Kowalski ester homologation. application to the synthesis of beta-amino esters

The Kowalski ester homologation protocol has been applied to a representative range of alpha-amino esters to provide beta-amino esters with excellent levels of enantio- and diastereocontrol. A key feature of this chemistry is the nature of the N-protecting group that is employed.     

Chemoselective Boronic Ester Synthesis by Controlled Speciation

Control of boronic acid solution speciation is presented as a new strategy for the chemoselective synthesis of boronic esters. Manipulation of the solution equilibria within a cross‐coupling milieu enables the formal homologation of aryl and alkenyl boronic acid pinacol esters. The generation of a new, reactive boronic ester in the presence of an active palladium catalyst also facilitates streamlined iterative catalytic C? C bond formation and provides a method for the controlled oligomerization of sp²‐hybridized boronic esters.     

An Improved Procedure for the Conversion of Carbonyl Compounds to α,β-Unsaturated Carboxylic Acids

The two carbon homologation of aldehydes and ketones to give α,β-unsaturated carboxylic acids is often carried out via the corresponding ester using metallated phosphonate reagents² (1) (Equation I). When alkyl esters (la) are employed problems are sometimes encountered in the hydrolysis step due to acid or base sensitivity but these problems can be overcome using the silyl ester (lb)³.     

Carbonylation of diols and their ethers and esters with ruthenium catalysts: synthesis of lactones and hydroxyacids ethers and esters

Diols and their formic or acetic esters can be carbonylated to give lactones or the corresponding hydroxyacid esters of ethers in the presence of carbonylruthenium iodide systems, [Ru(CO)₃I₃]⁻/alkyl or metal iodide, at a temperature of 200°C and CO pressure of 10-20 MPa. The reaction in the case of 1,3-propanediol gives γ-butyrolactone, with a selectivity of 60-% . Side reactions of homologation to 1,4-butanediol derivatives and hydrogenolysis to n-propyl derivatives by H₂ produced by the water gas shift reaction (WGSR) also occur, together with acid-catalyzed dehydration to give linear polypropylene glycols, α,ω-diols with more than 3 carbon atoms in the chain preferentially give hydroxyacid esters and ethers.The cyclic ether by-products and linear polyether by-products can be further activated and carbonylated under the reaction conditions to give lactones or hydroxy-acid derivatives thus increasing the total yield of carbonylation products. The formation of H₂ by WGSR involving water produced by the acid-catalyzed dehydration reactions, and the subsequent hydrogenolysis and homologation reactions cannot be avoided.     

Speciation Control During Suzuki–Miyaura Cross‐Coupling of Haloaryl and Haloalkenyl MIDA Boronic Esters

Boronic acid solution speciation can be controlled during the Suzuki–Miyaura cross‐coupling of haloaryl N‐methyliminodiacetic acid (MIDA) boronic esters to enable the formal homologation of boronic acid derivatives. The reaction is contingent upon control of the basic biphase and is thermodynamically driven: temperature control provides highly chemoselective access to either BMIDA adducts at room temperature or boronic acid pinacol ester (BPin) products at elevated temperature. Control experiments and solubility analyses have provided some insight into the mechanistic operation of the formal homologation process.     

Decomposition of 2‐Mercaptoethyl O‐Ester: SN2 Displacement or Acyl Transfer? A Theoretical Study

The mechanism for the decomposition of 2‐mercaptoethyl O‐ester was theoretically investigated. The mechanism that 2‐mercaptoethyl O‐ester undergoes an S_N2 displacement of the O atom by the S atom on α‐C is much favored over the mechanism of N‐to‐S acyl transfer. The length of the alcohol moiety has large effects on the decomposition efficiency of thiol‐substituted alkyl O‐esters. The reactivities of these esters are controlled by distortion energies. Only 2‐mercaptoethyl O‐ester can undergo the decomposition at room temperature due to the low distortion energy to achieve the transition state geometry. If the thiol group of 2‐mercaptoethyl O‐ester is replaced by an amino group, the N‐to‐N acyl transfer mechanism is more favored than the S_N2 displacement mechanism.     

