A Facile Construction of Eudesmanolide Denebutenolide via SmI2—catalyzed Rearrangement of an Enol Ester Epoxide

A very convenient construction procedure of the α，β-unsaturated lactone has been carried out in 98% yield. SmI2 (10%) was used as catalyst for rearrangement of an enol ester exoxide of eudesmanolide sesquiterpene. The possible mechanism was also discussed.     

Reactivite de carbanions β cetophosphorothiolate: Réarrangement phosphorothiolate-phosphate d'énol

The stabilization of S-alkanoate O,O-dialkyl phosphates carbanions has been tried. A enol phosphate-phosphorothiolate rearrangement is observed and the mechanism is described. This new transposition has been applied to the synthesis on various enol phosphates with thioether of enol function of phytosanitaire interest.     

双环烯醚的合成及其性质的研究

由内酯2和Tebbe试剂反应合成了双环烯醚化合物5-苄氧基-2,8-二亚甲基-3,4,9,10-四氢-2H,8H-苯并[1,2-b; 3,4-b']二吡喃(3), 环烯醚在酸性条件下很容易水解和异构化, 在酸存在下外环烯醚3于10 min内转化为内环烯醚4, 较长的反应时间只能得到水解物或其它分解产物.     

Suzuki coupling reactions of 2,4,6-trialkoxyphenylboronic acids with enol triflates: asymmetric synthesis of a lactone template for calyxin assemblage

Suzuki coupling reactions between 2,4,6-trialkoxyphenylboronic acids and enol triflates have been found to occur in excellent yield, while the use of an enol tosylate failed to give any of the desired product. This coupling reaction has led to the synthesis of a lactone which could serve as a precursor to several calyxin analogues.     

Nickel nanoparticles catalyzed chemoselective Knoevenagel condensation of Meldrum’s acid and tandem enol lactonizations via cascade cyclization sequence

A novel protocol has been reported wherein, polyethylene glycol (PEG) stabilized nickel nanoparticles have been used as catalyst for chemoselective Knoevenagel condensation of aromatic aldehydes and Meldrum’s acid to give 5-arylidene Meldrum’s acid which underwent tandem enol lactonization by Michael addition/cyclization sequence with active methylene compounds in the presence of Ni nanoparticles to give corresponding enol lactone derivatives.     

Gas Chromatographic and Mass Spectrometric Study of Trimethylsilyl Ethers of Cardiac Aglycones

Abstract

Cardenolides may be converted to volatile derivatives by silylation. Three levels of silylation have been defined. The use of HMDS-TMCS leads to TMSi ether formation for all relatively unhindered hydroxyl groups (3β, 12β, 16β). When BSA-TMCS is employed, the unsaturated lactone ring is converted to an enol form, with subsequent formation of a TMSi enol ether. All hydroxyl groups, including the tert. 14β-hydroxyl group and the enol derived from the lactone side chain, are converted to TMSi ethers with TSIM-BSA-TMCS at 60°. The derivatives have good gas chromatographic properties. Gas phase analytical methods can be used for the identification and characterization of these substances.     

A general synthesis of substituted indoles from cyclic enol ethers and enol lactones

[reaction: see text] X = CH2, C[double bond]O, R2 = H, alkyl. A general method was developed for the one-pot synthesis of highly functionalized indoles from simple, commercially available aryl hydrazines and cyclic enol ethers. Enol lactones were also used as substrates, affording substituted indole acetic acid or indole propionic acid derivatives. This procedure affords 2,3-disubstituted indoles as single regioisomers from the appropriately substituted enol ether or enol lactone. This method was highlighted in the efficient synthesis of the antimigraine drug sumitriptan and the antiinflammatory drug indomethacin.     

Nouvelle synthese de trifluoromethylcetones et d' /ga-bromo trifluoromethylcetones

We report a new method for the preparation of trifluoromethylketones, as an alternative to the use of organometallics. The condensation of phosphonium ylides with trimethylsilyl trifluoroacetate provides silyloxy enol ethers whose hydrolysis leads to trifluoromethylketones. Bromation of the same silyloxy enol ether is also a convenient preparation of /ga-bromo trifluoromethylketone.     

