this trigger in other PLP-enzyme active sites.

Stereospecific Synthesis of Fluoroalkenes by Silver-Mediated Fluorination of Functionalized Alkenylstannanes

The known procedures for the conversion of alkenylstannanes into the corresponding fluoroalkenes suffer from largely variable yields and a limited compatibility with functional groups; most notably, protodestannation becomes a serious issue whenever protic sites are present in the substrate. Outlined in this paper is a convenient alternative with a much improved application profile, which is largely unperturbed by free alcohols and amides of all sorts. Key to success is the use of F-TEDA-PF₆ in combination with non-hygroscopic and bench-stable silver phosphinate (AgOP(O)Ph₂ ) that acts as an essentially neutral, non-nucleophilic promotor and effective tin-scavenger at the same time. This new method opens many opportunities for late-stage fluorination of elaborate compounds far beyond the scope of the literature procedures, as witnessed by the preparation of a fluorinated macrolide antibiotic, a fluorinated prostaglandin derivative, and a set of fluorinated amino acid surrogates and peptide isosteres.

Enantioselective Synthesis of β-Fluoro Amines via β-Amino α-Fluoro Nitroalkanes and a Traceless Activating Group Strategy

Preparation of a range of enantioenriched β-fluoro amines (α,β-disubstituted) is described in which the nitrogen and fluorine atoms are attached to sp3-hybridized carbons. The key finding is a chiral bifunctional Brønsted acid/base catalyst that can deliver β-amino-α-fluoro nitroalkanes with high enantio- and diastereoselection. A denitration step renders the nitro group "traceless" and delivers secondary, tertiary, or vinyl alkyl fluorides embedded within a vicinal fluoro amine functional group. A synthesis of each possible stereoisomer of a β-fluoro lanicemine illustrates the potential ease with which fluorinated small molecules relevant to neuroscience drug development can be prepared in a stereochemically comprehensive manner.

E- or Z-Selective synthesis of 4-fluorovinyl-1,2,3-triazoles with fluorinated second-generation Julia-Kocienski reagents

A highly modular approach to N-substituted 4-(1-fluorovinyl)triazoles is described. In situ desilylation and Cu-catalyzed ligation reaction of TMS-protected α-fluoropropargyl benzothiazole sulfone with aryl, alkyl, and metallocenyl azides furnished second-generation Julia-Kocienski reagents in good to excellent yields. Condensation reactions of these reagents with aldehydes can be tuned to yield E or Z-alkenes selectively. Under mild conditions with DBU as the base, reactions of aldehydes furnished E-alkenes as the major isomer. On the other hand, in condensation reactions with LHMDS as the base and in appropriate solvents, both aldehydes and ketones reacted to yield fluoroalkenes with Z-selectivity. Stereochemical assignment of E/Z olefins obtained in the reaction of a ketone with two Julia reagents was performed via X-ray crystallographic analysis and comparisons of NMR data. The method allows efficient and ready diversification of the N1-substituent and substituents at the double bond.

Facile one-pot fabrication of a silica gel-supported chiral phase-transfer catalyst—N-(2-cyanobenzyl)-O(9)-allyl-cinchonidinium salt

A novel silica gel-supported cinchona alkaloid-based PTC catalyst was prepared by one-pot synthesis for the first time. Excellent enantioselectivities (72.0–96.9% ee) and 80–96% yields in enantioselective α-alkylation were achieved without significant loss in the catalytic performance for five runs.

Synthesis of regiospecifically fluorinated conjugated dienamides

Modular synthesis of regiospecifically fluorinated 2,4-diene Weinreb amides, with defined stereochemistry at both double bonds, was achieved via two sequential Julia-Kocienski olefinations. In the first step, a Z-α-fluorovinyl Weinreb amide unit with a benzothiazolylsulfanyl substituent at the allylic position was assembled. This was achieved via condensation of two primary building blocks, namely 2-(benzo[d]thiazol-2-ylsulfonyl)-2-fluoro-N-methoxy-N-methylacetamide (a Julia-Kocienski olefination reagent) and 2-(benzo[d]thiazol-2-ylthio)acetaldehyde (a bifunctional building block). This condensation was highly Z-selective and proceeded in a good 76% yield. Oxidation of benzothiazolylsulfanyl moiety furnished a second-generation Julia-Kocienski olefination reagent, which was used for the introduction of the second olefinic linkage via DBU-mediated condensations with aldehydes, to give (2Z,4E/Z)-dienamides in 50%–74% yield. Although olefinations were 4Z-selective, (2Z,4E/Z)-2-fluoro-2,4-dienamides could be readily isomerized to the corresponding 5-substituted (2Z,4E)-2-fluoro-N-methoxy-N-methylpenta-2,4-dienamides in the presence of catalytic iodine.

Enantioselective synthesis of γ-tetrasubstituted nitrosulfonyl carboxylates and amides vial-tert-leucine-derived-squaramide catalyzed conjugate addition of nitrosulfones to acrylates and acrylamides

A t-leucine-derived squaramide catalyzed reaction of α-nitrosulfones with acrylates/acrylamides provides γ-tetrasubstituted γ-nitro-γ-sulfonyl carboxylates/amides in excellent yield and enantioselectivity.

Recent developments in asymmetric phase-transfer reactions

Phase-transfer catalysis has been recognized as a powerful method for establishing practical protocols for organic synthesis, because it offers several advantages, such as operational simplicity, mild reaction conditions, suitability for large-scale synthesis, and the environmentally benign nature of the reaction system. Since the pioneering studies on highly enantioselective alkylations promoted by chiral phase-transfer catalysts, this research field has served as an attractive area for the pursuit of "green" sustainable chemistry. A wide variety of asymmetric transformations catalyzed by chiral onium salts and crown ethers have been developed for the synthesis of valuable organic compounds in the past several decades, especially in recent years.

Stereoselective synthesis of conjugated fluoro enynes

Metalation-electrophilic fluorination of TMS- and TIPS-protected 1,3-benzothiazol-2-yl (BT) propargyl sulfones gave corresponding BT fluoropropargyl sulfones, Julia-Kocienski reagents for the synthesis of fluoro enynes. Both reagents reacted with aldehydes under mild DBU- or LHMDS-mediated conditions, giving high yields of conjugated fluoro enynes with E-stereoselectivity. In comparison to DBU-mediated reactions, stereoselectivity was higher in low-temperature LHMDS-mediated reactions. Two ketones were shown to react as well, using LHMDS as base. In situ removal of the TMS group gave terminal conjugated 2-fluoro 1,3-enynes. Synthetic utility of the fluoro enynes was demonstrated by conversion to internal alkynes and to stereoisomeric fluoro dienes via Sonogashira and Heck couplings.

Practical synthesis of β-oxo benzo[d]thiazolyl sulfones: scope and limitations

In this paper, we discuss our new synthetic approach towards functionalized benzo[d]thiazolyl (BT) sulfones, based on the reunion of alkyl BT sulfones and various electrophiles (e.g. R-CO-X, RO-CO-X, RS-CO-X, Ts-X…). All important aspects of this coupling reaction, including relevant and undesirable side reactions, are evaluated by means of calculations and competitive experiments. The scope and limitations of this method are established.