Efficient and Selective Formation of Mixed Acetals by Nafion‐H SAC‐13 Silica Nanocomposite Solid Acid Catalyst

Abstract

Various types of hydroxy compounds can readily be converted to the corresponding mixed acetals with dialkoxymethanes in the presence of SAC‐13 solid superacid. The transformation is almost instantaneous, product acetals are isolated in good to excellent yields, and the catalyst can be reused with minor loss of activity. Comparative studies were also carried out with p‐toluenesulfonic acid and BF₃ · OEt₂.     

Relative activities of tetrahydrofurfural acetals in the detachment of a hydrogen atom by tert-butoxyl radicals

The reactivities of linear and cyclic acetals of tetrahydrofurfural in the reaction with tert-butoxyl radicals were studied. The k_a/k_d parameters, which are the ratios of the rate constants for the accumulation of tert-butyl alcohol to the rate constants for the formation of acetone, are close to the parameters for 2-alkyl-substituted acetals. Linear acetals are less active than cyclic acetals, the activities of which increase on passing from 2-tetrahydrofuryl-1,3-dioxolane to its sulfur- and nitrogen-containing analogs.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1597–1599, December, 1981.     

A selective and convenient ruthenium mediated method for the synthesis of mixed acetals and orthoesters

Addition of alcohols and phenols to O-allyl compounds (allyl ethers and acrolein acetals), catalyzed by [RuCl₂(PPh₃)₃], was examined. Intramolecular addition of an OH group, leading to the formation of cyclic acetals and orthoesters, was also investigated. As a result, a new, selective and convenient method for the synthesis of symmetrical and unsymmetrical (mixed) acetals and orthoesters was developed.     

Regio- and diastereoselective SN2′ or SN2″ reactions on chiral acetals of cyclic aldehydes promoted by PhCu,BF3

The regio- and diastereoselectivity of the addition of PhCu, BF₃ reagent on chiral acetals derived from several mono or dienic aldehydes was studied. It was found that monoethylenic acetals react regio- and diastereoselectively via an overall anti S_N2′ reaction. The regioselectivity observed with the dienic acetals seems to be strongly dependent on the nature of the acetal. In all cases the S_N2″reaction was the result of an anti process.     

Mild and efficient method for the cleavage of benzylidene acetals using HClO4-SiO2 and direct conversion of acetals to acetates

HClO₄-SiO₂ has been used successfully for the deprotection of benzylidene acetals and the direct conversion of benzylidene acetals to the corresponding di-O-acetates. The reactions are very fast and yields are excellent.     

Unique chemoselective Mukaiyama aldol reaction of silyl enol diazoacetate with aldehydes and acetals catalyzed by MgI2 etherate

Functionalized diazo acetoacetates are prepared by an efficient Mukaiyama aldol reaction between 3-TBSO-2-diazo-3-butenoate with aldehydes and acetals under mild reaction conditions. A variety of substituted aldehydes and the corresponding acetals are both accessible in good to excellent yields through this methodology. MgI₂ etherate (MgI₂·(OEt₂)_n) is the preferred catalyst and, the addition proceeds without decomposition of the diazo moiety. In addition, this MgI₂·(OEt₂)_n-catalyzed Mukaiyama aldol reaction shows unique chemoselectivity towards aldehydes and acetals.     

Synthesis and some properties of gem-dichlorocyclopropane carboxaldehyde acetals and cyclopropane carboxaldehyde acetals

α,β-Unsaturated aldehyde acetals add :CCl₂ (obtained in accordance with Makosza procedure) to form gem-dichlorocyclopropanecarboxaldehyde acetals (1) in high yields when the ratio of reactants is no less than 1:4. The addition of :CCl₂ to polyunsaturated aldehyde acetals in the above conditions proceeds regioselectively at double bonds enriched by electrons. Compounds 1 are reduced by sodium in liquid ammonia to give cyclopropanecarboxaldehyde acetals (2). Both 1 and 2 are hydrolized by dilute H₃P0₄ to the corresponding aldehydes and are added to vinyl ethers in the presence of BF₃·Et₂O to produce only 1:1 adducts which are hydrolyzed by a mixture of AcOH-AcONa-H₂O to give β-formylcyclopropanes which were previously unknown.     