Asymmetric homologation of boronic esters bearing azido and silyloxy substituents

In the asymmetric homologation of boronic esters with a (dihalomethyl)lithium, substituents that can bind metal cations tend to interfere. Accordingly, we undertook the introduction of weakly basic oxygen and nitrogen substituents into boronic esters in order to maximize the efficiency of multistep syntheses utilizing this chemistry. Silyloxy boronic esters cannot be made efficiently by direct substitution, but a (hydroxymethyl)boronic ester has been silylated in the usual manner. Conversion of alpha-halo boronic esters to alpha-azido boronic esters has been carried out with sodium azide and a tetrabutylammonium salt as phase-transfer catalyst in a two-phase system with water and either nitromethane or ethyl acetate. These are safer solvents than the previously used dichloromethane, which can form an explosive byproduct with azide ion. Boronic esters containing silyloxy or alkoxy and azido substituents have been shown to react efficiently with (dihalomethyl)lithiums, resulting in efficient asymmetric insertion of the halomethyl group into the carbon-boron bond.     

Mechanism of beta-silyl diacyl peroxide decomposition: a mild and stereoselective synthesis of beta-silyl esters

A novel method for the formation of beta-silyl esters is presented. Mechanistic studies were carried out on the formation and decomposition of beta-silyl diacyl peroxides, showing that the decomposition pathway involves an ionic mechanism that is influenced by the presence of the beta-silyl moiety. These studies exclude a free radical decomposition pathway as evidenced by the absence of chemically induced dynamic nuclear polarization (CIDNP) during the reaction and a strong correlation of the resulting regioisomeric product distribution to sigma(+). This reaction allows for the formation of beta-silyl esters in 45-50% isolated yield with predictable regioselectivity and good to excellent diastereoselectivity. Studies demonstrate that ester products which are formed at benzylic centers have the erythro configuration, whereas ester products formed at alkyl centers have the threo configuration.     

Vinylidene Homologation of Boronic Esters and its Application to the Synthesis of the Proposed Structure of Machillene

Alkenyl boronic esters are important reagents in organic synthesis. Herein, we report that these valuable products can be accessed by the homologation of boronic esters with lithiated epoxysilanes. Aliphatic and electron‐rich aromatic boronic esters provided vinylidene boronic esters in moderate to high yields, while electron‐deficient aromatic and vinyl boronic esters were found to give the corresponding vinyl silane products. Through DFT calculations, this divergence in mechanistic pathway has been rationalized by considering the stabilization of negative charge in the C?Si and C?B bond breaking transition states. This vinylidene homologation was used in a short six‐step stereoselective synthesis of the proposed structure of machillene, however, synthetic and reported data were found to be inconsistent.     

Gallium Trihalide Catalyzed Sequential Addition of Two Different Carbon Nucleophiles to Esters by Using Silyl Cyanide and Ketene Silyl Acetals

A sequential addition of silyl cyanide and ketene silyl acetals to esters was achieved by a gallium trihalide catalyst to produce β‐cyano‐β‐siloxy esters. This is the first example of the sequential addition of two different carbon nucleophiles to esters. The employment of lactones provided α,α‐disubstituted cyclic ethers with a cyano group and an ester moiety. A variety of esters and lactones are applicable to this reaction system.     

Synthesis of (2-Ethoxy-2-oxoethylthio)methyl Derivatives of Furoic Acids and Attempts of Their Intramolecular Cyclizations under the Conditions of the Claisen Reaction

Based on reactions of phosphorylated derivatives of halomethylfuroic acid esters with thioglycolic acid ester in the presence of bases, a method for the synthesis of mono-and bisphosphosphorylated esters of (2-ethoxy-2-oxoethylthio)methyl derivatives of furoic acids, containing an ester group in the position adjacent to the sulfide moiety was developed. Obtained compounds are relatively easily metallized with potassium tert-butylate, but intramolecular cyclization proceeds smoothly only in the case of 5-(diethoxyphosphorylmethyl)-2-(ethoxycarbonylmethylthiomethyl)furan-3-carboxylic acid. The resulting 1,3-dicarbonyl derivative of 7H-thiopyrano[3,4-b]furan is hydrolytically unstable, but its enol O-methyl ether is stabile. Alkaline hydrolysis of the latter compound was studied, and it was shown that it proceeds at the ester as well as at the phosphonate group.     

