Indium triflate mediated tandem acetalisation-acetal exchange reactions under solvent-free conditions

Acyclic acetals and ketals undergo exchange reactions in the presence of catalytic quantities of indium(III) triflate and diols to generate the corresponding cyclic acetals and ketals in excellent yield. The protocol is rapid, employs mild conditions and can be adapted to employ solvent-free reaction conditions. We have further developed this methodology to encompass a tandem acetalisation-acetal exchange protocol which provides facile access to cyclic ketals from unreactive ketones also under very mild, solvent-free reaction conditions.     

GaCl3-catalyzed insertion of isocyanides into a C-O bond in cyclic ketals and acetals

The reaction of cyclic ketals or acetals with 2,6-dibromophenylisocyanide in the presence of a catalytic amount of GaCl(3) results in the insertion of isocyanide into the carbon-oxygen bond of cyclic ketals and acetals. [reaction: see text]     

Indium(III)trifluoromethanesulfonate as a mild, efficient catalyst for the formation of acetals and ketals in the presence of acid sensitive functional groups

Aldehydes and ketones, including acetophenone and benzophenone, are readily protected under mild, neutral conditions in the presence of various alcohols or orthoformates and catalytic amounts of indium(III) trifluoromethanesulfonate (<0.8 mol %) under either room temperature or mild heating conditions to give the corresponding cyclic and acyclic acetals and ketals in good to excellent yields. Acid sensitive functional groups, N-Boc, THP, and TBDMS do not undergo competitive deprotection under the reported conditions.     

Highly efficient chemoselective deprotection of O,O-acetals and O,O-ketals catalyzed by molecular iodine in acetone

An extremely convenient method for deprotection of acetals and ketals catalyzed by molecular iodine (10 mol %) in acetone is reported. The protocol achieved the deprotection of acyclic or cyclic O,O-acetals and O,O-ketals in excellent yields within a few minutes under neutral conditions. The double bond, hydroxyl group, and acetate group remained unchanged, and the highly acid-sensitive furyl, tert-butyl ethers, and ketone-oxime stayed intact under these conditions.     

Reactions of Enone Ethylene Ketals with Methyl Diazomalonate/Bis(acetylacetonato)copper(II)

Several cyclic and acyclic enones and their ethylene ketals/acetals were reacted with dimethyl diazomalonate under bis(acetylacetonato)copper(II) catalysis. Cyclohex-2-en-1-one ( 1 ) yielded only C–H insertion products 2 and 3 , whereas but-3-en-2-one gave a cyclopropane albeit in very low yield. The ethylene ketals 6 of cyclopent-2-en-1-one and cyclohex-2-en-1-one gave the corresponding cyclopropanes 7 , which were in turn cleaved to the ketones 8 . The acetals 9 and 10 of crotonaldehyde ((E)-but-2-enal) and cinnamaldehyde ((E)-3-phenylprop-2-enal), respectively, yielded C–O insertion and [2,3]-sigmatropic rearrangement products 11b, c and 12b, c , as well as cyclopropanes 11a and 11b , all of which are polyfunctional and synthetically useful compounds.     

732^#强酸性阳离子交换树脂催化缩醛和缩酮的合成

本文研究了732＃强酸性阳离子交换树脂对缩醛和缩酮的催化作用，系统地探讨了各反应因素对反应的影响，研究发现，732＃强酸性阳离子交换树脂具有较高的催化活性，并使后处理简化。     

An Efficient Photoinduced Deprotection of Aromatic Acetals and Ketals

A novel type of photodeprotection reaction of simple aromatic acetals and ketals is described. The deprotection is highly efficient under optimized conditions. The aromatic ring confers the photoreactivity to the compounds. The efficiency of the process depends on the structure of the acetal moiety. The common minor side reaction, the photooxidation, becomes the major reaction pathway in the cases of cyclic acetals. The use of photon as only reagent makes this procedure especially attractive for acetal deprotection.     