Lactone enols are stable in the gas phase but highly unstable in solution

2-Hydroxyoxol-2-ene (C(5)-1), the enol tautomer of gamma-butyrolactone, was generated in the gas phase as the first representative of the hitherto elusive class of lactone enols and shown by neutralization-reionization mass spectrometry to be remarkably stable as an isolated species. Ab initio calculations by QCISD(T)/6-311+G(3df,2p) provided the enthalpies of formation, proton affinities, and gas-phase basicities for gaseous lactone enols with four- (C(4)-1), five- (C(5)-1), and six-membered rings (C(6)-1). The acid-base properties of C(4)-C(6) lactones and enols and reference carboxylic acid enols CH(2)=C(OH)(2) (3) and CH(2)=C(OH)OCH(3) (4) were also calculated in aqueous solution. The C(4)-C(6) lactone enols show gas-phase proton affinities in the range of 933-944 kJ mol(-)(1) and acidities in the range of 1401-1458 kJ mol(-)(1). In aqueous solution, the lactone enols are 15-20 orders of magnitude more acidic than the corresponding lactones, with enol pK(a) values increasing from 5.6 (C(4)-1) to 14.5 (C(6)-1). Lactone enols are moderately weak bases in water with pK(BH) in the range of 3.9-8.1, whereas the lactones are extremely weak bases of pK(BH) in the range of -10.5 to -17.4. The acid-base properties of lactone enols point to their high reactivity in protic solvents and explain why no lactone enols have been detected thus far in solution studies.     

7─羟基喹啉与甲醇、乙醇形成1：2桥式氢键化合物的AM1法研究

利用半经验ＡＭ１方法计算了基态与第一激发态７－羟基喹啉的两种异构体及其与甲醇等溶济分子形成１：２桥式氢键化合物的结构与稳定性。在基态，烯醇式结构比酮式结构稳定；而在第一激发态酮式比烯醇式稳定。１：２桥式氢键的形成使得酮式能量降低比烯醇式多。烯醇式１：２桥式氢键化合物呈交叉式结构，酮式１：２桥式氢键化合物呈折叠式结构，激发态的形成对氢键结构影响不大。在７－羟基喹啉羟基（或羰基）的邻位和间位引入取代基后，对喹啉环和桥式氢键结构的影响均不明显。     

The hetero-Diels-Alder addition of ethyl 2-nitrosoacrylate to electron-rich alkenes as a route to unnatural α-amino acids

A new method for the stereoselective synthesis of nonproteinogenic α-amino acids has been developed, which involves hetero-Diels-Alder addition of ethyl 2-nitrosoacrylate to electron-rich alkenes, such as enol ethers, enamines and allylsilanes, and a further two or three step manipulation of the resulting oxazines.     

7─羟基喹啉与甲醇、乙醇形成1：2桥式氢键化合物的AM1法研究

利用半经验ＡＭ１方法计算了基态与第一激发态７－羟基喹啉的两种异构体及其与甲醇等溶济分子形成１：２桥式氢键化合物的结构与稳定性。在基态，烯醇式结构比酮式结构稳定；而在第一激发态酮式比烯醇式稳定。１：２桥式氢键的形成使得酮式能量降低比烯醇式多。烯醇式１：２桥式氢键化合物呈交叉式结构，酮式１：２桥式氢键化合物呈折叠式结构，激发态的形成对氢键结构影响不大。在７－羟基喹啉羟基（或羰基）的邻位和间位引入取代基后，对喹啉环和桥式氢键结构的影响均不明显。     

The Richest Collection of Tautomeric Polymorphs: The Case of 2‐Thiobarbituric Acid

Five new polymorphs and one hydrated form of 2‐thiobarbituric acid have been isolated and characterised by solid‐state methods. In both the crystalline form II and in the hydrate form, the 2‐thiobarbituric molecules are present in the enol form, whereas only the keto isomer is present in crystalline forms I (reported in 1967 by Calas and Martinex), III , V and VI . In form IV , on the other hand, a 50:50 ordered mixture of enol/keto molecules is present. All new forms have been characterised by single‐crystal X‐ray diffraction, 1D and 2D (¹H, ¹³C, and ¹⁵N) solid‐state NMR spectroscopy, Raman spectroscopy and X‐ray powder diffraction at variable temperature. It has been possible to induce keto–enol conversion between the forms by mechanical methods. The role of hydrogen‐bond interactions in determining the relative stability of the polymorphs and as a driving force in the conversions has been ascertained. To the best of the authors’ knowledge, the 2‐thiobarbituric family of crystal forms represents the richest collection of examples of tautomeric polymorphism so far reported in the literature.     

Synthesis of 2-arylacrylic esters from aryl methyl ketones via Wittig reaction/singlet oxygen ene reaction

An efficient synthetic method has been developed for the synthesis of 2-arylacrylic esters from the corresponding aryl methyl ketones via Wittig reaction and singlet oxygen ene reaction. Wittig reaction to aryl methyl ketones with (methoxymethyl)triphenylphosphonium chloride in basic condition afforded the methyl enol ethers, and then 2-arylacrylic esters were obtained by singlet oxygen ene reaction, followed by tosylation and elimination in one-pot to the methyl enol ethers in good yields.     