Characterization of long-chain carboxylic esters with CH3OBOCH 3 + in a small fourier-transform ion cyclotron resonance mass spectrometer

The usefulness of CH₃OBOCH ₃ ⁺ as a chemical ionization reagent was examined by allowing the ion to react with carboxylic esters of various chain lengths in a small Fourier-transform ion cyclotron resonance mass spectrometer equipped with a permanent magnet. CH₃OBOCH ₃ ⁺ is a strong electrophile and readily abstracts an oxygen-containing group from the carboxylic esters. Long-chain esters exclusively lose the alkoxide moiety to give the acylium ion. The same reaction was observed for saturated, unsaturated, branched and cyclic esters. In each case, the acylium ion reacts further with a neutral ester molecule by proton transfer to yield the protonated ester as a secondary product. This remarkably simple product distribution reveals the molecular weight of the ester, the chain length of its acid moiety, and the degree of unsaturation in the acid and alcohol moieties.     

Isolation and Characterization of Wax Esters in Fennel and Caraway Seed Oils by SPE-GC

A rapid method for the isolation and quantitative determination of wax esters in vegetable oils was developed. For the first time wax esters in oils were separated from the triglyceride matrix by means of solid-phase extraction, which allows rapid sample preparation in parallel and therefore a high sample throughput. The thus obtained wax ester fractions of fennel and caraway seed oils were analyzed by high temperature gas chromatography. GC-MS analyses were carried out using electron impact ionization in order to characterize the wax ester fraction. With respect to the results of the GC-MS analyses different isomers of saturated wax esters with the same carbon number were observed. Additional monounsaturated wax esters with an unsaturated fatty acid moiety were identified.     

CeCl(3) x 7H(2)O-NaI catalyzed hydrooxacyclization of unsaturated 3-hydroxy esters

[reaction: see text] Cerium(III) chloride heptahydrate and sodium iodide in boiling acetonitrile promote cyclization of 3-hydroxyalkenoic acids esters giving 5-substituted tetrahydrofuranacetic acid esters and 6-substituted tetrahydropyranacetic acid esters in fair to good yield and with complete retention of the absolute configuration of the starting 3-hydroxy ester.     

Gold(I)-catalyzed isomerization of allenyl carbinol esters: an efficient access to functionalized 1,3-butadien-2-ol esters

A study concerning the gold(I)-catalyzed rearrangement of diversely substituted allenyl carbinol esters into functionalized 1,3-butadien-2-ol esters is described. The mild conditions employed allow the efficient, rapid, and stereoselective synthesis of a variety of such compounds via a new 1,3-shift of an ester moiety onto a gold-activated allene. [structure: see text]     

Competitive homolytic and heterolytic decomposition pathways of gas‐phase negative ions generated from aminobenzoate esters

An alkyl‐radical loss and an alkene loss are two competitive fragmentation pathways that deprotonated aminobenzoate esters undergo upon activation under mass spectrometric conditions. For the meta and para isomers, the alkyl‐radical loss by a homolytic cleavage of the alkyl‐oxygen bond of the ester moiety is the predominant fragmentation pathway, while the contribution from the alkene elimination by a heterolytic pathway is less significant. In contrast, owing to a pronounced charge‐mediated ortho effect, the alkene loss becomes the predominant pathway for the ortho isomers of ethyl and higher esters. Results from isotope‐labeled compounds confirmed that the alkene loss proceeds by a specific γ‐hydrogen transfer mechanism that resembles the McLafferty rearrangement for radical cations. Even for the para compounds, if the alkoxide moiety bears structural motifs required for the elimination of a more stable alkene molecule, the heterolytic pathway becomes the predominant pathway. For example, in the spectrum of deprotonated 2‐phenylethyl 4‐aminobenzoate, m/z 136 peak is the base peak because the alkene eliminated is styrene. Owing to the fact that all deprotonated aminobenzoate esters, irrespective of the size of the alkoxy group, upon activation fragment to form an m/z 135 ion, aminobenzoate esters in mixtures can be quantified by precursor ion discovery mass spectrometric experiments. Copyright © 2016 John Wiley & Sons, Ltd.     

