Stereoselective synthesis of the C7C21 segment of epothilone A via asymmetric alkoxyallyl- and crotylboration

The synthesis of the C₇C₂₁ fragment of epothilone A involving asymmetric alkoxyallyl- and crotylboration using α-pinene-derived reagents is described.     

Synthesis of the human adrenal cortex hormone ( h ACTH) with a revised amino acid sequence

We demonstrated recently that the amino acid sequence of human ACTH (α_h-ACTH¹) differs in four positions from the structure reported in 1961 by Lee and coworkers. The synthesis of the revised sequence and a method of verifying the identity of the synthetic peptide with the naturally occurring hormone are described. The two preparations can most readily be compared by examining the mixture of fragments obtained upon degradation with trypsin. In this mixture, fragments 22–39 of natural α_h-ACTH and of the synthetic peptide are identical and easily distinguishable from the corresponding synthetic fragment of the previously postulated sequence. This is demonstrated by thin layer-chromatography and -electrophoresis as well as by reference to the rate of deamidation under alkaline conditions.     

Komplexkatalyse. X. α-Olefin-Einschub in die Trihapto-allyl-nickel(II)-Gruppierung: Synthese und Eigenschaften von Bis(pentahapto-1,8-nonadien-5-yl-nickel(II)-difluorophosphat)

Complex Catalysis. X. α-Olefin Insertion into the Trihapto-allyl-nickel(II) Group: Synthesis and Properties of Bis(pentahapto-1,8-nonadiene-5-yl-nickel(II)-difluorophosphate) By reaction of (h³-C₃H₅)₂Ni with difluorophosphoric acid in ether under addition of 1,5-hexadiene bis(pentahapto-1,8-nonadiene-5-yl-nickel(II)-difluorophosphate) could be synthesized in good yield, proving thereby preparatively the 1,2-addition of the trihapto-allyl-nickel(II) fragment to the double bond of an α-olefin for the first time.     

Effect of chromium(III) oxide on corundum formation from products of shock-wave synthesis of aluminum oxide

Formation of α-Al₂O₃ in thermal treatment of ultradispersed aluminum oxide powders produced by shock-wave synthesis with addition of Cr₂O₃-(NH₄)₂Cr₂O₇ as a precursor was studied. The acceleration of the conversion of δ-and θ-Al₂O₃ to α-Al₂O₃ upon introduction of additives in the stage of synthesis of aluminum oxide was examined.     

A FACILE SYNTHESIS OF CONJUGATED ACETYL KETONES BY Pd(II)-Cu(I) DOPED KF/Al2O3-CATALYZED UNDER MICROWAVE IRRADIATION

A rapid and efficient method for synthesis of α,β-conjugated acetylenic ketones involves reaction ofphenylacetylene with aroyl chlorides in the presence of acatalytic amount Pd(PPh₃)₂Cl₂-CuIdoped KF/Al₂O₃ under microwave irradiationto give the corresponding α,β-conjugated acetylenicketones 3a–g in 81–94% yield.     

Efficient one-pot synthesis of α-aminophosphonates from aldehydes and ketones catalyzed by ytterbium(III) triflate

An efficient three component one-pot synthesis of N-silylated α-aminophosphonates and α,α-disubstituted α-aminophosphonates was developed using Yb(OTf)₃ as a catalyst at room temperature under mild conditions.     

Synthesis and transformations of metallacycles 46. Catalytic cycloalumination reaction in the synthesis of bis(phospholanes)

An efficient one-pot synthesis of bis(phospholanes) was elaborated, which included a sequential Cp₂ZrCl₂-catalyzed cycloalumination of α,ω-diolefins with AlEt₃, giving the corresponding bis(aluminacyclopentanes), and their in situ reaction with dihalophosphines to furnish the target α,ω-bis(phospholanes). The reaction of these compounds with H₂O₂ or elementary sulfur gave the corresponding bis(phospholane 1-oxides) and bis(phospholane 1-sulfides).

     

Studies towards the total synthesis of (−)-callystatin A

The synthesis of C₁-C₁₂ and C₁₃-C₂₂ fragments of (−)-callystatin A is accomplished employing desymmetrization strategy for the creation of five chiral centres of the polypropionate fragment and application of cross-metathesis (CM) reaction for the first time for this molecule.     

Dehydroascorbic acid (DHAA) capped magnetite nanoparticles as an efficient magnetic organocatalyst for the one-pot synthesis of α-aminonitriles and α-aminophosphonates

A dehydroascorbic acid capped magnetite (DHAA-Fe₃O₄) catalyst is prepared and used for the one-pot synthesis of α-aminonitriles and α-aminophosphonates. Different derivatives of these compounds are synthesized in good yields.     

Dibromomethane as one-carbon source in organic synthesis: total synthesis of (±)- and (−)-methylenolactocin

A general method was developed to construct monocyclic α-methylene-γ-butyrolactone moiety. The key step is to introduce the α-methylene group by the ozonolysis of mono-substituted alkenes followed by reacting with a preheated mixture of CH₂Br₂-Et₂NH. Application of this key step in the total synthesis of the (±)- and (−)-methylenolactocin was described.     

