Steric hindrance in synthesis : Dealkylation of hindered carboxylic esters via the action of organolithium reagents. synthetic and mechanistic aspects

The reaction of n-propyllithium with a number of hindered aliphatic carboxylic esters in ether at 0° produces cleanly the corresponding carboxylic acid in satisfactory yields. This reaction represents a facile dealkylation of hindered carboxylic esters. The mechanistic aspects are briefly examined.     

Kinetic Resolution of Racemic Secondary Benzylic Alcohols by the Enantioselective Esterification Using Pyridine‐3‐carboxylic Anhydride (3‐PCA) with Chiral Acyl‐Transfer Catalysts

Pyridine‐3‐carboxylic anhydride (3‐PCA) was found to function as an efficient coupling reagent for the preparation of carboxylic esters from various carboxylic acids with alcohols under mild conditions by a simple experimental procedure. This novel condensation reagent 3‐PCA was applicable not only for the synthesis of achiral carboxylic esters catalyzed by 4‐(dimethylamino)pyridine (DMAP) but also for the production of chiral carboxylic esters by the combination of chiral nucleophilic catalyst, such as tetramisole (=2,3,5,6‐tetrahydro‐6‐phenylimidazo[2,1‐b][1,3]thiazole) derivatives. An efficient kinetic resolution of racemic benzylic alcohols with achiral carboxylic acids was achieved by using 3‐PCA in the presence of (R)‐benzotetramisole ((R)‐BTM), and a variety of optically active carboxylic esters were produced with high enantiomeric excesses by this new chiral induction system without using a tertiary amine.     

Facile synthesis of diphenylmethyl esters from 2-diphenylmethoxypyridine using catalytic boron trifluoride·diethyl etherate

A practical method for the direct preparation of diphenylmethyl (DPM) esters from 2-diphenylmethoxypyridine is described. The reaction was readily performed in the presence of a catalytic amount of boron trifluoride-diethyl etherate at room temperature. Using this reaction protocol, various carboxylic acids were converted to DPM esters with high yields. This method is highly effective for the protection of carboxylic acids and the synthesis of DPM esters, and offers a promising approach for facile esterification of a variety of carboxylic acids.     

Lipase-catalyzed kinetic resolution of β-borylated carboxylic esters

The kinetic resolution of chiral β-borylated carboxylic esters via lipase-catalyzed hydrolysis and transesterification reactions was studied. The enantioselective hydrolysis catalyzed by CAL-B furnished the β-borylated carboxylic acid with reasonable enantiomeric excess (62% ee), while both methyl and ethyl β-borylated carboxylic esters were recovered with excellent ee (>99%). Meanwhile, the transesterification reaction of β-borylated carboxylic esters and several alcohols, catalyzed by CAL-B, only indicated a high selectivity when ethanol and methyl-(β-pinacolylboronate)-butanoate were used as substrates, which gave ethyl-(β-pinacolylboronate)-butanoate with >99% ee.     

The first asymmetric esterification of free carboxylic acids with racemic alcohols using benzoic anhydrides and tetramisole derivatives: an application to the kinetic resolution of secondary benzylic alcohols

A variety of optically active carboxylic esters are produced by the kinetic resolution of racemic secondary benzylic alcohols using free carboxylic acids with benzoic anhydride and tetramisole derivatives. 4-Methoxybenzoic anhydride (PMBA) is the best reagent to use in producing the corresponding esters in high ee when the reaction is catalyzed by (+)-benzotetramisole (BTM); by contrast, when non-substituted benzoic anhydride is used as a coupling reagent, the resulting optically active alcohols are obtained with high selectivities. This protocol directly produces chiral carboxylic esters from free carboxylic acids and racemic secondary alcohols by utilizing the trans-acylation process to generate mixed anhydrides from acid components and benzoic anhydride derivatives under the influence of chiral catalysts.     