有机磷化合物的研究 X:五价膦(磷)酸酯与卤代烷在卤化钠存在下的酯烷基交换反应

A detailed study on the ester alkyl exchange reaction of various types of quinique-valent phosphorus esters with alkyl halide in the presence of sodium bromides was reported. This ester alkyl exchange reaction was evidently influenced by the structure of phosphorus esters and alkyl halides as well as by the nature of the halides of metal ions. In contrast with the reaction without sodium hadlide, the alkyl phosphinmate is more reactive than phosphonate and phosphate by treatment with alkyl halide in the presence of sodium halide. This is consistent with the high nucleophilicity of >P(CO)O- as leaving group. The reactivity of butyl halides was decreased in the following order: n-BuBr>i-BuBr=s-BuBr>t-BuBr. Alkyl iodide was proved to be more reactive than the corresponding bromide and chloride. However, the use of iodioe is limited by the formation of alkene resulted from the elimination of HI. These structural effects show the general characteristics of a nucleophilic substitution reaction. A reaction mechanism involving the formation of sodium salt intermecutiate was proposed based on the concept of HSAB principle. This reaction may, however, be used as a convenient method for the preparation of mixed esters of quinque-valent phosphorus acids.     

Cationic ring‐opening polymerization of six‐membered cyclic carbonates with ester groups

This work deals with the cationic ring‐opening polymerization of the ester‐substituted cyclic carbonates 5‐methyl‐5‐benzoyloxymethyl‐1,3‐dioxan‐2‐one ( CC1 ) and 4‐benzoyloxymethyl‐1,3‐dioxan‐2‐one ( CC4 ). The polymerization was carried out with trifluoromethanesulfonic acid, methyl trifluoromethanesulfonate, boron trifluoride etherate, or methyl iodide as the initiator. The reactivity of CC1 and CC4 was higher than that of 5,5‐dimethyl‐1,3‐dioxan‐2‐one, which had no ester moiety. These results suggest that this ring‐opening polymerization was accelerated by the intramolecular ester group. CC1 showed a higher polymerizability than CC4 , affording a polymer with a higher molecular weight. Additionally, using methyl iodide as the initiator was effective for increasing the molecular weight of the obtained polycarbonate and decreasing decarboxylation. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1305–1317, 2001     

Stereospecific preparation of (Z)- and (E)-2,3-difluoro-3-stannylacrylic ester synthons and a new, efficient stereospecific route to (Z)- and (E)-2,3-difluoroacrylic esters

[reaction: see text] The (Z)-2,3-difluoro-3-stannylacrylic ester is readily prepared from (Z)-1,2-difluorovinyltriethylsilane via stereospecific stannyl/silyl exchange with KF/(Bu3Sn)2O or Bu3SnCl in DMF at 70 degrees C. The corresponding (E)-2,3-difluoro-3-stannylacrylate is prepared by stereospecific carbonylation of (E)-1,2-difluorovinyl iodide followed by low temperature/in situ stannylation of the resultant (Z)-2,3-difluoroacrylic ester. With Cu(I) iodide and Pd(PPh3)4 catalysis, the (Z)- and (E)-stannylacrylate esters readily couple with aryl iodides and vinyl bromides, as well as 2-iodothiophene, at room temperature to stereospecifically produce the respective (E)- and (Z)-2,3-difluoro-3-aryl substituted acrylic esters or conjugated dienes in high yields.     

Reaction of methylmagnesium iodide with perfluorocarboxylate esters

Conclusions In the reaction of methylmagnesium iodide with esters of perfluorocarboxylic acids, the magnesium salt of the hemiketal was found to be more stable and the ketone yield was found to be higher with increasing bulk of the alkoxy group of the ester.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1163–1165, May, 1988.