2,4,4,6-Tetrabromo-2,5-cyclohexadienone (TABCO), N-Bromosuccinimide (NBS) and Bromine as Efficient Catalysts for Dithioacetalization and Oxathioacetalization of Carbonyl Compounds and Transdithioacetalization Reactions

The use of 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TABCO), N-bromosuccinimide (NBS), and bromine as efficient catalysts for conversion of carbonyl compounds to their cyclic and acyclic dithioacetals and 1,3-oxathiolanes under mild reaction conditions are described. These catalysts are also used for efficient transdithioacetalization of acetals, diacetals, ketals, acylals, enamines, hydrazones, and oximes with high yields in the presence of thiols.     

Synthesis of [60]fullerene acetals and ketals: reaction of [60]fullerene with aldehydes/ketones and alkoxides

The reaction of C(60) with propionaldehyde (butyraldehyde or phenylacetaldehyde) and MeONa-MeOH or EtONa-EtOH in anhydrous chlorobenzene in the presence of air at room temperature unexpectedly gave rare fullerene acetals 2aa-cb, while the reaction of C(60) with acetone (acetophenone, cyclohexanone, or cyclopentanone) and MeONa-MeOH or EtONa-EtOH under the same conditions afforded the uncommon fullerene ketals 4aa-db. A possible reaction mechanism for the formation of the fullerene acetals and ketals is proposed based on further experimental results.     

Simple and efficient chemoselective mild deprotection of acetals and ketals using cerium(III) triflate

A new and chemoselective method for the cleavage of alkyl and cyclic acetals and ketals at room temperature in wet nitromethane by using catalytic cerium(III) trifluoromethane sulfonate is presented. The high yields, the observed selectivity, the very gentle reaction conditions, and the almost neutral pH make this procedure particularly attractive for multistep synthesis.     

Pd(II)-catalyzed acetal/ketal hydrolysis/exchange reactions

PdCl₂(CH₃CN)₂ catalyzes the hydrolysis of dioxolane acetals and ketals in moist CH₃CN, while in acetoze, efficient and more reproducible exchange reactions take place.     

Unzipping and scrambling reaction-induced sequence control of copolymer chains via temperature changes during cationic ring-opening copolymerization of cyclic acetals and cyclic esters

A temperature change-dependent sequence transformation of copolymer chains was demonstrated by a method based on tandem depolymerization and transacetalization reactions during the cationic ring-opening copolymerization of cyclic acetals and cyclic esters. In this study, the position of polymerization-depolymerization equilibrium was controlled by the reaction temperature rather than by the decrease in monomer concentration under vacuum conditions, as in our previous study. First, the conditions for efficient copolymerization were optimized, with a particular focus on the structures of cyclic acetals and cyclic esters. Subsequently, sequence transformation induced by temperature change was examined during the copolymerization of 2-methyl-1,3-dioxepane (generated in situ from 4-hydroxybutyl vinyl ether) and δ-valerolactone using EtSO₃H. The homosequence length of cyclic acetals decreased during depolymerization (unzipping) at the oxonium chain ends upon increasing the temperature from 30 to 90 °C, while transacetalization (scrambling) of the main chain transferred midchain cyclic acetal homosequences to the oxonium chain ends. As a result of the cycle of unzipping and scrambling reactions, an alternating-like copolymer was obtained. Interestingly, the possibility of reversible sequence transformation upon heating and cooling was also demonstrated.     

Potassium Ferrate Supported on Montmorillonite K-10: A Mild and Efficient Reagent for Oxidative Deprotection of Acetals and Ketals

Summary.  A variety of acetals and ketals were oxidatively deprotected to their parent compounds using potassium ferrate(VI) supported on montmorillonite K-10 under non-aqueous conditions. Received January 23, 2001. Accepted (revised) March 14, 2001     

Replacement of C−O by P−O in Cyclic Acetals and Ketals

Efficiency and structural specificity earmark the reaction of phosphonium ions 1 with cyclic acetals and ketals to yield 1,3,2‐dioxaphospholanium ions 2 [Eq. (1)]. Potential applications of this reaction are in monitoring trace levels of organophosphorus esters and in developing novel carbonyl deprotection agents. R=OCH₃, CH₃; R¹=H, CH₃; R²=CH₃, C₆H₅; R³=H, CH₃.     

