Pyrylium Mediated Transformations of Primary Amino Groups into Other Functional Groups. New Synthetic Methods (41)

The first pyrylium salt was isolated some 80 years ago, yet up to the 1950s only moderate interest was taken in the preparation, properties and uses of such salts. However, the past thirty years has seen a phenomenal growth in the literature pertinent to this area of chemistry: the importance of pyrylium salts as intermediates has been realized. They are readily prepared by a variety of generally applicable routes, and they are highly reactive towards nucleophiles. Together, this enables the convenient synthesis of a great variety of acyclic and heterocyclic compounds. We have used highly substituted pyrylium salts for the two-step conversion of the amino group in alkylamines RNH₂ into numerous other functionalities. In the first step, the pyrylium salts are converted with the amines into N-substituted pyridinium salts, which, in the second step, react with Nu^? to give the desired products RNu. In some cases the R moiety is also changed, e.g. by elimination. Studies of the reactions of these pyridinium salts have allowed interesting insights into the mechanisms of nucleophilic substitution, in addition to rendering aliphatic amines important synthetic intermediates. Thus, the method complements the diazotization procedure for the transformation of arylamines.     

A study of the thermal decarboxylation of three perfluoropolyether salts

The thermal decarboxylation of three dicarboxylic perfluoropolyether potassium salts of relatively short chain length has been investigated and the products and kinetics of the main reactions have been defined. From the rate constants and Arrhenius parameters data, the second decarboxylation appears to be quantitatively rather close to the first.     

Recyclization of pyrimidinium salts into 1,2,4-triazole derivatives

The interaction of the iodomethylates of pyrimidinyl-2-acetic acid derivatives with monosubstituted hydrazines, in addition to the products of a Kost-Sagitullin rearrangement, leads also to N-substituted triazoles. The structure of the triazoles was demonstrated by NOESY NMR experiments. The structure of the reaction products was determined on the basis of the response observed in the spectra between the methyl group protons of the triazole ring and the spatially close proton of the substituent in position 1 and a conclusion was drawn on the direction of the primary attack of nucleophile in the recyclization process of the pyrimidinium salts into a 1,2,4-triazole. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1175–1179, August, 2007.     

Catalytic olefination reaction of carbonyl compounds. A study on stereoselectivity of alkene formation

The mechanism of formation of alkene stereoisomers in the catalytic olefination reaction of carbonyl compounds was studied. 4-Chlorobenzaldehyde hydrazone 1 stereoselectively reacts with a number of F-, Cl-, Br-, and I-containing polyhaloalkanes in the presence of catalytic amounts of CuCl to give -substituted styrenes 2 with the more thermodynamically stable alkene isomer being the major product. A model for the formation of the stereoisomers of alkenes 2 in the olefination reaction is proposed. Stereoselectivity of the reaction is determined by elimination of copper(ii) halides from the lowest-lying conformers of organocopper intermediates II. According to quantum-chemical calculations, the elimination should involve the staggered conformations with antiperiplanar arrangement of C—Hal and C—Cu bonds and proceed by the E2 anti-elimination mechanism. The results of quantum-chemical calculations are in good agreement with the experimental E/Z alkene isomer ratios.     

Über die Darstellung von Di(i-propyl)thiosulfonium-Salzen

On the Preparation of Di(i-propyl)thiosulfonium Salts [1] The preparation of the mercaptosulfonium salts (i-C₃H₇)₂SSX⁺SbCl₆^? (X = H, D) and of the chlorothiosulfonium salts (i-C₃H₇)₂SSCl⁺MF₆^? (M = As, Sb) and (i-C₃H₇)₂SSSCl⁺SbCl₆^? is reported. They are formed by reaction of chlorinated or protonated (i-C₃H₇)₂S with excess H₂S or SCl₂ and S₂Cl₂. The thiosulfonium compounds are characterized by vibrational and NMR spectroscopic methods.     

Easy synthesis of heterocyclic carbene complexes by activation of chalcogenopyrones and benzopyrones to pyrylium salts and subsequent addition of carbanion of methoxy(methyl)pentacarbonyltungsten carbene complex

Methylenechalcogenopyran and benzopyran Fischer carbene complexes are easily obtained from commercially available chalcogenopyrones or benzopyrones and carbanion of methoxy(methyl)carbene tungsten complex. The key of the heterocyclic carbene formation is the activation of the carbonyl group by alkylation with alkyl trifluoromethanesulfonate reagent.     

A mild hydrolysis of esters mediated by lithium salts

When treated with amine bases such as triethylamine and various lithium salts in wet solvents, esters are efficiently hydrolyzed to the corresponding acids in good yields. Esters incorporating an α- or β-heteroatom with respect to the ester carbonyl group are hydrolyzed rapidly even at room temperature. To further demonstrate the usefulness of this method, one example is provided where hydrolysis of acetylated camphorsultam is mediated by LiBr.     

Carbamoylimidazolium and thiocarbamoylimidazolium salts: novel reagents for the synthesis of ureas, thioureas, carbamates, thiocarbamates and amides

Carbamoylimidazolium salts act as efficient N,N-disubstituted carbamoylating reagents. These salts are readily prepared by the sequential treatment of secondary amines with N,N′-carbonyldiimidazole (CDI) and iodomethane. The carbamoylimidazolium salts are more efficient carbamoyl transfer reagents than the intermediate carbamoylimidazoles, as a result of the ‘imidazolium’ effect. Kinetic studies on the base promoted hydrolysis of both carbamoylimidazoles and carbamoylimidazolium salts reveal over a hundred-fold rate acceleration. The salts react with amines, thiols, phenols/alcohols, and carboxylic acids in high yields, without the need for subsequent chromatographic purification of the products, producing ureas, thiocarbamates, carbamates, and amides, respectively. Analogous thiocarbamoylimidazolium salts were also synthesized from secondary amines and N,N′-thiocarbonyldiimidazole (TCDI), followed by methylation with iodomethane.     

Über die Kristallstrukturen von CH3PF2H+AsF6− und CH3PF2H+SbF6− und eine einfache Darstellung von CH3PF2

On the Crystal Structures of CH₃PF₂H⁺AsF₆^? and CH₃PF₂H⁺SbF₆^? and a simple Method for Preparation of CH₃PF₂ A simple method for preparation of CH₃PF₂ from CH₃PCl₂ is reported. The phosphonium salts CH₃PF₂H⁺MF₆^? are obtained by the reaction of CH₃PCl₂ with superacidic systems HF/MF₅ (M = As, Sb). CH₃PF₂H⁺SbF₆^? crystallizes in the space group P1 with a = 548.4(4) pm, b = 695.5(8) pm, c = 960.2(9) pm, α = 94.68(5)°, β = 97.19(6)°, γ = 94.41(6)° and Z = 2. CH₃PF₂H⁺SbF₆^? crystallizes in P1 with a = 554.3(3), b = 724.2(4), c = 970.4(5), α = 94.73(4)°, β = 96.14(5)°, γ = 95.30(4)°.     

Diastereoselective synthesis of 2-oxiranyl and 2-aziridinyl thiazoles

2-Chloroalkylthiazolyllithiums, prepared by deprotonating the 2-chloroalkylthiazoles with n-BuLi at −78°C in THF, add to ketones or imines affording in ‘one-pot’ oxiranes and aziridines in a Z stereoselective manner.