An effective method for the synthesis of carboxylic esters and lactones using substituted benzoic anhydrides with Lewis acid catalysts

An efficient mixed-anhydride method for the synthesis of carboxylic esters and lactones using benzoic anhydride having electron withdrawing substituent(s) is developed by the promotion of Lewis acid catalysts. In the presence of a catalytic amount of TiCl₂(ClO₄)₂, various carboxylic esters are prepared in high yields through the formation of the corresponding mixed-anhydrides from 3,5-bis(trifluoromethyl)benzoic anhydride and carboxylic acids. The combined catalyst consisting of TiCl₂(ClO₄)₂ together with chlorotrimethylsilane functions as an effective catalyst for the synthesis of carboxylic esters from free carboxylic acids and alcohols with 4-(trifluoromethyl)benzoic anhydride. Various macrolactones are prepared from the free ω-hydroxycarboxylic acids by the combined use of 4-(trifluoromethyl)benzoic anhydride and titanium(IV) catalysts together with chlorotrimethylsilane under mild reaction conditions. The lactonization of trimethylsilyl ω-(trimethylsiloxy)carboxylates using 4-(trifluoromethyl)benzoic anhydride is also promoted at room temperature in the presence of a catalytic amount of TiCl₂(ClO₄)₂. An 8-membered ring lactone, a synthetic intermediate of cephalosporolide D, is successfully synthesized according to this mixed-anhydride method using 4-(trifluoromethyl)benzoic anhydride by the promotion of a catalytic amount of Hf(OTf)₄.     

Electrochemical synthesis of network polygermanes and polysilanes

Network σ-conjugated polymers, i.e., a butyl substituted network polygermane, (BuH_xGe)_n (x=0–2), a butyl substituted network polysilane, (BuH_xSi)_n (x=0–2), and an octyl substituted network polysilane, (OctH_xSi)_n (x=0–2), were synthesized by electrochemical reduction of butyltrichlorogermane, butyltrichlorosilane, and octyltrichlorosilane, respectively, for the first time. A polymer of (BuH_xGe)_n (x=0–2) had absorption which extended to 900 nm, while a polymer of (BuH_xSi)_n (x=0–2) and a polymer of (OctH_xSi)_n (x=0–2) had absorption which extended to 500 and 800 nms, respectively. It was found that the reported electrochemical method gave σ-conjugated polymers with more highly developed network structures compared to the other synthetic pathways. The structure of the polymers was successfully controlled from linear to highly developed network structures by controlling the charge for electrosynthesis.     

A new and efficient method for the facile synthesis of N-acyl sulfonamides under Lewis acid catalysis

The N-acylation of sulfonamides with carboxylic acid anhydrides in the presence of Lewis acids is described. Several Lewis acids such as BF₃·Et₂O, ZnCl₂, MoCl₅, TiCl₄, B(C₆F₅)₃, Sc(OTf)₃ and I₂ were found to catalyze the reaction efficiently to furnish the N-acylated products in good yields under solvent-free conditions. The reactions of various sulfonamides were studied with different carboxylic acid anhydrides including the less reactive benzoic and pivalic anhydrides, in the presence of 3 mol % ZnCl₂ as the catalyst. Carboxylic acids were also successfully used as acylating agents via the mixed anhydride method.     

Lewis acid catalyzed nenitzescu indole synthesis

A novel method for Lewis acid catalyzed Nenitzescu indole syntheses of 5‐hydroxyindoles bearing different substituents in positions 1 )Alk, Bn, Ar), 2 )Me, Et, Ph), and 3 )COOEt, COMe, CONHPh) as well as tricyclic derivatives are reported. The method is simple, rapid, efficient, and allows preparation of hydroxyindoles from 1,4‐benzoquinone and enamines in good to excellent yields with the use of low‐polar solvents in the presence of weak Lewis acids catalysts. The formation of 5‐hydroxyindoles under such mild conditions is explained in terms of a non‐redox mechanism.     

