Enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes catalyzed by chiral dirhodium(II) carboxylates: asymmetric synthesis of phenylglycine derivatives

The first catalytic enantioselective amination of silylketene acetals with (N-arylsulfonylimino)phenyliodinanes is described. The reaction of silylketene acetals derived from methyl phenylacetates with [N-(2-nitrophenylsulfonyl)imino]phenyliodinane (NsN = IPh) under the catalysis of dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], Rh₂(S-TCPTTL)₄, proceeds in benzene at room temperature to give N-(2-nitrophenylsulfonyl)phenylglycine derivatives in high yields and with enantioselectivities of up to 99% ee.     

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.     

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.     

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.     

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.     

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.     

(Triisopropylsilyl)acetaldehyde acetal as a novel protective group for 1,2-diols

(Triisopropylsilyl)acetaldehyde dimethyl acetal (TIPS-ADMA) was synthesized from chlorotriisopropylsilane in three steps. Cyclic and acyclic 1,2-diols can be transformed to (triisopropylsilyl)ethylidene acetals (TIPS-AA). Removal of the acetal by LiBF₄ regenerates the starting diol in excellent yield even in the presence of an acetonide of 1,2-diol. The TIPS-AA group can survive under the deprotection conditions of the acetonide in acetic acid at 80 °C. Selective protection of 2,3- and 4,6-diols for O-methyl d-mannoside with TIPS-ADMA and selective deprotection of the acetals have been achieved.     

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.     

Acetals of γ-oxo-α,β-unsaturated esters in nitrone cycloadditions. Regio- and stereochemical implications

Several acetals of γ-oxo-α,β-unsaturated esters have been prepared, mainly from enantiopure C₂-symmetric diols, and their 1,3-dipolar cycloaddition to 2,3,4,5-tetrahydropyridine 1-oxide has been studied. All the reactions showed complete regioselectivity and a high preference for the endo approach of the reactants in the transition state. The enantiopure acetals derived from (cis,cis)-spiro[4.4]nonane-1,6-diol gave the highest diastereofacial selectivity.     

Proximity effects and cyclization in oxaalkanes+CCl4 mixtures DISQUAC characterization of the Cl–O interactions. Comparison with Dortmund UNIFAC results

Thermodynamic properties, vapor–liquid equilibria (VLE), molar excess Gibbs energies (G^E), molar excess enthalpies (H^E) and natural logarithms of activity coefficients at infinite dilution (ln γ_i^∞) or partial molar excess enthalpies at infinite dilution (H_i^E,∞) of mixtures of oxaalkanes, linear or cyclic monoethers, diethers or acetals, and CCl₄ are studied in the framework of DISQUAC. The oxygen/CCl₄ contacts are characterized by dispersive (DIS) and quasichemical (QUAC) interaction parameters, which are reported. Contacts of the type (polar group)/CCl₄ are usually characterized by DIS parameters only. The effects of proximity and cyclization on the interchange coefficients are examined. In comparison with systems of oxaalkanes and n-alkanes, some differences exist; e.g., linear monoethers and diethers+CCl₄ mixtures are represented by different interaction parameters due to proximity effects of oxygen atoms (i.e., –O–C–C–O–) in diethers. In solutions with cyclic molecules, ring strain seems to be now more important. DISQUAC results are compared with those obtained using the Dortmund version of UNIFAC. From this comparison, it is concluded that it is necessary to distinguish at least between monoethers, diethers and acetals when treating mixtures with oxaalkanes and that each cyclic molecule should be characterized by its own set of interaction parameters.     

Synthesis of 3-sulfanylpropanols containing three consecutive stereocenters via tandem Michael-aldol reaction of enoylthioamides with acetals as key reaction

(2S,3S,1′R)-2-(α-Methoxybenzyl)-3-phenyl-3-sulfanylpropionamides were diastereoselectively prepared by the reactions of N-cinnamoyl-4S-isopropyl-5,5-dimethyloxazolidinethione with acetals in the presence of SnCl₄. The absolute configuration of the three newly created contiguous stereocenters was determined by the X-ray analysis of the disulfide. The amides were transformed into propanols by the reductive removal of the oxazolidinone moiety.     

