Showing papers on "Claisen rearrangement published in 1997"
147 citations
TL;DR: In this article, N-protected amino acid allylic esters with LDA at −78°C and subsequent addition of a metal salt presumably results in the formation of a chelated metal enolate that undergoes Claisen rearrangement upon warming up to room temperature, giving rise to γ, Δ-unsaturated amino acids.
Abstract: Deprotonation of N-protected amino acid allylic esters with LDA at −78°C and subsequent addition of a metal salt presumably results in the formation of a chelated metal enolate that undergoes Claisen rearrangement upon warming up to room temperature, giving rise to γ,δ-unsaturated amino acids. Many different metal salts can be used for chelation, but in general the best results are obtained with zinc chloride. Due to the fixed enolate geometry, as a result of chelate formation, and a strong preference for the chair-like transition state, the rearrangement proceeds with a high degree of diastereoselectivity. This method can be applied to acyclic as well as to cyclic substrates, and even to peptides, and allows the synthesis of amino acids containing quaternary carbon centers. Esters of chiral allylic alcohols produce optically active amino acids via chirality transfer from the ester moiety to the α-carbon center of the amino acid. Another possibility to introduce chirality is given by the rearrangement in the presence of chiral ligands. The best results are obtained if Al(OiPr)3 is used as the chelating metal salt and the Chinchona alkaloids as bidentate ligands. Quinine gives rise to (2R)-configured amino acids, whereas quinidine delivers the (2S) derivatives in very high yields and excellent enantioselectivities (80–93% ee). The ligands can easily be recovered nearly quantitatively from the reaction mixture.
68 citations
TL;DR: The mechanism of the thermal rearrangement of 1-aryl-5-allyloxytetrazoles to give 1aryl-4-allyltetrazolones with a very high yield has been investigated through kinetic studies in one672polar and one less polar solvent as discussed by the authors.
Abstract: The mechanism of the thermal rearrangement of
1-aryl-5-allyloxytetrazoles 1 to give 1-aryl-4-allyltetrazolones 2 in
very high yield has been investigated through kinetic studies in one
polar and one less polar solvent. The results suggest mainly a concerted
[3,3] sigmatropic process, in which a partially positively charged allyl
group migrates from oxygen to nitrogen, similar to the polar transition
state found in the Claisen rearrangement.
63 citations
TL;DR: In this article, transition structures, activation energies, and reaction energies were calculated by ab initio quantum mechanical methods for the Claisen rearrangements of five hydroxy-substituted allyl vinyl ethers.
Abstract: Transition structures, activation energies, and reaction energies were calculated by ab initio quantum mechanical methods for the Claisen rearrangements of five hydroxy-substituted allyl vinyl ethers. The RHF, DFT(Becke3LYP), and CASSCF methods with the 6-31G* basis set were carried out. There is good agreement with activation energies measured for alkoxy-substituted compounds. The activation energies were separated into thermodynamic and intrinsic effects using Marcus theory as adapted by Murdoch for pericyclic reactions. Intrinsic effects were analyzed by frontier molecular orbital theory. The deuterium kinetic isotope effects calculated at the CASSCF/6-31G* level for the 2-OH allyl vinyl ether are in good agreement with the experimental results for the 2-OSiMe3 derivative, and these calculated isotope effects show much more bond-breaking and less bond-making than those at both the RHF/6-31G* and Becke3LYP/6-31G* levels.
61 citations
TL;DR: In this paper, a thermodynamic investigation of the Claisen rearrangement of chorismate2-(aq) to prephenate2(aq) has been performed using microcalorimetry and high-performance liquid chromatography.