One-step novel synthesis of methylene bisphenols and methylene bisnaphthols using Lewis acid mediated rearrangement

Mono- and di-substituted isomeric methylene bisphenols and methylene bisnaphthols have been synthesized by rearrangement of the corresponding O-methoxyacetyl derivatives of phenols and naphthols, respectively, in presence of aluminium chloride under dry conditions. The chemistry observed is different from the usual Fries rearrangement reaction and involves an intermolecular rearrangement. The reactions reported here also reflect the influence of substituents present in the substrate as is supported by the substitution of the bridging methylene at a position meta to the phenolic hydroxyl in some of the minor products formed along-side the majorly formed ortho substituted products.     

An efficient synthesis of chiral isoquinuclidines by Diels-Alder reaction using Lewis acid catalyst

The Diels-Alder reaction of 1,2-dihydropyridine derivatives (1-phenoxycarbonyl-1,2-dihydropyridine 1 or 1-methoxycarbonyl-1,2-dihydropyridine 4) with N-acryloyl (1S)-2,10-camphorsultam (1S)-2 {or N-acryloyl (1R)-2,10-camphorsultam (1R)-2} in the presence of Lewis acid, such as titanium tetrachloride, zirconium tetrachloride, and hafnium tetrachloride afforded the endo-cycloaddition product, 2-azabicyclo[2.2.2]octane derivatives in good yields with excellent diastereoselectivity. The absolute stereochemistry assignment of the endo-cycloaddition product (1S)-5a starting from N-acryloyl (1S)-2,10-camphorsultam (1S)-2 has been established to be (1S,4R,7S) and the reaction mechanism was proposed.     

Lewis acid facilitated regioselective synthesis of τ-histidinoalanine

τ-Histidinoalanine, with an unusual cross-link between His and Ala, is the central component of theonellamides, a family of bioactive peptidic natural products. Previous syntheses of this residue were plagued with low regioselectivity in the alkylation step. Herein, we report two novel routes to τ-histidinoalanine, involving alkylation of Boc-His-OMe with a serine-derived β-lactone and β-bromoalanine, respectively, as the electrophiles. The use of Mg(OTf)₂ as a catalyst was found to be essential to ensure high regioselectivity for the τ-isomer, presumably due to the formation of a six-membered ring chelation involving the π-nitrogen atom of histidine.     

Lewis acid mediated highly regioselective intramolecular cyclization for the synthesis of β-lapachone

A highly regioselective intramolecular cyclization of lapachol mediated by Lewis acids including NbCl₅, AlCl₃, and FeCl₃ was developed for synthesizing β-lapachone in excellent yields without any formation of the isomer α-lapachone. This procedure was efficient, mild, and easily scalable that avoided using highly hazardous concd H₂SO₄. In the case of ZrCl₄ the cyclization was found to give α-lapachone as the main product. A possible mechanism for the Lewis acid mediated cyclization was also discussed.     

Novel solid acid with both Brønsted and Lewis acid sites for biodiesel synthesis

The solid acid with both Brønsted and Lewis acid sites has been synthesized from resorcinol-formaldehyde (RF) resin, hydroxyethylsulfonic acid and copper hydroxyethylsulfonate. The solid acid was applied to catalyze the transesterification of rapeseed oil. The results showed that the novel solid acid was very efficient for the biodiesel synthesis with the yield over 99% even for the rapeseed oil with high water content and both the free fatty acid and triglyceride could be efficiently transformed to the biodiesel products. The regular sphere structure, low cost, high acidity and catalytic activities gave the solid acid great potential for green chemical processes.     