Solvolysis of 1-Chloro-1-Alkenyl Phenyl Sulfides. Synthesis of α-Bromo Phenyl Thiocarboxylic Esters, α-Bromo Alkyl Carboxylic Esters and α-Phenylthio Methyl Carboxylic Esters

1-chloro-1-alkenyl phenyl sulfides treated with bromine followed by hydrolysis or methanolysis give α-bromo phenyl thiocarboxylic esters and α-phenyl-thio methyl carboxylic esters. Direct oxidative solvolysis with bromine and alcohol give α-bromo alkyl carboxylic esters.     

典型表面活性剂结构和组成的波谱表征——山梨醇酐脂肪酸酯和聚氧乙烯山梨醇酐脂肪酸酯的波谱

用核磁共振波谱、质谱和红外光谱表征了非离子表面活性剂中山梨醇酐脂肪酯和聚氧乙烯山梨醇酐脂肪酸酯的结构特征和它们的组成。     

Dimethylsulfoxide-iodine catalysed deprotection of allyl carboxylic esters

A convenient method for deprotection of allyl carboxylic esters has been developed by using the inexpensive and environmentally friendly reagent dimethylsulfoxide-iodine. A variety of carboxylic esters were deprotected to the carboxylic acids. This method is more efficient and operationally simple in comparison to methods using transition metal complexes.     

An enantioselective synthesis of benzylidene-protected syn-3,5-dihydroxy carboxylate esters via osmium, palladium, and base catalysis

[reaction: see text]. The enantioselective syntheses of several protected syn-3,5-dihydroxy carboxylic esters have been achieved from the corresponding achiral 1,3-dieneoates. The route relies upon an enantio- and regioselective Sharpless dihydroxylation and a palladium-catalyzed reduction to form delta-hydroxy-1-enoates. The resulting delta-hydroxy-1-enoates are subsequently converted into benzylidene-protected 3,5-dihydroxy carboxylic esters in one step. The benzylidene-protected 3,5-dihydroxy carboxylic esters are produced in good overall yields (25% to 51%) and high enantiomeric excesses (80% to >95%).     

Direct lactonization of α-amino γ,δ-unsaturated carboxylic acid esters via olefin activation: stereo- and regioselective production of homoserine lactone scaffolds having contiguous stereocenters

Triflic acid-mediated stereoselective direct lactonization of a variety of α-amino γ,δ-unsaturated carboxylic acid esters and the construction of new γ-butyrolactone structural motifs are reported. Several α-amino γ,δ-unsaturated carboxylic acid esters underwent stereo- and regioselective 1,5-cyclization and afforded a variety of highly substituted homoserine lactone scaffolds having contiguous stereocenters. The direct lactonization of the chiral α-amino γ,δ-unsaturated carboxylic acid esters with triflic acid led to the enantioselective synthesis of the novel homoserine lactones. A plausible mechanism for the direct lactonization of α-amino γ,δ-unsaturated carboxylic acid esters is presented. The stereochemistry of major isomers 3f, 7a, 7b, and 7d was unambiguously established from the X-ray structure analysis.     

Synthesis of enol and vinyl esters catalyzed by an iridium complex

Enol and vinyl esters were successfully synthesized by the use of an iridium complex as a catalyst. The reaction of carboxylic acids with terminal alkynes in the presence of catalytic amounts of [Ir(cod)Cl]₂ and Na₂CO₃ gave the corresponding 1-alkenyl esters. The addition of carboxylic acids to alkynes principally took place in the Markovnikov fashion. In addition, by the use of an Ir complex combined with NaOAc various vinyl esters were prepared through the transvinylation between carboxylic acids and vinyl acetate.     

De novo synthesis of 2-substituted syn-1,3-diols via an iterative asymmetric hydration strategy

The enantioselective syntheses of several protected 4-substituted syn-3,5-dihydroxy carboxylic esters have been achieved from the corresponding achiral (E,E)- or (E,Z)-1,3-dienoates. The route relies upon an enantio- and regioselective Sharpless dihydroxylation and a palladium-catalyzed reduction to form gamma-substituted delta-hydroxy-1-enoates. The resulting delta-hydroxy-1-enoates are subsequently converted into benzylidene-protected 4-substituted syn-3,5-dihydroxy carboxylic esters in one step. The benzylidene-protected 3,5-dihydroxy carboxylic esters are produced in good overall yields (20-54%) and high enantiomeric excess (73-97% ee).     

