Acid-Catalyzed Cyclodehydration of Quinazolone Prepolymers

The effect of p-toluenesulfonic acid and phosphoric acid on the cyclodehydration of quinazolone prepolymers has been investigated. Isothermal weight loss during the curing process was monitored closely to follow the extent of cyclization. The optimum weight loss corresponding to 2 mol water is used as an indication of the formation of a fully cyclized quinazolone structure without concurrent decarboxylation. This can be achieved by incorporating the catalyst at 2 wt% concentration, which permits a lower curing temperature than those previously reported. The polymers formed in the presence of catalyst have thermal stability superior to those prepared without catalyst. The glass transition temperature is also raised. This we conclude to be a direct result of the decrease in the amount of incomplete quinazolone rings along the polymer backbone as a result of more efficient cyclization catalyzed by the acids.     

1,4-丁二醇气相环化脱水合成四氢呋喃

以1,4-丁二醇为原料,采用气相环化脱水法合成了四氢呋喃.考察了催化剂(γ-Al2O3,ZrO2,CeO2和ZSM-5),反应温度,进料速率和反应时间等因素对环化脱水反应的影响.结果表明,在以γ-Al2O3为催化剂,进料速率为2.4 mL*min-1,于320 ℃反应2 h的最佳反应条件下,1,4-丁二醇的转化率接近100%,四氢呋喃的选择性达99.8%.     

AlPO4-Al2O3 Promoted Cyclodehydration of Diols

A convenient method for the preparation of cyclic ethers by distillation of diols over solid AlPO₄[sbnd]Al₂O₃ is presented.     

Cyclodehydration of 1,4-butanediol catalyzed by metal(IV) phosphates

Metal(IV) phosphates of tin, zirconium and titanium in different morphological forms, viz. amorphous, calcined and crystalline, have been used as catalysts for selective cyclodehydration of 1,4-butanediol to tetrahydrofuran. A comparative study of the three catalysts for the above reaction has been carried out to understand the effect of their composition and morphology on the catalytic activity.     

Cyclodehydration of N-(2-nitroaryl)-1,2,3,4-tetrahydroisoquinoline derivatives

Compounds 1a-1g gave the corresponding N-oxide derivatives 3a-3g when heated with acetic or propionic acid.     

Cyclodehydration reaction of polyhydrazides. III. Influence of the sample history

The influence of preparation history upon the thermal cyclodehydration reaction of polyhydrazide samples has been investigated. Solid polyhydrazide samples were prepared from DMSO solutions using the phase-inversion technique. Significant differences in conversion rates were observed between samples prepared by nonsolvent immersion precipitation and by evaporation of solvent. It appeared that contact with the nonsolvent water during the sample preparation process has considerable influence on the conversion rate. Not only does the immersion in and washing with water lead to a better removal of the solvent also the pH of the water determines the rate of conversion. A distinct change in the conversion rate is found for washing water at a pH of 7. Basic washing solutions showing lower conversion rates than acid ones. A mechanism explaining these phenomena is proposed.     

Cyclodehydration of 1,4-butanediol to tetrahydro- furan catalyzed by supported silicotungstic acid

Summary Acid strength, acid density and the combination ability between silicotungstic acid and support were investigated when supported silicotungstic acid was used as a catalyst for cyclodehydration of 1,4-butanediol to tetrahydrofuran. The interaction between heteropoly acid and support influenced the catalytic activity. TiO2-supported silicotungstic acid catalyst exhibited higher activity than the other catalysts supported on kaolin, kieselguhr, and activated charcoal.     

Nonclassical Mechanism in the Cyclodehydration of Diols Catalyzed by a Bifunctional Iridium Complex

1,4- and 1,5-diols undergo cyclodehydration upon treatment with cationic N-heterocyclic carbene (NHC)–Ir^III complexes to give tetrahydrofurans and tetrahydropyrans, respectively. The mechanism was investigated, and a metal-hydride-driven pathway was proposed for all substrates, except for very electron-rich ones. This contrasts with the well-established classical pathways that involve nucleophilic substitution.     

Brønsted酸性离子液体中的二甘醇脱水环化反应

首次报道了Brønsted酸性离子液体介质中的二甘醇的脱水环化反应,考察了不同的离子液体、离子液体/二甘醇摩尔比、反应温度和时间对反应的影响。结果表明,Brønsted酸性离子液体作为反应介质能够促进脱水环化反应的有效进行,且在离子液体1-(3-磺酸根丙基)-3-甲基咪唑硫酸氢盐([SPmim]HSO4)中,二甘醇的转化率和1,4-二氧六环的选择性更高。采用Hammett指示剂法测定了离子液体的酸度函数H0,其酸性强弱顺序为[SPmim]HSO4 > [Bmim]HSO4 > [Amim]HSO4 > [Hmim]BF4> [Bmim]H2PO4 >[Amim]H2PO4 > [Hmim]Tsa,这与离子液体在脱水环化反应中的催化效果一致。当温度为170 ˚C,离子液体/二甘醇摩尔比为1:1时,二甘醇在[SPmim]HSO4中反应2 h,转化率可达到97.0%,1,4-二氧六环的选择性为89.3%。     

Cyclodehydration of N-[(2-heteroarylcarbamoyl)methyl] benzamides. Preparation of 5-(heteroarylamino)oxazoles

The cyclodehydration of N-[heleroarylcarbamoyl)methyl]benzamides was found to yield 5-(heteroarylamino)oxazoles     

Cyclodehydration of cis-2-butene-1,4-diol with active methylene compounds catalyzed by a diphosphinidenecyclobutene-coordinated palladium complex

(π-Allyl)palladium triflate coordinated with 1,2-bis(4-methoxyphenyl)-3,4-bis(2,4,6-tri-t-butylphenylphosphinidene)cyclobutene (DPCB-OMe), [Pd(η³-C₃H₅)(DPCB-OMe)]OTf, efficiently catalyzes cyclodehydration of cis-2-butene-1,4-diol with active methylene compounds such as acetylacetone and ethyl acetoacetate in toluene in the presence of pyridine. The reactions can be performed in air, giving 2-vinyl-2,3-dihydrofurans in good to high yields.     