Abstract: A thermodynamic investigation of the Claisen rearrangement of chorismate2-(aq) to prephenate2-(aq) has been performed using microcalorimetry and high-performance liquid chromatography. The study used a well-characterized monofunctional chorismate mutase from Bacillus subtilis that was devoid of prephenate dehydrogenase and prephenate dehydratase activities. The calorimetric measurements led to a standard molar enthalpy change = −(55.4 ± 2.3) kJ mol-1 at 298.15 K for this reaction. An estimated value of the standard molar entropy change = 3 J K-1 mol-1 for the above reaction was used together with the experimental value of to obtain a standard molar Gibbs free energy change ≈ −56 kJ mol-1 and an equilibrium constant K ≈ 7 × 109 for the conversion of chorismate2-(aq) to prephenate2-(aq) at 298.15 K. Thus, for all practical purposes, this reaction can be considered to be “irreversible”. Quantum mechanics (Gaussian 94 with a B3LYP functional and a 6-31G* basis set) was used to calculate values of absolute and...
55 citations
TL;DR: In this article, the authors describe a catalytic enanti-selective synthetic route to the bicyclic tetraene ester 3, a key intermediate for the synthesis of the naturally occurring adenosine diphosphate transport inhibitor atractyligenin.
Abstract: In this paper, we describe a novel catalytic enantioselective synthetic route to the bicyclic tetraene ester 3, a key intermediate for the synthesis of the naturally occurring adenosine diphosphate transport inhibitor atractyligenin (2). The success of this route depended on the extension of the oxazaborolidine-catalyzed (CBS) reduction of an achiral β-stannyl-substituted α,β-enone (6c) to form a chiral allylic alcohol and further steps to effect simultaneous transfer of chirality, carbocycle formation, and quaternary stereocenter formation, which led to the triene acid 13. The conversion of 13 to 3 was carried out efficiently by a four-step sequence involving iodolactonization, double elimination, and esterification. The combined use of the CBS reduction of appropriate α,β-enones and Claisen rearrangement provides an important synthetic avenue to many types of natural products containing quaternary stereocenters embedded in cyclic networks.
55 citations
TL;DR: In this article, model studies towards the synthesis of the cytotoxic marine natural product diazonamide A are described, and three 3-phenylbenzo[b]furan derivatives 5, 8 and 10 were prepared using rhodium(II) catalysed decomposition of the diazophenylacetate 3, Claisen rearrangement of the ether 7, and a classic intramolecular Friedel-Crafts reaction as key steps.
Abstract: Model studies towards the synthesis of the cytotoxic marine natural
product diazonamide A are described. Three model
3-phenylbenzo[b]furan derivatives 5, 8 and 10 were prepared
using rhodium(II) catalysed decomposition of the
diazophenylacetate 3, Claisen rearrangement of the ether 7, and a
classical intramolecular Friedel–Crafts reaction as key steps.
Only the aromatic benzofuran system proved satisfactory in palladium
coupling reactions; diazoacetyl(benzofuranyl)indole 18 was prepared by
Suzuki coupling of (benzofuran-7-yl)boronic acid 11 with 4-bromoindole
14 to give 17, followed by diazo-transfer. Rhodium(II)
catalysed decomposition of 18 in acetonitrile resulted in the formation
of the 3,4-bridged indole 19 rather than the desired oxazole 20.
55 citations
TL;DR: In this paper, an enantioselective synthesis of (+)-epoxydictymene (1) is reported, which efficiently constructs the strained oxabicyclo[3.0] octane subunit of 1.3.
Abstract: An enantioselective synthesis of (+)-epoxydictymene (1) is reported. Condensation of the enantiopure aldehydo ester 5 with (S)-3-isopropylcyclopentenyllithium proceeds selectively to afford 13. Once this lactone was methylenated with the Tebbe reagent, the newly formed allyl vinyl ether was induced into Claisen rearrangement under catalysis with triisobutylaluminum. Sequential hydroboration−oxidation of the resulting dicyclopentacyclooctenone derivative was followed by angular methylation and deoxygenation of the carbonyl functionality. Following epimerization at C-11, an α-hydroxyl was introduced regio- and stereoselectively. Some functional group manipulation led to 57 and 58, both of which underwent efficient cyclization to deliver the complete framework of the target molecule when irradiated with visible light in cyclohexane solution containing iodosobenzene diacetate and iodine. The generality of this key reaction, which efficiently constructs the strained oxabicyclo[3.3.0]octane subunit of 1, is dem...
