Palladium-catalyzed direct α-arylation of α-fluoroketones: A straightforward route to α-fluoro-α-arylketones

The palladium-catalyzed direct α-arylation of both open-chain and cyclic α-fluoroketones by using P(o-tolyl)₃ or RuPhos as ligand and K₃PO₄·3H₂O as mild base has been developed. This method allows a variety of quaternary α-aryl-α-fluoroketones to be easily prepared.     

Simple, chemoselective, and diastereoselective Reformatsky-type synthesis of α-bromo-α-fluoro-β-lactams

The chemoselective and diastereoselective synthesis of syn-α-bromo-α-fluoro-β-lactams was achieved using the diethylzinc-mediated Reformatsky-type reaction of ethyl dibromofluoroacetate with imines. The reaction led to diastereomerically pure β-lactams in good to moderate yields (up to 78% yield) with only small amounts of aziridine derivatives. Noncyclized 3-amino-2-bromo-2-fluoro carboxylic esters, usual Reformatsky adducts, were not formed. In contrast, reactions carried out under typical Reformatsky conditions using zinc metal were poorly chemoselective, leading to mixtures of β-lactams and aziridine derivatives.     

Synthesis of α-Cyano-β-fluoro-α-hydroxyesters

The ring opening fluorination of glycidic gem-cyanoesters was achieved by action of pyridine polyhydrofluoride at 25°C in dichloromethane. The regioselective nucleophilic substitution reaction allows the synthesis of a new class of fluorohydrins with OH, CN and CO₂R on the same carbon atom.     

Synthesis of α-trifluoromethylstyrene derivatives via Ni-catalyzed cross-coupling of 2-bromo-3,3,3-trifluoropropene and aryl Grignard reagents

The Ni-catalyzed cross-coupling of 2-bromo-3,3,3-trifluoropropene and aryl Grignard reagents was investigated. When NiCl₂(PPh₃)₂ was used as a catalyst, the highest yield of α-trifluoromethylstyrene (89%) from 2-bromo-3,3,3-trifluoropropene and PhMgBr was obtained in 1,3-dimethyl-2-imidazolidinone at 50 °C for 30 min. Various α-trifluoromethylstyrene derivatives could be produced in satisfactory yields by NiCl₂(PPh₃)₂-catalyzed coupling using aryl Grignard reagents.     

Practical synthesis of 16α-bromo-17α-hydroxysteroids via a Raney Ni-catalyzed bromide exchange reaction

D-ring modified glucocorticoids are attractive synthetic targets owing to their broad application in medicinal chemistry. Herein, we reported a practical synthesis of 16α-bromo-17α-hydroxysteroids from easily available 16β-bromo isomers via a Raney Ni-catalyzed bromide exchange reaction. The catalytic Finkelstein-type reaction features high yield, mild reaction condition, short reaction time and simple operation. The method provided an efficient approach to prepare 17α-hydroxy-15-pregnen-20-ones.     

Novel α-arylnitriles synthesis via Ni-catalyzed cross-coupling of α-bromonitriles with arylboronic acids under mild conditions

An applicable and easy-handling Ni-catalyst can be used to promote direct arylation of α-bromonitriles with various arylboronic acids to construct α-arylnitriles under mild conditions. The methodology tolerates β-hydrogens and functional groups in the substrates.     

Selective synthesis of α-fluoro-β-keto- and α-fluoro-β-aminophosphonates via electrophilic fluorination by selectfluor

A series of α-mono- and α,α-difluoro-β-ketophosphonates were synthesized in moderate to good yields with excellent selectivities via electrophilic fluorination by Selectfluor. Subsequently, synthetic potential of the obtained α-monofluoro-β-ketophosphonates was demonstrated by their application in synthesis of α-monofluoro-β-aminophosphonates, useful building blocks in the preparation of phosphapeptides.     

Regioselective synthesis of α-bromo-α,β-unsaturated carbonyl compounds via photocatalytic α-bromination reactions

A visible light-mediated approach for the preparation of α-bromo-α,β-unsaturated ketones and aldehydes was developed. In comparison to traditional methods that generally take two steps to afford the above compounds, this protocol was highlighted by its operational simplicity, avoiding using hazardous bromine and mild reaction conditions.     

Synthesis and reactions of α-fluoro-α-amino amides

N-((S)-1-Phenylethyl)halofluoroethanamides have been investigated as precursors to N-protected α-fluoro-α-amino amides by nucleophilic displacement of halide with nitrogen nucleophiles such as potassium phthalimide, sodium succinimide, sodium glutarimide, trimethylamine and sodium azide. With single diastereoisomers of the iodofluoroethanamide, clean inversion of configuration occurs at room temperature, but subsequent epimerisation may occur as a result of the liberated iodide. The α-fluoro-α-amino amides made underwent a wide variety of reactions depending on conditions, but in many cases the carbon-fluorine bond was compromised. However, reacting trimethylamine and N-((S)-1-phenylethyl)iodofluoroethanamide gave the corresponding α-fluorobetaine amide, and subsequent acidic hydrolysis led to α-fluorobetaine as the first example of an ‘unprotected’ α-fluoroamino acid.     

Enantioselective synthesis of α-fluoro-β(3)-amino esters: synthesis of enantiopure, orthogonally protected α-fluoro-β(3)-lysine

The scope of a tandem conjugate addition-fluorination sequence performed on α,β-unsaturated esters using the enantiopure lithium amide derived from (S)-N-benzyl-N-(α-methylbenzyl)amine, and the electrophilic fluorinating agent N-fluorobenzenesulfonimide has been investigated. Using this method, α-fluoro-β(3)-amino esters can be obtained in up to quantitative yield and 80:20 to >99:1 dr. This simple methodology does not rely on the use of α-amino acids from the chiral pool and thus provides the potential for the preparation of enantiopure α-fluoro-β(3)-amino acids with a wide variety of side chains. Its utility was demonstrated through the synthesis of orthogonally protected (2S,3S)-α-fluoro-β(3)-lysine.     

