A highly stereospecific synthesis of (E)-α,β-unsaturated esters

CrCl₂-induced olefination of aldehydes using methyl dichloroacetate exclusively generates (E)-α,β-unsaturated esters in excellent yields. The intermediate α-chloro-β-hydroxy adducts could also be isolated in good yields under conditions of limited reagent.     

Stereoselective synthesis of (Z)-axino- and (Z)-debromoaxinohydantoin

(Z)-Axinohydantoin and (Z)-debromoaxinohydantoin, two pyrrole-imidazole alkaloids isolated from different marine sponges, possess moderate activities in inhibiting the progress of the cell cycle at different phases. A stereoselective synthesis of both natural products was achieved. The key step in the synthetic pathway was the installation of the hydantoin northern ring by using 1-benzoyl-2-methylsulfanyl-1,5-dihydroimidazol-4-one.     

Convenient preparation of (Z)-α-halo-α,β-unsaturated aldehydes: synthesis of a Laurencia flexilis toxin

The CrCl₂-mediated two-carbon halo-homologation of aryl, alkenyl, and aliphatic aldehydes with chloral ethyl hemiacetal or bromal affords (Z)-α-chloro- and (Z)-α-bromo-α,β-unsaturated aldehydes, respectively, in good to excellent yields and high stereoselectivity. The utility of this methodology was illustrated by a synthesis of 2-chloropentadec-2(Z)-enal, a toxin isolated from the marine red alga Laurencia flexilis.     

Stereoselective synthesis of methyl (Z)-α-methoxyacrylates via two-carbon homologation of aldehydes

Methyl (Z)-α-methoxyacrylates are generated in good yields by a mild, stereospecific two-carbon homologation of a wide variety of aldehydes utilizing commercial methyl 2,2-dichloro-2-methoxyacetate and CrCl₂ under Barbier conditions at room temperature. A rational mechanism based upon a Reformatsky-type addition pathway or an in situ generated (E)-trioxo-chromium vinylidene carbenoid is proposed.     

Stereoselective synthesis of fluoroalkenes via (Z)-2-fluoroalkenyliodonium salts

Stereoselective synthesis of fluoroalkenes is described. (Z)-2-Fluoro-1-alkenyl(phenyl)iodonium tetrafluoroborates (1) were synthesized stereoselectively in good yields by Michael-type addition of HF to 1-alkynyl(phenyl)iodonium tetrafluoroborates (2) with a commercially available HF reagent, hydrofluoric acid or Et₃N-3HF. Pd-catalyzed cross-coupling reactions using 1 gave (Z)-2-fluoro-1-alkene derivatives in moderate yields. The treatment of 1 with KI in the presence of a catalytic amount of CuI gave (Z)-2-fluoro-1-iodo-1-alkenes (3). Pd-catalyzed cross-coupling reactions of 3 gave better results than that of 1, and a variety of (Z)-2-fluoro-1-alkene derivatives were synthesized in good yields.     

Pd-catalyzed cross-coupling reaction of (Z)- and (E)-bromoenyne: unusual stereochemical outcome

Pd-catalyzed cross-coupling reaction of (E)-bromoenyne 1Z with 1-alkyne and tributylvinyl-stannane occurs with retention of the configuration in benzene but with an inversion of the configuration in DMF or CH₂Cl₂. On the other hand, that of (Z)-bromoenyne 1E occurs with retention of the configuration in these solvents.     

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.     

Stereoselective preparation of trisubstituted (Z)-alkenes; synthesis of the C17-C27 fragment of (−)-laulimalide

A Ni-catalyzed cross-coupling reaction of (Z)-5-(tert-butyldiphenylsilyl)oxy-3-bromo-1-trimethylsilyl-3-penten-1-yne (1) with alkyl Grignard reagent gives (Z)-3-alkyl-5-(tert-butyldiphenylsilyl)oxy-1-trimethylsilyl-3-penten-1-ynes (2) stereospecifically in good yields. The (Z)-enyne 2a is transformed in four steps to (Z)-3-methyl-5-silyloxy-3-pentenal (3), which is coupled with ketophosphonate 4 to give enone 13. The η-hydroxyallyl methanesulfonate derived from 13 is cyclized to 3,6-dihydro[2H]pyran by an intramolecular SN2′ reaction stereoselectively, furnishing a C17-C27 carbon unit of (−)-laulimalide.     