Enantioselective enol lactone synthesis under double catalytic conditions

[reaction: see text] The reaction of dimedone with 1-(2-alkenoyl)-4-bromo-3,5-dimethylpyrazoles in THF, catalyzed by catalytic amounts of both DBFOX/Ph-nickel(II) perchlorate trihydrate and 2,2,6,6-tetramethylpiperidine, in the presence of acetic anhydride in THF produces the corresponding enol lactones in high enantioselectivities through enantioselective Michael additions followed by cyclization with removal of the pyrazole auxiliary. Other related nucleophile precursors can be successfully applied in the enantioselective enol lactone synthesis under the double catalytic conditions.     

1,4-Benzodioxin-2(3H)-one and some of its derivatives

The methods of synthesis of 1,4-benzodioxin-2(3H)-one have been reexamined. Frequently quoted in the literature the method of Ghosh has been found to give impure (2-hydroxyphenoxy)acetic acid rather than the lactone. Using various methods some simple derivatives of the title lactone substituted in the benzene ring have been prepared and characterized. The bromination of the lactone with NBS gives predominantly 7-bromo-1,4-benzodioxin-2(3H)-one under electrophilic conditions, while 6-bromo-1,4-benzodioxin-2(3H)-one is obtained as a major isomer under photolytic conditions.     

Synthetic approaches to thienamycin: Carbon-carbon bond formation at C-4 of azetidin-2-ones

A mild and efficient method of β-lactam alkylation has been achieved using 4-acetoxy-2-azetidinone and a variety of silyl enol ethers.     

Oxidative cyclization based on reversing the polarity of enol ethers and ketene dithioacetals. Construction of a tetrahydrofuran ring and application to the synthesis of (+)-nemorensic Acid

The utility of oxidative cyclization reactions for the construction of tetrahydrofuran rings has been examined. In these experiments, alcohol nucleophiles were found to be effective traps for radical cation intermediates generated from both enol ether and ketene dithioacetal groups. The reactivity of the alcohol trapping group appeared to lie between that of an enol ether and an allylsilane trapping group. The stereochemical outcome of cyclization reactions originating from the oxidation of an enol ether was found to be controlled by stereoelectronic factors. The utility of these cyclization reactions was illustrated with the synthesis of a key tetrahydrofuran building block for the synthesis of linalool oxide and rotundisine. Cyclization reactions triggered by the oxidation of a ketene dithioacetal led to far greater levels of stereoselectivity. The stereochemical outcome of these reactions was shown to arise from steric factors involving the larger ketene acetal group. The synthetic utility of cyclizations utilizing ketene dithioacetal derived radical cations was demonstrated by completing an asymmetric synthesis of (+)-nemorensic acid. Finally, the reactions were shown to be compatible with the use of an amide nucleophile and the direct formation of a lactone product.     

Crystal structure, spectroscopy, and quantum chemical studies of (E)-2-[(2-chlorophenyl)iminomethyl]-4-trifluoromethoxyphenol

The Schiff base compound (E)-2-[(2-chlorophenyl)iminomethyl]-4-trifluoromethoxyphenol has been synthesized and characterized by IR, UV-vis, and X-ray single-crystal determination. The molecular geometry from X-ray experiment in the ground state has been compared using the density functional theory (DFT) with the 6-311++G(d,p) basis set. The calculated results show that the DFT can well reproduce the structure of the title compound. Using the TD-DFT method, electronic absorption spectra of the title compound have been predicted, and a good agreement is determined with the experimental ones. To investigate the tautomeric stability, optimization calculations at the B3LYP/6-311++G(d,p) level were performed for the enol and keto forms of the title compound. Calculated results reveal that its enol form is more stable than its keto form. The predicted nonlinear optical properties of the title compound are much greater than those of urea. The changes of thermodynamic properties for the formation of the title compound with the temperature ranging from 200 to 500 K have been obtained using the statistical thermodynamic method. At 298.15 K, the change of Gibbs free energy for the formation reaction of the title compound is -824.841 kJ/mol. The title compound can spontaneously be produced from the isolated monomers at room temperature. The tautomeric equilibrium constant is also computed as 3.85 × 10(-4) at 298.15 K for enol?keto tautomerization of the title compound. In addition, a molecular electrostatic potential map of the title compound was performed using the B3LYP/6-311++G(d,p) method.     

Solvent effects on hydrogen abstraction reactions from lactones with antioxidant properties

Solvent effects on the kinetics for hydrogen abstraction from a lactone antioxidant were determined for alkoxyl and nitroxyl radicals; their reactivity differ by about 7 orders of magnitude. A decrease by approximately 12 and approximately 35 were determined for H-abstraction by tert-butoxyl and nitroxyl radicals, respectively, upon changing the solvent from hexane to acetonitrile. Results of solvent and isotope studies indicate that the antioxidant properties of lactone antioxidants should be attributed to the enol, not the lactone. [reaction: see text]