Regio- and chemoselective reductive cleavage of 4,6-O-benzylidene-type acetals of hexopyranosides using BH3·THF-TMSOTf

Benzylidene-type cyclic acetals of carbohydrates undergo efficient reductive ring opening using BH₃·THF and a catalytic amount of TMSOTf at room temperature. 4,6-O-Benzylidene-hexopyranosides afford the corresponding 4-O-benzyl ethers exclusively, in high yields. Other benzylidene-type acetals, such as naphthylmethylene and 4-methoxybenzylidene acetals are also cleaved with the same reagent. The conversions are highly regio- and stereoselective and afford benzyl-type ethers in excellent yields.     

Reactivity of RCu,BF3 and R2CuLi,BF3 towards the ether linkage. Epoxides,acetals and orthoformates

The association of BF₃ to organocopper and cuprate reagents increases dramatically their reactivity towards epoxides. The same reagents cleave acetals to afford the product of substitution of one alkoxy group, whereas orthoformates lead to acetals under conditions where no further attack occurs.     

Polymerization of α-Methylstyrene at High Temperatures in Tetrahydrof uran with Potassium as Initiator. I. Thermodynamic Study and Gel-Permeation Chromatographic Analyses of the Polymers

The acetals of aryl aldehydes react with aryl amines to produce Schiff bases in quantitative yields. This reaction was also facile with diamines and diacetals. It involves a two-step elimination of alcohol; kinetic data indicate that k ₂ is equal to or greater than k ₁ and this complicates the isolation of any intermediate compound.

Because of overlap of absorption bands, the existence of intermediates was not confirmed by infrared spectral measurements. Adducts of the acetals and the aniline hydrochlorides were isolated as hydrochlorides; their molecular weights, as well as the products obtained by neutralization, indicated that the intermediate is not a monoalkoxy compound. The acetals react, also, with N-acyl aniline by the elimination k ₁ of the alcohol and k ₂ of the ester, in which k ₁ k ₂, to produce Schiff bases in less than quantitative yields. These reactions of acetals with amines and their N-acyl derivatives are of interest in the syntheses of polymers.     

Alkylation of acetals using manganate-BF3·OEt2 mixed reagent

A mixture of `R<sub>3</sub>MnMgBr' and BF<sub>3</sub>·OEt<sub>2</sub> prepared in advance only by stirring both reagents in ether converted acetals to alkylation products, where an alkoxy group of acetals was substituted by the alkyl group of manganese reagent used. Ketals also reacted with the `mixed reagent' to afford the corresponding alkylation products in high yield. α-Alkoxy-substituted cyclic ethers and acetoxy-substituted cyclic ethers were selectively converted to ring-opening alkylation products and α-alkyl-substituted cyclic ethers, respectively.     

A Convenient Synthesis of Aminoacids by ZnBr2 Promoted Reaction of Ketene bis(Trimethylsilyl) Acetals with Aldimines

The ZnBr₂ promoted addition of ketene bis(trimethylsilyl) acetals to aromatic aldimines affords β-arylaminoacids in good to excellent yields. Under the same reaction conditions vinylic ketene bis(trimethylsilyl) acetals give exclusively or mainly δ - phenylaminoacids.     

Vanadium(III) Chloride (VCl3): Efficient Reagent for the Introduction of Tetrahydrofuran‐Based Acetal Protecting Groups for Alcohols

Treatment of different types of alcohols with tetrahydrofuran (THF) in the presence of VCl₃ and CCl₄ smoothly afforded the corresponding THF‐based acetals in excellent yields. The reaction is fast at room temperature, and several functional groups are tolerated, with no racemization being observed. A radical mechanism, based on Cl₃C^. as the active species, is proposed for this novel kind of transformation, which complements the classical tetrahydro‐2H‐pyran‐2‐yl (THP) protocol.     