Decarboxylative Conjunctive Cross-coupling of Vinyl Boronic Esters using Metallaphotoredox Catalysis

The synthesis of complex alkyl boronic esters through conjunctive cross-coupling of vinyl boronic esters with carboxylic acids and aryl iodides is described. The reaction proceeds under mild metallaphotoredox conditions and involves an unprecedented decarboxylative radical addition/cross-coupling cascade of vinyl boronic esters. Excellent functional-group tolerance is displayed, and application of a range of carboxylic acids, including secondary α-amino acids, and aryl iodides provides efficient access to highly functionalized alkyl boronic esters. The decarboxylative conjunctive cross-coupling was also applied to the synthesis of sedum alkaloids.     

Carbon-13 chemical shifts of some model carboxylic acids and esters

Selected model carboxylic acids and esters dissolved in deuteriochloroform have been studied by carbon-13 nuclear magnetic resonance under standardized conditions. Assignments of the chemical shifts have been made for all samples, and the spin–lattice relaxation times and nuclear Overhauser effects measured for a few selected esters. The results are intended to provide a practical aid for the analysis of carboxylic acids and esters of a more complex nature (e.g. polymers, natural products, etc.).     

一种使羧酸增加两个碳原子的合成方法

自由基反应已普遍应用于有机合成。涉及电子转移过程的一个重要试剂是 S_2O_8~(2-),S_2O_8~(2-)在银盐催化下可氧化羧酸形成烷基自由基:     

Convenient Method for the Preparation of Carboxylic Esters Using (Chloromethylene)dimethylammonium Chloride

(Chloromethylene)dimethylammonium chloride (Vilsmeier reagent) works as an efficient condensation reagent in the formation of carboxylic esters from carboxylic acids and alcohols under mild conditions in a one-pot procedure. Secondary alcohols are converted successively into the corresponding esters in moderate to high yields with inversion of configurations.     

Chemical transformations of tetracyclo[3.3.1.1.<Superscript>3,7</Superscript>.0.<Superscript>1,3</Superscript>]decane (1,3-dehydroadamantane): I. Reaction of 1,3-dehydroadamantane with carboxylic acids esters

Adamantylation of carboxylic acids esters was performed with 1,3-dehydroadamantane for the first time. The reaction proceeds under mild conditions and can be used as a convenient single stage procedure for the synthesis of esters of branched carboxylic acids having an adamantyl group in the α-position to the carbonyl group.     

Iron-catalysed green synthesis of carboxylic esters by the intermolecular addition of carboxylic acids to alkenes

Iron triflate, in situ-formed from FeCl3 and triflic acid, or FeCl3 and silver triflate efficiently catalyse the intermolecular addition of carboxylic acids to various alkenes to yield carboxylic esters; the reaction is applicable to the synthesis of unstable esters, such as acrylates.     

Lipase-Catalyzed Synthesis of Carboxylic Amides: Nitrogen Nucleophiles as Acyl Acceptor

 The lipase-catalyzed aminolysis of carboxylic esters is a fairly general reaction that has been performed with a wide range of esters and amines, generally in anhydrous organic media to avoid undesirable hydrolysis of the ester. Alternatively, carboxylic amides can be synthesized by lipase mediated condensation of carboxylic acids and amines if an excess of either reactant is avoided. Chiral carboxylic esters have been resolved by lipase-catalyzed aminolysis. In the majority of these resolutions, Candida antarctica lipase B has been employed as the catalyst. A range of chiral amines has been resolved by lipase mediated acylation, using mainly the lipases from C. antarctica (B type) and Pseudomonas species. The enantiorecognition was frequently found to depend critically on the acylating agent and the reaction medium.     

α-(1H-Imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters as prodrugs of carboxylic acid containing drugs

Synthesis of α-(1H-imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters have been reported in 2-3 simple steps. α-(1H-Imidazol-1-yl)alkyl (IMIDA) carboxylic acid esters were found to be chemically labile and thus serve as novel prodrugs of carboxylic acids.