53 citations
52 citations
TL;DR: In this article, the Hartree−Fock method was applied to separate the intrinsic and thermodynamic contributions to the activation energies of 2-and (E)-6-substituted allyl vinyl ethers with CN, CH3 and NH2 groups.
Abstract: Transition structures for the Claisen rearrangements of 2- and (E)-6-substituted allyl vinyl ethers with CN, CH3, and NH2 groups were located with the Hartree−Fock method and the 6-31G* basis set. Energies were obtained using MP2 single points and B3LYP/6-31G* calculations. A Marcus theory type analysis was applied to separate the intrinsic and thermodynamic contributions to the activation energies. The calculations predict a decrease in the activation barrier for the 2-CN, 2-NH2, and 6-NH2 derivatives, while a large increase in the activation energy is predicted for the 6-CN derivative. The 2-CN and 6-NH2 groups decrease the barrier height due to intrinsic contributions, while the rest are controlled by thermodynamic factors. To estimate solvent effects on rates, SCRF calculations for benzene and acetonitrile were performed.
49 citations
TL;DR: In this paper, a three-step synthesis of (R)-(−)-baclofen is described, where the key step is the orthoester Claisen rearrangement of enantiopure allylic alcohol (S,E)- (−)- 1a, affording γ,δ-unsaturated ester with high stereoselectivity.
Abstract: A three step synthesis of (R)-(−)-baclofen is described. The key step is the orthoester Claisen rearrangement of enantiopure allylic alcohol (S,E)-(−)- 1a , affording γ,δ-unsaturated ester (S,E)-(+)- 6 with high stereoselectivity. This latter derivative is converted into ( R )-(−)-baclofen through a high yield one-pot reaction.
TL;DR: In this paper, two different stragies for the synthesis of pentacyclic Strychnos alkaloids in enantiomerically pure form are explored, which involve the use of enantiopure 2-(4-piperidylmethyl)indoles prepared by kinetic resolution of 1-(3-pyridyl)ethanol, followed by partial reduction of the pyridine ring to the tetrahydropyridine level, Claisen rearrangement of the resulting allylic alcohol, and finally Smith indolization.
Abstract: Two different stragies for the synthesis of pentacyclic Strychnos alkaloids in enantiomerically pure form are explored. Both of them involve the use of enantiopure 2-(4-piperidylmethyl)indoles prepared by kinetic resolution of 1-(3-pyridyl)ethanol, followed by partial reduction of the pyridine ring to the tetrahydropyridine level, Claisen rearrangement of the resulting allylic alcohol, and finally Smith indolization. Whereas 2-(4-piperidylmethyl)indole 6 could not be converted to tetracyclic ABDE substructures of Strychnos alkaloids, photocyclization of chloroacetamide 14 , derived from (piperidylmethyl)indole 13 , satisfactorily afforded the stemmadenine-type tetracycle 15 , which was then converted to the alkaloids (−)-tubifoline, (−)-tubifolidine, and (−)-19,20-dihydroakuammicine.
TL;DR: In this article, a series of crownophanes containing two phenolic moieties within a macrocyclic ring have been successfully synthesized by a one-step thermal reaction from the corresponding macro-cyclic polyethers by using a "Tandem Claisen Rearrangement".
TL;DR: The ortho ester Claisen rearrangement of trisubstituted allylic alcohols exhibits significant levels of diastereoselection and the relative stereochemistry of the syn isomer was confirmed independently by the synthesis of the mycotoxin botryodiplodin.
