Regio- and stereoselective synthesis of Z-vinylic tellurides from propargylic alcohols: a route to chiral Z-enynes

tert-Butyldimethylsilyl ethers of propargylic alcohols are hydrotellurated regioselectively to give 1,2-Z-vinylic tellurides. Enantiomerically pure propargylic alcohols give enantiomerically pure vinylic tellurides, which are coupled with alkynes under Pd catalysis to give enantiomerically pure allylic enynols.     

Preparation and reactivity of phenyltelluroalkylphosphine oxides. Vinylic tellurides

Phenyltelluromethyldiphenylphosphine oxide was prepared by the reaction of methyldiphenylphosphine oxide with base and PhTeBr. The telluromethylphosphine oxide reacts with aldehydes and ketones to give the corresponding vinylic tellurides, with preferential E stereochemistry for aryl aldehydes and Z for alkyl aldehydes.     

Chemoselective cross-coupling Suzuki-Miyaura reaction of (Z)-(2-chlorovinyl)tellurides and potassium aryltrifluoroborate salts

An ultrasound-assisted synthesis of functionalized vinylic chlorides is described by palladium-catalyzed cross-coupling reaction of potassium aryltrifluoroborate salts and (Z)-2-chloro vinylic tellurides. This procedure offers easy access to vinylic chlorides architecture that contains sterically demanding groups in good yields.     

Diastereoselective synthesis of γ-hydroxy α,β-epoxyesters and their conversion into β-hydroxy α-sulfenyl γ-butyrolactones

The diastereoselectivity of the nucleophilic epoxidation of γ-hydroxy-α,β-unsaturated esters has been studied. The γ-hydroxy-α,β-unsaturated esters were obtained through treatment of ethyl (E)-4-oxo-2-butenoate with the corresponding Grignard reagent and were used as a racemic mixture. The resulting γ-hydroxy α,β-epoxyesters were treated with thiophenol for transformation into α-phenylsulfanyl trisubstituted γ-butyrolactones. The syn,syn-lactones isomerize easily in basic media into the syn,anti structures. In order to explain this interconversion, a retroaldol-aldol sequence has been proposed and a sulfur-oxygen interaction has been invoked to explain the syn stereochemical preference of the α-sulfured aldols resulting from the intramolecular aldol reaction.     

Synthesis and Reaction of Vinylic Tellurides

Vinylic tellurides were synthesized by the addition of diorganyl tellurides to acetylenes in the presence of a radical initiator or by visible light irradiation. Vinylic tellurides underwent transmetalation by the reaction with triethylaluminium or diethylzinc to form the corresponding vinylmetal reagents with retention of the double bond configuration. Oxidation of vinylic tellurides formed by carbotelluration reactions followed by pyrolysis gave internal acetylenes in good yields.     

One-pot synthesis of aryl butyl tellurides from tellurium tetrachloride and activated aromatics through a solventless step

A solventless preparation of aryl tellurium trichlorides from activated aromatic compounds avoiding the use of hazardous solvents as carbon tetrachloride and chloroform is described. The trichlorides were reduced and alkylated leading to aryl butyl tellurides in a one-pot procedure. Transmetallation of these tellurides with n-butyllithium followed by reaction with benzaldehyde gave the corresponding benzhydrols in good yields.     

gem-Difluoromethylation of α- and γ-ketoesters: preparation of gem-difluorinated α-hydroxyesters and γ-butyrolactones

PhSCF₂SiMe₃ has been demonstrated as difluoromethyl carbanion synthon (⁻CF₂H). It reacts chemoselectively with α- and γ-ketoesters at the keto group in the presence of a catalytic amount of TBAF in THF to give the corresponding α-hydroxy ester adducts as well as γ-gem-difluorophenylsulfanylmethylated-γ-butyrolactones in good yields. Reductive cleavage of the phenylsulfanyl group of these products employing Bu₃SnH/AIBN gives the corresponding gem-difluoromethylated α-hydroxyesters and γ-butyrolactones in good yields.     

Lipase-mediated resolution of the hydroxy-cyclogeraniol isomers: application to the synthesis of the enantiomers of karahana lactone,karahana ether,crocusatin C and γ-cyclogeraniol

A comprehensive study on the lipase PS-mediated resolution of different hydroxy-geraniol isomers is reported. A number of α-, β- and γ-isomers bearing a 2-, 3- or 4-hydroxy functional group were synthesised regioselectively and then submitted to the lipase-mediated kinetic acetylation. The latter experiments showed that the 2-hydroxy isomers 4, 5 and 14 (α, γ and β, respectively) as well as cis-3-hydroxy α-cyclogeraniol 7 and cis-4-hydroxy γ-cyclogeraniol 10 could be easily resolved by this procedure. The enantiomeric purity of the main part of these compounds was increased by recrystallisation and the enantiopure diols obtained were used as building blocks for the synthesis of the natural terpenoids karahana lactone, karahana ether and crocusatin C and for the preparation of the synthetic intermediate γ-cyclogeraniol. The absolute configurations of the enantiomers of the diols 7, 10, 14 and 19 were determined by chemical correlation with the known compounds 40, 41, 39 and 41, respectively.     

Diastereoselective synthesis of enantioenriched homopropargyl amino alcohols from α-dibenzylamino aldehydes and their use as chiral synthons

Homochiral α-dibenzylamino aldehydes, prepared from the corresponding α-amino acids, react with propargyl bromide and zinc in DMF/THF (1:1) or DMF/Et₂O (1:1) at 20 °C to afford, in good yields and dr, homopropargylic 1,2-amino alcohols. anti Diastereomers were always formed as major products in this reaction. These compounds are versatile intermediates for a variety of synthetic targets: γ-amino-β-hydroxy-ketones, 4-amino-1,3-diols, 1,7-diamino-2,6-diols, and ω-amino-δ-hydroxy esters.     

