Abnormal Beckmann rearrangement in 23-hydroxyiminodiosgenin acetate

A new transformation of the spiroketal side chain of diosgenin is reported: treatment of 23-hydroxyiminodiosgenin acetate with phosphorous oxychloride in pyridine produced an abnormal Beckmann rearrangement directing to the cleavage of the spiroketal side chain and generating 23,24-bisnorchol-5-enic skeletons: (2′R)-3′-cyano-2′-methylpropyl 3β-acetoxy-16α-chloro-23,24-bisnorchol-5-en-22-oate as the main product, and small amounts of (2′R)-3′-cyano-2′-methylpropyl 3β-acetoxy-16β-hydroxy-23,24-bisnorchol-5-en-22-oate and vespertilin acetate.     

Studies on the Lossen-type rearrangement of N-(3-phenylpropionyloxy) phthalimide and N-tosyloxy derivatives with several nucleophiles

The reaction of N-(3-phenylpropionyloxy)phthalimide (1a) and N-tosyloxy (5a,b) derivatives with nucleophiles was examined and found to give the products via Lossen-type rearrangement. In order to obtain the scope of this reaction mechanism, further studies the reaction of several N-sulfonyloxyimide derivatives with various nucleophiles under similar conditions were carried out and found to afford the corresponding same types of products in high yields.     

[2,3]-Wittig rearrangement approach to iminosugar C-glycosides: 5-epi-ethylfagomine, 2-epi-5-deoxyadenophorine and formal synthesis of indolizidine 167B and 209D

A new strategy for the synthesis of 1,2-dideoxy iminosugar C-glycosides and indolizidines involving highly stereoselective [2,3]-Wittig rearrangement from Garner aldehyde has been developed. This rearrangement yielded an optically pure, highly functionalized key intermediate, which has been further utilized for the synthesis of 5-epi-ethylfagomine, 2-epi-5-deoxyadenophorine, and 5-hydroxymethyl indolizidine.     

Barrelene-semibullvalene rearrangement induced by visible light: synthesis of dicorannulenosemibullvalene dimethyl dicarboxylate

A clean, quick conversion of biscorrannulenobarrelene dicarboxylate to the respective semibullvalene is observed upon irradiation of solutions of the former with a sun lamp. This represents the first reported example of such rearrangement induced by visible light. Hybrid density functional theory calculations predict the exo-endo conformer of the semibullvalene as the most stable, with the bis-exo and bis-endo conformers being higher in energy by ca. 1 and 4 kcal/mol, respectively.     

Synthesis of vinylcyclopropanes by allylation/ring-closing metathesis/Claisen rearrangement

A facile double allylation/ring-closing metathesis/Claisen rearrangement route for preparing vinylcyclopropanes 6 is developed. The efficient synthesis includes O-allylation of α-allyl-α-sulfonylketones 8 with allylic bromides, ring-closing metathesis of diallyl compounds 9 and sequential Claisen rearrangement of the resulting oxepines.     

Asymmetric synthesis of trans-4,5-dioxygenated cyclopentenone derivatives by organocatalyzed rearrangement of pyranones and enzymatic dynamic kinetic resolution

Dioxygenated cyclopentenones are versatile building blocks for the synthesis of several natural products. Herein we report a direct asymmetric synthesis of trans-4,5-dioxygenated cyclopentenone derivatives through base-catalyzed rearrangement of pyranones followed by dynamic kinetic resolution. Milder conditions than previously reported for this rearrangement have been found regarding amine base catalysis, solvent and temperature effects. All data supports a mechanism involving cyclization of an intermediate formed by electrocyclic ring opening of a pyranone-derived enol. We have developed conditions for asymmetric synthesis of trans-4-tert-butoxy-5-hydroxycyclopent-2-enone, in 81% yield and 95% ee, and analogous dioxygenated cyclopentenones, via a lipase induced dynamic kinetic resolution.     

Theoretical study on the mechanism for the thermal rearrangement reactions of 2‐silylethyl acetate H3SiCH2CH2OCOCH3

The thermal rearrangement mechanisms of 2‐silylethylacetate H₃SiCH₂CH₂OOCCH₃ were investigated by ab initio molecular orbital theory for the first time. All structures of reactant, transition states, and products were located and fully optimized at the B3LYP/6‐311+G(d, p) levels, and harmonic vibrational frequencies for the involved stationary points on the potential energy surface were obtained. The reaction pathways were analyzed and confirmed by intrinsic reaction coordinate (IRC) calculations. Furthermore, atomic charges were determined by using the natural bond orbital (NBO) analysis. The calculational results show that H₃SiCH₂CH₂OOCCH₃ can rearrange thermally in two ways. One is [1,3] rearrangement (Reaction A), in which silyl group transfers from carbon to oxygen(in C? O? C) via a four‐membered ring transition state, forming silyl acetate and ethylene, the other way, [1,5] rearrangement (Reaction B), happens with transferring of silyl group from carbon to oxygen (in C?O) via a six‐membered ring transition state, forming the same products as in Reaction A. The energy barriers of the Reactions A and B were calculated to be 188.9 and 191.6 kJ/mol at the B3LYP/6‐311+G(d,p) levels, respectively. Changes in thermodynamic functions (ΔS, ΔH, and ΔG), equilibrium constant K(T), as well as preexponential factor A(T), and reaction rate constant k(T) in Eyring transition state theory were calculated over a temperature range of 200–1600 K, and then thermodynamic and kinetic properties of the reactions were analyzed. It can be suggested that Reactions A and B are noncompetitive, and both happen only at elevated temperature. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Claisen rearrangement induced by low-energy collision of ESI-generated, protonated benzyloxy indoles

The Claisen rearrangement is a well-known process occurring in condensed phase. In the gas-phase protonated allyl phenyl ethers, propargyl phenyl ethers, and N-allyl aniline produced by positive ion chemical ionization undergo Claisen rearrangement. This reaction has been observed even in the case of odd-electron molecular ions. Phenyl allenyl ether molecular ions actually undergo Claisen rearrangement, producing intense [M - CO](+*) ions. In this investigation, the behavior of protonated benzyloxy indole and some of its derivatives, obtained in electrospray conditions, is described. Low-energy MS/MS experiments carried out on [M + H](+) species show CO loss and an unexpected water loss: both can be justified only by the occurrence of Claisen rearrangement. Deuterium labeling experiments confirm this mechanism. The influence of different substituents in the indole moiety is discussed.     