Abstract: The ortho ester Claisen rearrangement of trisubstituted allylic alcohols exhibits significant levels of diastereoselection. In E allylic alcohols, a 1,3-diaxial interaction develops in the chairlike transition state leading to the anti isomer, rendering the reaction syn selective by a factor of 3−5 to 1. In Z allylic alcohols, the 1,3-diaxial interaction develops in the transition state leading to the syn isomer, generating an anti:syn selectivity of 6−15 to 1. The relative stereochemistry of the syn isomer was confirmed independently by the synthesis of the mycotoxin botryodiplodin.
TL;DR: In this paper, the synthesis of the aromatic segment 4 was achieved starting from commercially available 5-hydroxyisophthalic acid (6) by utilizing Claisen rearrangement of 9, bromolactonization of 12, and modified Curtius rearrangements of 16 as the key steps.
TL;DR: A highly enantioselective and diastereoselectives Claisen rearrangement of N-arylimidates derived from an axially chiral binaphthylamine auxiliary is reported.
Abstract: A highly enantioselective and diastereoselective Claisen rearrangement of N-arylimidates derived from an axially chiral binaphthylamine auxiliary is reported. Upon deprotonation of the imidates with lithium diethylamide, the resultant azaenolates rearrange at 0 °C to give anti α,β-disubstituted, γ,δ-unsaturated N-binaphthyl amides. A iodolactonization/zinc reduction sequence readily converts these amides into the corresponding carboxylic acids of 91−95% ee and allows an efficient recovery of the chiral auxiliary.
TL;DR: In this article, a highly enantioselective aromatic Claisen rearrangement was achieved by the reaction of catechol mono allylic ethers with chiral boron reagent.
TL;DR: In this article, a series of monofunctional propargylic compounds were investigated using 1 H and 13 C nuclear magnetic resonance, high performance liquid chromatography, Fourier transform infrared spectroscopy, and thermal analysis in the temperature range of 50-350°C.
TL;DR: The cis-fused decalin segment 6 required for the total synthesis of popolohuanone E (1) was efficiently synthesized starting from the enantiomerically pure (−)-Wieland-Miescher ketone derivative 9; the method features ortho ester Claisen rearrangement of 15 and Ir-catalyzed hydrogenation of 17.
TL;DR: In this paper, a convenient procedure for the preparation of allyl vinyl ethers is described, which is the precursors for the Claisen rearrangement, and a procedure has been described for allyl Vinyl ethers.
TL;DR: In this article, a novel type of ketene-Claisen rearrangement in which the precursor is generated in situ by reaction of optically active allyl thioethers with dichloroketene is described.
Abstract: A novel type of ketene-Claisen rearrangement in which the precursor of the rearrangement is generated in situ by reaction of optically active allyl thioethers with dichloroketene is described. A characteristic feature of this rearrangement is the excellent chemoselectivity in favor of allyl thioethers vs. allyl ethers, i.e., exclusive chirality transfer of the allylic sulfur moiety is observed with 12, 13, and 25--27. The cyclic, optically active allyl thioethers (+)-(R)-4 and (−)-(S)-4 and the open-chain allyl thioethers 11--13 rearrange with in situ generated dichloroketene to the optically active thioesters (−)-(S)-28, (+)-(R)-28, and 31-33, respectively. A chirality-transfer of > 99% in the cyclic cases (+)-(R)-4 and (−)-(S)-4, and 96--98 % in the open-chain cases 11--13 is observed. Furthermore, the dichloroketene-Claisen rearrangement is characterized by a high asymmetric 1,2-induction. The chiral allylic sulfides 25--27 give the optically active thioesters 36--38 with a 1,2-induction > 99% as determined by NMR-shift experiments.
TL;DR: Fumagillol, a hydrolysis product of fumagillin, has been synthesized in a highly stereoselective manner utilizing a glycolate Claisen rearrangement and an intramolecular ester enolate alkylation as key steps starting from carbohydrate-based precursor 5.