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.     

Asymmetric C–H insertion of Rh(II) stabilized carbenoids into acetals: A C–H activation protocol as a Claisen condensation equivalent

The dirhodium tetraprolinate, Rh₂(S-DOSP)₄ is an efficient catalyst in an enantioselective C–H activation protocol. Rh₂(S-DOSP)₄ catalyzed decomposition of aryldiazoacetates or vinyldiazoacetates results in the formation of transient rhodium carbenoid intermediates. These intermediates are capable of selectively inserting into the C–H bond of acetals. The resulting products are protected β-keto esters, and so the C–H activation protocol can be considered as strategically equivalent to the Claisen condensation.     

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.     

A facile synthetic method of α-quaternary-β,γ-unsaturated aldehydes via the stereoselective 1,4-elimination and α-regioselective Ferrier reaction

The 1,4-elimination reaction of 2-substituted-(2Z)-4-methoxy-O-alkenyl acetals with n-butyllithium is shown to afford the 2-substituted-(1Z,3E)-O-1,3-dienyl acetals in high stereoselectivities. The Ferrier reaction of the O-1,3-dienyl acetals thus obtained provides the corresponding α-quaternary-β,γ-unsaturated aldehydes in excellent yields with high α-regioselectivities.     

Convergent procedure for the synthesis of trifluoromethyl-containing N-(pyridinyl-triazolyl)pyrimidin-2-amines

This study describes a simple and efficient procedure to synthesize a novel series of fourteen 4-substituted N-(5-pyridinyl-1H-1,2,4-triazol-3-yl)-6-(trifluoromethyl)pyrimidin-2-amines, where the 4-substituents are H, CH₃, C₆H₅, 4-FC₆H₄, 4-CH₃C₆H₄, 4-CH₃OC₆H₄ and 2-Furyl; from the cyclocondensation reaction of N-[5-(pyridinyl)-1H-1,2,4-triazol-3-yl]guanidines with 4-alkoxy-4-alkyl(aryl/heteroaryl)-1,1,1-trifluoroalk-3-en-2-ones. The reactions were carried out in ethanol under reflux for 18 h and led to 40-68% yields. N-(Pyridyl-triazolyl)guanidine precursors were further obtained from reactions of cyanoguanidine with nicotinic or isonicotinic acid hydrazides and the halogenated enones from trifluoroacetylation of enolethers or acetals.     

A simple TiCl4 promoted arylation of orthoformate and benzyl ethers by N,N-dialkylarylamines

N,N-Dialkylarylamines react with trimethyl orthoformate and TiCl₄ under ambient conditions to give the corresponding formyl derivatives in 75-89% yields, whereas the corresponding arylated products are obtained from benzyl ethers and acetals in 42-78% yields.     

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.     

Thermodynamics of vinyl ethers—XVIII : Unsaturated acetals

A thermodynamic study of geometrical isomerization reactions of unsaturated acetals R¹R²C(OCHCHMe)₂ and MEOC(R¹R²)OCHCHMe is reported. The values of the thermodynamic parameters of isomerization are rather insensitive to the bulkiness of R¹ and R² suggesting that the favored rotamers of these acetals are not the s-cis but the s-trans rotamers. This conclusion is supported by IR and NMR data.     

Addition of activated olefins to cyclic N-acyliminium ions in ionic liquids

Organoindate(III) ionic liquid (BMI·InCl₄) was successfully employed in the nucleophilic addition of allyltrimethylsilane, silyl enol ethers and ketene silyl acetals to in situ generated cyclic N-acyliminium ions at room temperature without the need of an external Lewis acid. The corresponding α-substituted heterocycles were obtained in good yields and the recovered ionic liquid phase could be reused at least three times.