Stereoselective synthesis of (Z)-β-fluoro-α,β-unsaturated esters from (Z)-2-fluoro-1-alkenyliodonium salts

(Z)-β-Fluoro-α,β-unsaturated esters were stereoselectively synthesized from (Z)-2-fluoro-1-alkenyliodonium salts by the Pd-catalyzed methoxycarbonylation reaction. The reaction proceeded at room temperature and various functional groups on the substrate can tolerate the reaction conditions.     

Reactions of alkynes with trisubstituted (Z)-bromoalkenyl-pinacolboronates towards the synthesis of 1,2,4,6-tetrasubstituted benzenes

Treatment of trisubstituted (Z)-bromoalkenyl-pinacolboronates with terminal alkynes in the presence of cuprous iodide, palladium(0) tetrakis(triphenylphosphine) and Hünig’s base leads to the formation of 1,2,4,6-tetrasubstituted aromatic compounds in a one-pot fashion. Substrate scope was shown for three trisubstituted (Z)-bromoalkenyl-pinacolboronates and four terminal alkynes.     

Stereoselective synthesis of functionalized trisubstituted olefins via palladium(0)-catalyzed cross-coupling reaction

[reaction: see text]. A highly flexible and stereoselective protocol for the synthesis of branched (E)- and (Z)-trisubstituted alkenes has been developed. The key steps are hydrozirconation-iodination of (1-alkynyl)trimethylsilane followed by Negishi-type cross-coupling. The resultant (Z)-vinyl silane is iododesilylated and subjected to a second cross-coupling reaction to give the trisubstituted olefin. Model studies aimed at the construction of the C14-C15 (Z)-trisubstituted olefin of discodermolide and the C8-C9 (Z)-trisubstituted olefin of callystatin A and analogues are also described.     

Efficient and regioselective chromium(0)-catalyzed reaction of 2-substituted furans with diazo compounds: stereoselective synthesis of (2E,4Z)-2-aryl-hexadienedioic acid diesters

Pentacarbonyl(η²-cis-cyclooctene)chromium(0) (1) catalyzes efficiently reactions of diazo compounds with electron-rich furans. The reaction of 2-methoxyfuran (2) with alkyl α-diazoarylacetate (3a-g) furnishes the (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g) in excellent yields. These reactions are highly regioselective. The cyclopropanation intermediates formed from 1 and diazo compounds 3a-g always arise from a carbene addition to the less substituted CC bond of 2. The resulting cyclopropanation product undergoes a ring opening reaction to form the corresponding (2E,4Z)-2-aryl-hexadienedioic acid diesters (4a-g). The pentacarbonylchromium(0)-catalyzed reactions of 2-alkylfuran (5a-b) with ethyl α-diazophenylacetate (3a) and 9-diazo-9H-fluorene (3h) produce the 1(E),3(E)-butadienes (6a-d) in very good yields.     

Chromium mediated stereoselective synthesis of (Z)-1-fluoro-2-alkenyl alkyl and trialkylsilyl ethers from dibromofluoromethylcarbinyl ethers

Reactions of 2,2-dibromo-2-fluoroethyl alkyl ethers and dibromofluoromethylcarbinyl silyl ethers with CrCl₂ and Mn powder provided 2-fluorovinyl alkyl ethers and 1-fluoro-2-alkenyl enol silyl ethers in high yields and in Z selective manner.     

Coordination chemistry of the activation of [(triazacyclohexane)CrCl3] with [PhNMe2H][B(C6F5)4] and AlR3

Complexes of N-substituted 1,3,5-triazacyclohexanes with CrCl₃ form 1:1 adducts with [PhNMe₂H][B(C₆F₅)₄] with increased solubility in toluene. Addition of AlⁱBu₃ leads to free PhNMe₂ and a complex with [B(C₆F₅)₄]⁻ weakly coordinated to chromium via a meta-fluorine atom. This complex can polymerise and/or trimerise olefins similar to methyl aluminoxane activated complexes. Decomposition of the active complex involves transfer of the triazacyclohexane to aluminium leading to [(triazacyclohexane)AlⁱBu₂][B(C₆F₅)₄] and [(arene)₂Cr][B(C₆F₅)₄]. These chromium(I) complexes have been characterised by X-ray crystallography and prove that chromium is reduced to the oxidation state +I during the catalysis.     