Stereocontrolled synthesis of α-amino-α'-alkoxy ketones by a copper-catalyzed cross-coupling of peptidic thiol esters and α-alkoxyalkylstannanes

A stereocontrolled synthesis of α-amino-α'-alkoxy ketones is described. This pH-neutral copper(I) thiophene-2-carboxylate (CuTC)-catalyzed cross-coupling of amino acid thiol esters and chiral nonracemic α-alkoxyalkylstannanes gives α-amino-α'-alkoxy ketones in good to excellent yields with complete retention of configuration at the α-amino- and α-alkoxy-substituted stereocenters.     

Enantioselective α-hydrazination of α-fluoro-β-ketoesters catalyzed by chiral nickel complexes

The catalytic enantioselective electrophilic α-hydrazination promoted by chiral nickel complexes is described. Treatment of α-fluoro-β-ketoesters with azodicarboxylates as electrophilic amination reagents under mild reaction conditions afforded the corresponding α-amino α-fluoro-β-ketoesters with high yields (80-96%) and enantioselectivities (up to 78% ee).     

N-heterocyclic carbene catalyzed reactions of α-bromo-α,β-unsaturated aldehydes/α,β-dibromoaldehydes with 1,3-dinucleophilic reagents

Carbenes ring true: N-Heterocyclic carbene (NHC) catalyzed reactions of α-bromo-α,β-unsaturated aldehydes/α,β-dibromoaldehydes with 1,3-dinucleophilic reagents, such as 1,3-dicarbonyl compounds, β-enamino ketones, and β-enamino esters through umpolung processes gave functionalized 3,4-dihydropyranones and 3,4-dihydropyridinones. The availability of the starting materials, lack of external oxidant, and usefulness of the products make this strategy attractive.     

Development of a scalable palladium-catalyzed α-arylation process for the synthesis of a CGRP antagonist

The Pd-catalyzed α-arylation of cycloheptapyridyl ketone is a key complexity-building step in the synthesis of BMS-846372, a CGRP antagonist. A first-generation process utilized Pd(OAc)₂/P^tBu₃·HBF₄ catalyst system with a strong base NaO^tBu. Although this process was demonstrated on multi-kilo scale, the harsh conditions led to non-selective metal catalyzed processes, which generated several operational, quality, and throughput issues. By acquiring detailed knowledge around several important process parameters, we were able to design an efficient and scalable second-generation α-arylation process using a Pd(OAc)₂/RuPhos catalyst system with the weaker base, K₃PO₄ in tert-amyl alcohol. This new weak base process was high yielding, efficient, and superior in several respects compared to the strong base process. The strategy behind the reaction and isolation development and the process considerations important to scaling a catalytic reaction from laboratory to manufacturing scale will be discussed.     

Organocatalytic synthesis of quaternary stereocenter bearing a fluorine atom: enantioselective conjugate addition of α-fluoro-β-ketoesters to nitroalkenes

The catalytic enantioselective conjugate addition reaction of α-fluoro-β-ketoesters to nitroalkenes promoted by chiral bifunctional organocatalysts is described. The treatment of α-fluoro-β-ketoesters with nitroalkenes under mild reaction conditions afforded the corresponding Michael adducts containing a fluorinated quaternary stereogenic center with excellent enantioselectivity (up to >99% ee).     

Total synthesis of sparstolonin B via a palladium-catalyzed aldehyde α-arylation

A concise and convergent total synthesis of sparstolonin B was developed. A palladium-catalyzed aldehyde α-arylation was utilized to construct the carbon skeleton of the natural product. A subsequent simple one-pot procedure effected global deprotection and closure of the final two rings via an unusual autoredox mechanism for the conversion of a bis-hydroquinone intermediate to the natural product. The 6 step synthetic sequence was realized in 18% overall yield.     

Enantioselective trapping of an α-chiral carbanion of acyclic nitrile by a carbon electrophile

An α-chiral nitrile carbanion generated by deprotonation of enantioenriched O-carbamoyl cyanohydrin was trapped in situ with ethyl cyanoformate to give the corresponding ester derivative in 92% yield and 90?:?10 er, providing the first example of trapping of an α-chiral acyclic nitrile carbanion that has been considered to be very configurationally labile.     

Phosphonium supported triphenylphosphine reagent: an improved access to α-fluoro-α,β-unsaturated esters

α-Fluoro-α,β-unsaturated esters 2 were efficiently synthetized via diethylzinc-promoted Wittig reaction using a phosphonium-supported triphenylphosphine SCG-PPh₃1, which possesses similar reactivity as its parent analog triphenylphosphine. The main advantage of this system is the use of a novel low-molecular-weight support that is soluble in solvents of medium polarities for the attachment of reagents and insoluble in solvents of low polarities.     

InX3-catalyzed haloamidation of vinyl arenes: a facile synthesis of α-bromo- and α-fluoroamides

A variety of alkenes are converted into the corresponding α-fluoroamides in high yields by selectfluor^TM in the presence of 10 mol % of InF₃ in nitrile solution. α-Bromoamides are obtained with NBS in the presence of 10 mol % InBr₃ under similar conditions. The use of Lewis acid in haloamidation significantly improved the yields and reaction rates.