氟烷基磺酰氟和(2S,3R)-2,3-二羟基-3-苯基丙酸甲酯的脱水环氧化反应的研究

考察了(2S,3R)-2,3-二羟基-3-苯基丙酸甲酯和几种常用的氟烷基磺酰氟试剂在有机碱存在下的脱水环氧化反应.实验结果表明,氟烷基磺酰氟能够诱导(2S,3R)-2,3-二羟基-3-苯基丙酸甲酯发生脱水环氧化反应而生成(2R,3R)-2,3-环氧-3-苯基丙酸甲酯,得率80%～92%,且产物的不对映过量(ee)值大于97%.该方法具有工业化应用前景.     

Gold(I)‐Catalyzed Cyclodehydration Enabled by the Triisopropylsilyl Group: A Synthetically Versatile Methodology

Introduction of a triisopropylsilyl group into allyl and allenyl carbinols greatly enhances the efficiency of gold(I)‐catalyzed cyclodehydration, which can provide rapid access to a library of various compounds including 1H‐indenes (Table 2 and Scheme 5), benzofulvenes (Table 3), indan‐2‐ones (Scheme 2), fulvenes (Table 4), cyclopentadienes (Table 4), 5H‐dibenzo[a,c][7]annulenes (Scheme 6) and dibenzosuberones (Scheme 6). The developed method enables unprecedented product generality for several classes of cyclodehydration reactions, which is particularly notable for the preparation of 1H‐indenes. The first synthesis of non‐benzo‐fused fulvenes via cyclodehydration of allenyl vinyl carbinols could be accomplished. The protocol is remarkable for mild conditions, operational convenience, and easy access to starting materials.     

Superacid‐Promoted Cyclodehydration Leading to the Imidazo[2,1‐a]isoquinoline Ring System

A series of heterocycle‐substituted acetophenones were prepared and reacted with the Brønsted superacid CF₃SO₃H (triflic acid=trifluoromethanesulfonic acid). Cyclodehydration provided aryl‐substituted imidazo[2,1‐a]isoquinolines and related products (28–85%, seven examples). A mechanism is proposed involving dicationic intermediates.     

A New Method of Cyclodehydration of 3,3-Dialkyl-1-Oxo-1-(2- hydroxyaryl)propan-3-ols to 2,2-Dialkyl-4-chromanones: Synthesis of Cannabinoid Synthon

A new method for cyclodehydration of o-hydroxyaryl-β-ketols to 4-chromanones using HMPT is described. The method has general applicability. It is particularly useful for the cyclisation of labile intermediates as exemplified by the synthesis of cannabinoid synthon, 2-methyl-2-(4-methyl-3-pentenyl)-7-methoxy-4-chromanone.     

Divergent Control of Point and Axial Stereogenicity: Catalytic Enantioselective C−N Bond‐Forming Cross‐Coupling and Catalyst‐Controlled Atroposelective Cyclodehydration

Catalyst control over reactions that produce multiple stereoisomers is a challenge in synthesis. Control over reactions that involve stereogenic elements remote from one another is particularly uncommon. Additionally, catalytic reactions that address both stereogenic carbon centers and an element of axial chirality are also rare. Reported herein is a catalytic approach to each stereoisomer of a scaffold containing a stereogenic center remote from an axis of chirality. Newly developed peptidyl copper complexes catalyze an unprecedented remote desymmetrization involving enantioselective C?N bond‐forming cross‐coupling. Then, chiral phosphoric acid catalysts set an axis of chirality through an unprecedented atroposelective cyclodehydration to form a heterocycle with high diastereoselectivity. The application of chiral copper complexes and phosphoric acids provides access to each stereoisomer of a framework with two different elements of stereogenicity.     

Acylation and Cyclodehydration of Benzofuran-, Benzothiophene-, and Indolyl-3-acetic Acid Arylamides. Synthesis of Novel Benzofuro[2,3-c]-, Benzothieno[2,3-c], and Indolo[2,3-c]pyrilium and Pyridine Derivatives

The acylation of benzo[b]furan-, benzo[b]thiophene, and indolyl-3-acetic acid arylamides using acetic anhydride in the presence of 70% perchloric acid occurs at the -position of the heterocycle to give 2-acetylbenzo[b]furan-, 2-acetylbenzo[b]thiophene, and 2-acetylindolyl-3-acetic acid arylamides. Depending on the amount of perchloric used in the reaction they undergo cyclodehydration to 3-arylamino-1-methylhetero[2,3-c]pyrilium salts and to N-aryl-1-methyl-3(2H)hetero[2,3-c]pyridones.