Unexpected highly chemoselective deprotection of the acetals from aldehydes and not ketones: TESOTf-2,6-lutidine combination

Acetal functions are recognized as good protecting groups of carbonyl groups. Although many deprotecting methods of acetals to carbonyl functions have already been developed, there is no methodology which can deprotect acetals in the presence of ketals because the usual acidic or radical reactions occur more easily via the more stable cationic or radical intermediates from the ketals. On the other hand, this new method can proceed in a reverse manner to that described in previous reports. That is, the method can deprotect aliphatic acetals in the presence of ketals. The reaction condition is common for silylation, i.e., the TESOTf-2,6-lutidine combinations. Although the TMSOTf-2,6-lutidine combination can also deprotect acetals, it lacks chemoselectivity in deprotection of the acetals from aldehydes and ketones. The treatment of acetals with TESOTf and 2,6-lutidine in CH2Cl2 followed by a H2O workup gave the corresponding aldehydes. Of course, the compounds, which have both acetal and hydroxyl functions afforded the compounds obtained by the usual silylation of an alcohol and deprotection of an acetal without any problem. However, deprotection of the ketals from ketones was not observed during the conversion reaction of acetals from aldehydes. This chemoselectivity was confirmed in the reactions of the compounds that have the acetal and ketal in the same molecule. In both cases, the acetal functions were deprotected to give aldehydes with intact ketals. Furthermore, under the conditions described here, many functional groups such as methoxy, acetoxy, allyl alcohol, and silyloxy ether are intact. This method is very mild and available for many compounds.     

Neighbouring-group Influence on the Ring Opening of Some 2-Alkyl-1,1,2-tribromocyclopropanes under Phase-transfer Conditions

Several 2-alkyl-1,1,2-tribromocyclopropanes were treated with sodium hydroxide and ethanol under phase-transfer conditions. Ring opening gave mixtures of the corresponding acetylenic diethyl ketals and acetals. When the steric bulk of the alkyl substituent was increased acetal formation dominated, and in the case of 1,1,2-tribromo-2-(tert-butyl)cyclopropane, the acetal was formed as the only product.     

Сhemo- and regioselective modification of adenosine with tertiary cyanopropargylic alcohols

Adenosine reacts with tertiary cyanopropargylic alcohols under mild conditions (K₂CO₃, DMF, 20-25 °C, 4-30 h) with 100% chemo- and regioselectivity by the way of two vicinal hydroxy groups of the ribose moiety to afford (in 52-72% yield) a novel family of adenosine cyclic ketals possessing cyano and hydroxyalkyl functions.     

Cerium(IV)-Catalyzed Deprotection of Acetals and Ketals under Mildly Basic Conditions

Smooth and quantitative deprotection of a wide range of acetals and ketals [Eq. (a); R, R(1)=alkyl, aryl, H] under neutral to mildly basic conditions was achieved with catalytic quantities of cerium ammonium nitrate (CAN). The reaction conditions are compatible with a variety of sensitive functional groups, and aldehydes can be liberated from acetals without being oxidized to the corresponding carboxylic acids.     

Neighbouring-group Influence on the Ring Opening of Some 2-Alkyl-1,1,2-tribromocyclopropanes under Phase-transfer Conditions

Summary. Several 2-alkyl-1,1,2-tribromocyclopropanes were treated with sodium hydroxide and ethanol under phase-transfer conditions. Ring opening gave mixtures of the corresponding acetylenic diethyl ketals and acetals. When the steric bulk of the alkyl substituent was increased acetal formation dominated, and in the case of 1,1,2-tribromo-2-(tert-butyl)cyclopropane, the acetal was formed as the only product.     

An efficient procedure for protection of carbonyls in Brønsted acidic ionic liquid [Hmim]BF4

Protection of carbonyls as acetals or ketals using Brønsted acidic ionic liquid [Hmim]BF₄ as catalyst as well as solvent was investigated. Satisfactory results were obtained for the protection of carbonyls as cycloacetals or ketals with diols. The product can be separated conveniently from the reaction system, and the ionic liquid can be reused after removal of water.