Chemoenzymatic method for the preparation of γ-amino alcohols from phenylfuran-based aldehydes; lipase-catalyzed kinetic resolution of β-hydroxy nitriles

The chemoenzymatic preparation of novel enantiopure phenylfuran-based γ-amino alcohols with N-Boc-protection starting from the corresponding aldehydes is described. Enantiopurity (ee 98–99%) is introduced using Thermomyces lanuginosus lipase as the IMMTLL-T1-1500 preparation with β-hydroxy nitriles in an acylation/alcoholysis sequence in tert-butylmethyl ether.     

Silylene transfer to α-keto esters and application to the synthesis of γ-lactones

Disubstituted α-hydroxy acids have been synthesized by metal-catalyzed silylene transfer to α-keto esters. A range of substituents are tolerated in the transformation with the exception of branched groups at the vinylic position. The α-hydroxy acid products can be converted into γ-lactones using a variety of lactonization conditions.     

Diastereoselective sulfonate-directed carbonyl reduction of γ-keto-sulfonates

The diastereoselective reduction of γ-keto-sulfonates to afford α,γ-substituted γ-hydroxy sulfonates has been investigated. Herein we report the first example of a diastereoselective carbonyl reduction whereby hydride attack is directed via chelation of a neighbouring sulfonate group to a boron atom, thus affording prevalently trans γ-hydroxy sulfonates.     

Palladium-catalyzed cross-coupling of vinylic tellurides and potassium vinyltrifluoroborate salt: synthesis of 1,3-dienes

Here we report an ultrasound-assisted synthesis of stereodefined 1,3-dienes by Suzuki-Miyaura cross-coupling reactions of vinylic tellurides and potassium β-styryl trifluoroborate salt.     

An efficient procedure for the synthesis of 2-N-Boc-amino-3,5-diols

We wish to describe here the diastereoselective reaction between chiral N-Boc-α-amino aldehydes with both achiral allyltrichlorostannanes leading to 1,2-syn-N-Boc-α-amino alcohols, which are easily converted to the corresponding 4-N-Boc-amino-3-hydroxy ketones after treatment with catalytic amounts of OsO₄ in the presence of NaIO₄. After reduction of the carbonyl function, these 4-N-Boc-amino-3-hydroxy ketones were converted to 1-deoxy-5-hydroxy sphingosine analogues.     

Stereoselective synthesis of vinylic (Z)-vic-bis(arylchalcogenides)

Alkyne-titanium complexes 3, readily prepared in situ by the reaction of alkynes with Ti(O-i-Pr)₄/2 i-PrMgCl, react with electrophilic chalcogen species under mild conditions to provide the corresponding addition products in fair to good yields. The obtained vinylic vic-bis(arylchalcogenides) 4 are useful synthetic intermediates for introducing vinyl functions into organic molecules.     

Diastereoselective synthesis of α,β-unsaturated systems

Functionalized Z-vinylic tellurides were used in substitution reactions with lower order cyanocuprates leading to α,β-unsaturated ketones and esters in good yields. In the case of acyclic tellurides, the product was obtained in high diastereoselectivity. The control of the stereoselectivity was achieved by simple change of the reaction temperature.     

Catalytic enantioselective allylation of aldehydes using β-amido functionalized allylstannanes with chiral In(OTf)3/i-Pr-pybox complexes

The enantioselective allylation of aldehydes using a variety of β-amido functionalized allyltributylstannanes proceeded smoothly with good to high yields and enantioselectivities in the presence of 10 mol % of a chiral catalytic complex prepared from In(OTf)₃ and 2,6-bis[(S)-4-isopropyloxazolin-2-yl]pyridine {(S)-i-Pr-pybox}, providing the corresponding chiral γ-hydroxy amides.     

Enantioselective total synthesis of iso-cladospolide B, cladospolide C and cladospolide B from tartaric acid

The enantioselective synthesis of the natural products cladospolide B, cladospolide C, and iso-cladospolide B has been accomplished from tartaric acid. Key reactions in the synthetic sequence include the elaboration of a γ-hydroxy amide derived from tartaric acid via alkene cross metathesis, Yamaguchi lactonization, and ring closing metathesis.     

An efficient synthesis of N-protected threo (2R,3S)-3-amino-1,2-epoxy phenylbutane

A precise and versatile method was developed for the synthesis of threo amino epoxide derivatives, which are useful intermediates for protease inhibitors. It involves the diastereoselective reduction of the carbonyl group of γ-N,N-dibenzyl amino α-hydroxy β-keto sulfide prepared from an amino acid, and its subsequent stereospecific conversion to an amino epoxide via acetoxy halogenation in high yield.     

Stereoselective allylation of acyclic and chiral α-amino-β-Hydroxy aldehydes part 2: Application to the formal synthesis of the polyhydroxylated γ-amino acid (+)-Detoxinine

Stereoselective allylations of acyclic, chiral α-amino-β-hydroxy aldehydes mediated by BF₃·OEt₂ and its application to the formal synthesis of the polyhydroxylated γ-amino acid (+)-detoxinine are described. The reactions of syn-α-NHCbz-β-OTBS substrates mediated by BF₃·OEt₂ afforded syn-selective products. The same reaction conditions gave anti-selective products from syn-α-NCbzBn-β-OTBS substrates. A hydrogen-bonded transition state and Felkin-Anh model have been suggested to account for the stereochemical outcomes of the two reactions, respectively. One of the allylation products was used for the formal synthesis of the polyhydroxylated γ-amino acid (+)-detoxinine.