Exploiting catalytic dehydrogenative coupling in the synthesis and study of polysilanes

A reinvestigation of the catalytic activity of Wilkinson's catalyst, (Ph₃P)₃RhCl ( 1 ), for the dehydrogenative coupling reactions of secondary silanes has pointed the way toward the synthesis of key oligosilane reagents for structure property correlation studies of polysilanes. Implications are discussed for understanding the mechanisms of coupling and redistribution reactions of silanes mediated by such late metal centres. Also described are attempts to derivatize Si-H bonds in the resulting oligosilanes, which is highly relevant to the development of improved methods for the post-polymerization functionalization of polysilanes containing Si-H bonds.     

Ultrasonic and Lewis acid ionic liquid catalytic system for Kabachnik-Fields reaction

A combination of ultrasonic (US) and [bmim]AlCl₄ ionic liquid is used as an alternative to conventional acid catalysts in the Kabachnik-Fields reaction of an amine and aryl aldehyde with phosphite leading to the formation of aminophosphonates. The reaction time was significantly reduced and the reaction progressed very smoothly.     

An N-heterocyclic carbene/Lewis acid strategy for the stereoselective synthesis of spirooxindole lactones

A cooperative catalysis approach for the enantioselective formal [3+2] addition of α,β-unsaturated aldehydes to isatins has been developed. Homoenolate annulations of β-aryl enals catalyzed by an N-heterocyclic carbene (NHC) require the addition of lithium chloride for high levels of enantioselectivity. This NHC-catalyzed annulation has been used for the total synthesis of maremycin B.     

The role of a Lewis acid in the Nenitzescu indole synthesis

A highly efficient Lewis acid-catalyzed method for the Nenitzescu synthesis of 5-hydroxyindoles with a range of substituents at N-1 and C-3 and symmetric 5,5′-dihydroxydiindoles has been developed. The amount of the catalyst (10-100 mol %) required depended on the nature of the enaminone component. It has been shown that Lewis acid plays a role in enaminone component activation through an enamine-ZnC1₂ complex followed by its deprotonation.     

Practical and efficient multigram preparation of a camphor-derived diol for the enantioselective Lewis acid catalyzed allylboration of aldehydes

[reaction: see text] Chiral diols are important molecules with widespread use as chiral auxiliaries and ligands in enantioselective synthesis. Therefore, efficient and practical syntheses of highly dissymmetrical nonracemic diols are still a meaningful pursuit. Two new routes to access camphor-derived chiral diol 1 have been developed. One route employs camphorquinone (3) as the starting material, affording in only two steps the desired diol in 55% overall yield. The second route, from camphor (2), leads to the desired diol in an efficient four-step synthesis, with an overall yield of 55%.     

Development of mild and efficient method for synthesis of substituted flavones using oxalic acid catalyst

Oxalic acid has been used as a catalyst for intramolecular cyclisation of 2-hydroxyehalcones to form flavones in good yields.The efficiency of the catalyst was proved with a variety of substrates ranging from electron-deficient to electron-rich aryl aldehydes and 2-hydroxychalcone.     

A new method for the synthesis of levulinic acid

A two- step method is described for the synthesis of levulinic acid from ethyl acetoacetate and ethyl chloroacetate in 70% yield.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 474–475, February, 1990.     

Conformational analysis of polysilanes by the empirical force field method

Empirical force field (EFF) parameters have been developed for molecules containing Si-Si bonds. These parameters have been adjoined to those of a standard EFF and the extended force field has been used to calculate structures and relative conformational energies for a variety of acyclic and cyclic catenated polysilanes.     

Efficient synthesis of gamma-alkylidenetetronic esters by sequential Lewis acid catalyzed

A new approach for the synthesis of gamma-alkylidenetetronic acids and esters is reported which involves Me3SiOTf-catalyzed, regio- and stereoselective cyclization of 4-alkoxy-1,3-bis(trimethylsilyloxy)-1,3-butadienes with oxalyl chloride. The alpha-hydroxy group of the butenolides is efficiently functionalized by palladium-catalyzed cross-coupling reactions via the corresponding enol triflates.