Stereoselective synthesis of 2-iodo-1-perfluoroalkyl-2(Z)-alkenes and E or Z-4-perfluoroalkylmethyl-4-en-2-ynol via Na2S2O4-promoted radical addition reaction of perfluoroalkyl iodides with allenes and the palladium-catalyzed kinetic resolution with Sonogashira coupling reaction

A Na₂S₂O₄-promoted radical addition reaction of perfluoroalkyl iodides with allenes has been studied in which a Z/E mixture of 2-iodo-1-perfluoroalkyl-2-alkenes 3 were afforded in 52-69% yields. A kinetic resolution using Sonogashira coupling reaction in MeCN using Et₂NH as the base was developed to synthesize the 2-iodo-1-perfluoroalkyl-2(Z)-alkenes (Z-3) and E-4-perfluoroalkylmethylalk-4-en-2-ynols (E-5) stereoselectively. A complete Sonogashira coupling procedure in Et₂NH at 40 °C was also developed affording a mixture of E and Z-4-perfluoroalkylmethyl-4-en-2-ynols (E-5 and Z-5), which may be easily separated by chromatography on silica gel.     

Solvent-free NbCl5 catalyzed condensation of 1,3-dicarbonyl compounds and aldehydes: a facile synthesis of trisubstituted alkenes

A simple preparation of trisubstituted alkenes by condensation of β-diketones or β-ketoesters with aldehydes in presence of NbCl₅ as a Lewis acid catalyst in solvent-free conditions is described.     

Stereoselective synthesis of (Z)-γ-cyano-β-perfluoroalkyl-β,γ-unsaturated esters

(Z)-γ-Cyano-β-perfluoroalkyl-β,γ-unsaturated esters have been synthesized by the reaction of perfluoroacylated phosphonates with organozinc reagent in 50-71% (three steps) yields.     

The CeCl3·7H2O-NaI system as promoter in the synthesis of functionalized trisubstituted alkenes via Knoevenagel condensation

The arylidene malonates with two different geminal carboxylate functions, a suitable class of substrates of several synthetic and pharmacological studies, are easily available through Knoevenagel condensation of ethyl tert-butyl malonate and different aromatic aldehydes. The results have increased the potentialities of CeCl₃·7H₂O-NaI system as a type of water-tolerant green Lewis acid promoter for carbon-carbon bond forming procedures.     

The stereoselective synthesis of (Z)-HFCCFZnI and stereospecific preparation of (E)-1,2-difluorostyrenes from (Z)-HFCCFZnI via an unusual Pd(PPh3)4-Cu(I)Br co-catalysis approach or (Z)-HFCCFSnBu3

(Z)-HFCCFZnI was stereoselectively synthesized from activated zinc dust and (Z)-HFCCFI that was synthesized from chlorotrifluoroethene in a sequential manner. Compared to (E)-HFCCFZnI, (Z)-HFCCFZnI was more challenging to prepare in terms of sluggish metallation and formation of by-products, and underwent slower and incomplete Negishi coupling with aryl iodides. In a modification of Negishi coupling, (E)-α,β-difluorostyrenes were stereospecifically prepared in good to excellent yields under mild conditions from aryl iodides and (Z)-HFCCFZnI with the co-catalysis of Pd(PPh₃)₄/Cu(I)Br. Experimental investigation and mechanistic rationalization suggested that Cu(I)Br would be a scavenger of free ligands for the facilitation of Pd(PPh₃)₂ formation, and a supplier of ligand for the metathesis process. Alternatively, (Z)-HFCCFSnBu₃ and aryl iodides with an electron-withdrawing group underwent Stille-Liebiskind coupling to afford (E)-α,β-difluorostyrenes.     

Stereoselective synthesis of (E)- and (Z)-3-hydroxy-2-methyl-1-alkenyl iodides by base-promoted ring-opening of iodomethylated epoxides

Both (E)- and (Z)-3-hydroxy-2-methyl-1-alkenyl iodides were stereoselectively synthesized from iodomethylated epoxides by treatment with sodium hexamethyldisilazane in DMF and with LDA in THF (or lithium 2,2,6,6-tetramethylpiperidide in THF), respectively.