AuCl3/AgSbF6-catalyzed rapid epoxide to carbonyl rearrangement

An efficient epoxide to carbonyl rearrangement using catalytic AuCl₃/AgSbF₆ has been presented. The reactions are fast and high yielding. β-Hydrogen migration takes place exclusively when hydrogen and methyl or substituted methyl groups are present at β-carbon of epoxide. When phenyl/acetyl/benzoyl and hydrogen are available at same carbon atom, migration of the former is preferred over the latter.     

Indirect electrochemical oxidation of piperidin-4-ones mediated by sodium halide-base system

Indirect electrochemical oxidation of 1-N-subsituted piperidin-4-ones in methanol in an undivided cell in the presence of sodium iodide/sodium methoxide system leads to the corresponding α-hydroxyketals in 50-80% substance yield (50-65% current yield). 2,2,6,6-Tetramethylpiperidin-4-one under the same conditions forms a mixture of methyl 2,2,5,5-tetramethyl-3-pyrrolidinecarboxylate and methyl 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrole-3-carboxylate in 70% substance yield (60-70% current yield) via electrochemically induced Favorskii rearrangement.     

Multicatalytic Beckmann rearrangement of 2-hydroxylarylketone oxime: Switchable synthesis of benzo[d]oxazoles and N-(2-hydroxylaryl)amides

A switchable synthesis route is developed for benzo[d]oxazole derivatives and (2-hydroxylaryl)benzamide from 2-hydroxylbenzeneketoxime using organomolecules (BOP-Cl, and CNC) and Lewis acid cocatalyzed Beckmann rearrangement (BR) reaction. Further, this reaction is switched using different organocatalysts.     

苯酞类化合物重排反应的研究——————3－位烷基取代苯 酞的重排反应

发现了3-正丁基苯酞在三氯化铝作用下进行重排反应生成1-甲基-5-羧基四氢萘,并提出了氢迁移的重排反应机制。研究了3-位不同烷基取代的苯酞类化合物在三氯化铝作用下的重排反应。在重排过程中,既有氢迁移,也会有烷基迁移,这取决于形成的碳正离子的稳定性。     

Synthetic studies towards zetekitoxin AB: preparation of 4,5-epi-11-hydroxy-saxitoxinol

A concise synthesis of 4,5-epi-11-hydroxy-saxitoxinol utilizing d-ribose to direct an asymmetric Mannich reaction. This approach allows many modes of reactivity, which can be used to access various analogs of saxitoxin.     

Synthesis and transformations of alkyl N-(1-cyclohex-3-enyl)carbamates prepared from cyclohex-3-ene carboxylic acid via Curtius rearrangement

The Curtius rearrangement of cyclohex-3-ene carboxylic acid using diphenylphosphoryl azide in the presence of triethylamime and ethanol, t-butanol or benzyl alcohol has been described. As a result the synthesis of ethyl, t-butyl or benzyl N-(1-cyclohex-3-enyl)carbamates has been achieved in one pot, in good chemical yield. A series of transformations of benzyl N-(1-cyclohex-3-enyl)carbamate, such as iodination and epoxidation, as well as opening of the corresponding ring epoxide, have been carried out leading to some useful oxygenated cyclohexylamimo building blocks.     

A highly stereoselective transformation of carboxylic esters to trisubstituted olefins via cationic cyclopropyl-allyl rearrangement of sulfonates of cis-1,2-disubstituted cyclopropanols

Methanesulfonates of readily available cis-1,2-disubstituted cyclopropanols on reaction with magnesium bromide in diethyl ether undergo cyclopropyl-allyl rearrangement to afford allyl bromides, with an (E)-trisubstituted double bond, as main products. The amount of the corresponding (Z)-isomers did not exceed 5% when the reaction was performed at 0 °C, and the major concomitant products were the regioisomeric secondary bromides. The latter are sufficiently less reactive towards sulfur, phosphorus and hydrogen nucleophiles than the dominant (E)-disubstituted allyl bromides and these reactions are suitable for the stereoselective preparation of trisubstituted olefins.     

Synthesis of lupiwighteone via a para-Claisen-Cope rearrangement

The lanthanide catalysed para-Claisen-Cope rearrangement has been used as the key step in a short synthesis of the prenylated isoflavone, lupiwighteone. The Mitsunobu reaction was employed for the 5-O-prenylation of the acid/base sensitive acetylated isoflavone to afford the allyl aryl ether precursor, which was then rearranged under mild conditions in good yield. Rearrangement of the isoflavone gave the 8-prenylisoflavone as a single product, in good yield.     

Formal synthesis of ent-dysiherbaine from sulfinyl dihydropyrans by sigmatropic rearrangement and tethered aminohydroxylation

A stereospecific [2,3]-sigmatropic rearrangement of a sulfinyl dihydropyran, followed by a tethered aminohydroxylation reaction, are the key steps of a formal synthesis of ent-dysiherbaine from an enantiopure sulfinyl dienol.