Rhenium-catalyzed reaction of carbonyl compounds with ketene silyl acetals

It was found that rhenium complex is an effective catalyst for the reaction of carbonyl compounds with ketene silyl acetals. A wide range of β-silyloxy esters is obtained by the treatment of carbonyl compounds with ketene silyl acetals in the presence of a catalytic amount of ReBr(CO)₅ in moderate to good yields.     

1,3-Polyol arrays via the stereoselective rearrangement of vinyl acetals

A series of 1,3-dioxanyl vinyl acetals were readily synthesized from the corresponding dioxanone by a reduction and in situ acylation followed by Petasis olefination. Treatment of these vinyl acetals with BF₃·OEt₂ results in an O to C rearrangement to form anti-3,5-dihydroxyketones while a mixture of Me₃Al and BF₃·OEt₂ provides the corresponding syn relationship via a stereoretentive rearrangement.     

A new method for the synthesis of mixed orthoesters from O-allyl acetals

Two new methods for the synthesis of orthoesters and compounds containing an orthoester moiety (dihydroisoxazoles) are presented. Mixed orthoesters of general formulas RC(OR¹)(OR²)₂ and RC(OR¹)(OR²)(OR³) were prepared via addition of ROH (R = Bu or m-methylphenyl) to O-allyl acetals (acrolein acetals: diethyl or cyclic, i.e., 2-vinyl-1,3-dioxanes or dioxolanes). The catalytic systems for these reactions were generated from [RuCl₂(PPh₃)₃] and Na₂CO₃; {[RuCl₂(COD)]_x} or {[OsCl₂(1,5-COD)]_x}, PPh₃, and Na₂CO₃. Compounds containing an orthoester moiety (dihydroisoxazoles) were prepared via tandem isomerization of O-allyl acetals (to O-vinyl acetals) catalyzed by ruthenium complexes followed by cycloaddition to in situ-generated 2,6-dichlorophenylnitrile oxide.     

Co2(CO)8-catalyzed carbonylation of acetals using N-silylamines

Formaldehyde dialkyl acetals and cyclic acetal, 1,3-dioxolane, are smoothly carbonylated using N-silylamines at 140–160 °C under 70–88 kg cm⁻² CO pressure in the presence of a catalytic amount of Co₂(CO)₈ to give the corresponding 2-alkoxyamides in moderate to good yields. In the carbonylation of formaldehyde dialkyl acetals using N-silylamines, addition of pyridine drastically enhances the catalytic activity.     

Direct synthesis of acetals by rhodium catalysed hydroformylation of alkenes in the presence of orthoformate

The two catalyst precursors [Rh₂(μ-penicillamine)₂(CO)₄][OTf]₂ and [Rh₂(μ-cysteine)₂(CO)₄][OTf]₂ in the presence of 4 equivalents of P(OPh)₃ in triethyl orthoformate as solvent and reactant, permit the low pressure hydroformylation of various alkenes into the corresponding acetals. Apart from a few low-yield by-products resulting from isomerization of the substrates, the carbonylated products obtained directly and exclusively are acetals.     

Efficient preparation of 4-methoxy-5,6-dihydro-2H-pyran

We report the efficient synthesis of 4-methoxy-5,6-dihydro-2H-pyran (MDHP) via the TiCl₄ driven elimination of MeOH from 4,4-dimethoxytetrahydropyran. The previous difficulty of preparing MDHP restricted the wider use of 4-methoxytetrahydropyran-4-yl (MTHP) acyclic acetals, which have desirable protecting group properties when compared to more commonly used MOM- and THP-acetals. The behaviour of the elimination on related acetals is also examined.     

Indium(III) triflate catalysed transacetalisation reactions of diols and triols under solvent-free conditions

Acyclic acetals and ketals undergo transacetalisation in the presence of catalytic quantities of indium(III) triflate (In(OTf)₃) and diols or triols under solvent-free conditions to generate the corresponding cyclic acetals and ketals in excellent yield. The methodology has been further developed to encompass a tandem acetalisation-acetal exchange protocol, which provides a facile and high yielding route to cyclic ketals from unreactive ketones under very mild reaction conditions.