Formal total synthesis of (−)-spongidepsin

The formal total synthesis of (−)-spongidepsin is described. Three fragments I, II, and III were first prepared from readily available starting materials and then assembled to the target compound. The key steps involved in the synthesis are asymmetric α-hydroxylation, Ender's alkylation, and ring-closing metathesis reactions. An alternative route for the fragment II is also achieved involving Sharpless asymmetric epoxidation and Gilman's alkylation as key reactions.     

Synthesis of the methylene bis-tetrahydropyran motif of (−)-exiguolide

A chiral-pool approach for the construction of methylene bis-tetrahydropyran subunit, C₁-C₁₆ fragment, of (−)-exiguolide is described. The synthesis was efficiently accomplished starting from l-glutamic acid and l-aspartic acid involving oxa-Michael reaction and an aldol-driven-reductive etherification as key steps for the formation of tetrahydropyran ring.     

The stereoselective total synthesis of (−)-cleistenolide

The stereoselective total synthesis of (−)-cleistenolide is described employing the Barbier allylation, MacMillan α-hydroxylation, Stille-Gennari olefination, and CeCl₃·7H₂O mediated lactonization as key steps.     

Synthesis and desilylation of some bis(trimethylsilyl)alkenes and polymers bearing bis(silyl)alkenyl groups

The synthesis of various vinylbis(silanes) from some aryl and heteroaryl aldehydes and (Me₃Si)₃CLi in Et₂O is described. Friedel-Crafts reaction of 1,1-bis(trimethylsilyl)-2-(2-naphthyl)ethene with various acyl chlorides (RCOCl, R = Me, Et, i-Pr, i-Bu, n-pent) gave the corresponding α-silyl-α,β-unsaturated enones with high E steroselectivity. Moreover, poly(styrene)-co-[2,2-bis(trimethylsilyl)ethenyl(styrene)] obtained via the reaction of polymers bearing pendant enone functions and (Me₃Si)₃CLi, reacts with the same acyl chlorides in the presence of catalytic amount of AlCl₃ to give the new macromolecules bearing α-silyl-α,β-unsaturated enones and α,β-unsaturated enones.     

Amino-zinc-ene-enolate cyclisation: a short access to (2S,3R)- and (2S,3S)-3-benzylprolines (3-benzylpyrrolidine-2-carboxylic acids)

The synthesis of 3-benzylprolines can be easily achieved in a diastereoselective and enantioselective way via amino-zinc-ene-enolate cyclisation. Transmetalation of the cyclic zinc intermediate with Pd₂(dba)₃/P(o-Tolyl)₃ allowed functionalisation with an aromatic ring. One-pot hydrogenolysis and Boc-protection led to the cis-isomer readily usable for peptide synthesis. The trans-isomer was obtained by epimerisation of the α-centre in a sealed tube at 200 °C.     

Synthesis of functionalized α-CF3-α-aminophosphonates via Cu(I)-catalyzed 1,3-dipolar cycloaddition

A convenient method for the preparation of α-CF₃-α-aminophosphonates bearing alkynyl group at the α-carbon atom has been described. New alkynylphosphonates have been further utilized in the synthesis of functionalized triazole-containing α-CF₃-α-aminophosphonates via copper-catalyzed (3+2)-cycloaddition to different organic azides.     

Dibromomethane as one-carbon source in organic synthesis: total synthesis of (±)-canadensolide

A diastereoselective total synthesis of (±)-canadensolide is described. The key step is to introduce the α-methylene group by the ozonolysis of mono-substituted alkenes followed by reaction with a preheated mixture of CH₂Br₂-Et₂NH.     

Stereoselective Michael-alkylation and Michael-oxidation reactions of chiral 1,3-dioxolanones

A highly diastereoselective Michael-alkylation/oxidation methodology has been developed for the synthesis of optically active α-hydroxy-1,5-diester subunits. Inverse stereochemistry at the C_2′ position could be achieved by using a Michael acceptor equipped with a suitable group followed by a highly stereoselective protonation. This methodology has been applied to the enantioselective synthesis of the upper fragment of (+)-retusine.     

Synthesis of α1-(Cbz-aminoalkyl)-α2-(hydroxyalkyl)phosphinic esters

A synthesis of α₁-(Cbz-aminoalkyl)-α₂-(hydroxyalkyl)phosphinic esters was achieved by the 1,2-addition of the appropriate aldehyde to Cbz-protected phosphinic analogues of amino acid esters in the presence of at least three equivalents of trimethylsilyl chloride and NEt₃. The complete deprotection of the product esters could be achieved in one step using 35% HBr in acetic acid.     

The asymmetric sythesis of (-)-actinonin using the iron chiral auxiliary [(η5-C5H5)Fe(CO)(PPh3)]

The asymmetric synthesis of the α-pentyl succinate fragment of (-)-Actinonin is achieved using the chiral iron acetyl S-(+)-[(η⁵-C₅H₅)Fe(CO)(PPh₃)COCH₃] and subsequently converted to (-)-Actinonin in an overall yield of 41%.