A study of gaseous 3-membered ring oxonium and sulphonium ions

The 1-methyl-1-thionia cyclopropane 3 and 1-phenyl-1-thionia cyclopropane 4 ions are stable, with lifetimes greater than 10^?5 sec, and can be identified from their collisional activation spectra. Their metastable counterparts (lifetime window 10^?6–10^?5 sec) have undergone ring opening to the isomeric structures CH₃S⁺CHCH₃9 and C₆H₅S⁺CHCH₃11 prior to decomposition. The 1-methyl-1-oxonia cyclopropane 1 and 1-phenyl-1-oxonia cyclopropane 2 ions could not be generated: instead acyclic structures CH₃O⁺CHCH₃5 and C₆H₅O⁺CHCH₃7 were found for both metastable and long living species. Loss of a phenoxy radical from C₆H₅OCH₂CH₂OCH₃ is shown to be preceded by a reciprocal hydrogen transfer and is not due to a SN_i-type reaction.     

Regio- and stereoselective ring opening of aziridines with nitric oxide

Reaction of N-tosyl aziridines with nitric oxide affords the corresponding ring-opened products in regio-, stereoselectivities and excellent yields.     

Regio- and stereoselectivity of diethylaluminum azide opening of trisubstituted epoxides and conversion of the 3 degrees azidohydrin adducts to isoprenoid aziridines

The regioisomer ratios (3 degrees,2 degrees /2 degrees,3 degrees ), and in some cases the stereochemistry, of vicinal azidohydrins formed in reactions of 11 trisubstituted terpene epoxides with Et(2)AlN(3) in toluene are reported. The more highly substituted azide usually predominated (3 degrees,2 degrees /2 degrees,3 degrees ratios >or= 40:1 to 2.5-1) in accord with a Markovnikov orientation and an S(N)1-like transition state. Reversed regioisomer ratios were observed with 6,7-epoxygeranyl acetate (1:2.5) and cis-1,2-epoxylimonene (1:3.3 to 1:10). The tertiary azido diols from 2,3-epoxygeraniol, 2,3-epoxyfarnesol, and 2,3-epoxynerol were formed as single isomers with inversion of configuration at C3 (>or= 35-40:1 for the C(10) azido diols). The regioselectivity was affected by the presence and proximity of oxy functional groups on the epoxide substrate (OH, OAc, and OSi-tBuMe(2)), the equivalents of Et(2)AlN(3), and additives (EtOAc or EtOH). The results and trends are rationalized by consideration of the structural and stereoelectronic characteristics of proposed diethylaluminum epoxonium ion intermediates and transition states, together with the nucleophilicity of the azide donor. Six of the 3 degrees,2 degrees azidohydrins were converted to the corresponding aziridines by primary-selective silylations of four azido diols, mesylations, and reductive cyclizations with LiAlH(4).     

Hydrolytic opening of the quinazoline ring in 3-furfuryl-4-oxo-3,4-dihydroquinazoline derivatives

Ethyl 3-furfuryl-4-oxo-3,4-dihydroquinazoline-2-carboxylate extremely readily undergoes hydrolysis in acid, alkaline, or neutral medium with formation of 2-(2-ethoxy-1,2-dioxoethylamino)-N-furfurylbenzamide. The reaction of ethyl 3-furfuryl-4-oxo-3,4-dihydroquinazoline-2-carboxylate with phenylmagnesium bromide yields N-furfuryl-2-(2-hydroxy-2,2-diphenyl-1-oxoethylamino)benzamide as a result of hydrolytic cleavage of the quinazoline ring.     

Regio- and stereoselectivity in the addition of metal n,n-dimethylphenylacetamide enolates to some conjugated carbonyl compounds

The nucleophilic addition of lithium, sodium, potassium and bromomagnesium N,N-dimethylphenylacetamide enolates to cinnamic aldehyde, chalcone and methyl cinnamate is studied. The regioselectivity of the reaction is found to depend on the metal counter ion and other reaction conditions. The stereoselectivity under kinetic and thermodynamic conditions does not depend on the metal. The reactivity of the amide-enolates is compared with the reactivity of other enolates already studied.     

Regio- and stereoselective ring opening of vinyl epoxides with MgBr2

The regio- and stereoselective ring opening of vinyl epoxides has been achieved by the use of Lewis acid, MgBr₂, affording bromohydrins in excellent yield, which are readily transformed to azidoalcohol, a key intermediate of several classes of pyrrolizidine and indolizidine alkaloids. The scope and limitations of the reaction are discussed.     

Mass-spectrometric study of the cleavage products of 3,4-epoxytetrahydropyrans by amines

Conclusions Only 4-methyl-3-amino-4-hydroxytetrahydropyrans are formed when the oxide ring of 4-methyl-3,4-epoxytetrahydropyran is opened by ammonia and amines. The structural isomers of 4-amino-3-hydroxy-and 3-amino-4-hydroxytetrahydropyran are formed in the analogous reaction of the unsubstituted 3,4-epoxy-tetrahydropyran, the mutual arrangement of the substituents in which was determined on the basis of analyzing the mass spectra.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 636–639, March, 1971.     

Regio- and stereoselective ring opening of epoxy alcohols with organoaluminium compounds leading to 1,2-diols

Introduction of alkyl, alkynyl group, or hydride occurs regioselectively at the 3 position of the epoxy alcohols with inversion of the configuration upon treatment with organoaluminium reagents to produce vic-diols.     

Chiral anion-mediated asymmetric ring opening of meso-aziridinium and episulfonium ions

Reactions proceeding through cationic intermediates that lack a Lewis or Br?nsted basic site present a challenge for traditional asymmetric catalysis based on chiral metals or organocatalysts. We present an enantioselective ring opening of tetrasubstituted meso-aziridinium ions with alcohol nucleophiles proceeding through a chiral ion pair with a binaphthol-phosphate anion. The reaction is initiated by silver-induced ring closure of beta-chloroamines using the Ag salt of the chiral anion as in situ generated catalyst. Use of insoluble Ag2CO3 as silver source is essential to obtain high enantioselectivity; we believe the chiral phosphate acts as a "chiral anion phase transfer catalyst" to bring silver ion into the organic phase. The chiral anion concept can also be extended to the related asymmetric opening of meso-episulfonium ions generated by protonation of trichloroacetimidates vicinal to sulfides.     

Alkylation and an unusual reductive ring opening of some thieno[3,4-b][1,5]benzoxazepin-10-ones

As part of a program towards the syntheses of novel tricyclic compounds, some derivatives of thieno[3,4-b]-[1,5]benzoxazepin-10-one system 1 were prepared and were subsequently subjected to reduction reactions using lithium aluminum hydride. Although thienobenzoxazines 4 were obtained as the sole product of the reduction in most cases, unusual ring-opened products, namely 3-hydroxymethylthiophenes 5 , were also formed during several reductions.     

The reductive ring opening of some derivatives of isatin

Treatment of two isatin-3-imines, 3-isopropyIamino-1-methylindoline-2-one and 3-cyclohexylamino-1-methylindolin-2-one with sodium borohydride produced the fully-reduced anilines 2-isopropylamino-2-(2-methylaminophenyl) ethanol and 2-cyclohexylamino-2-(2-methylaminophenyl) ethanol, respectively. Reductive ring-opening of derivatives of isatin is not observed in related examples when either catalytic or lithium aluminum hydride reduction are employed and it is concluded that the present effect is dependant upon the softness of sodium borohydride as a reducing agent.     

Studies of amine-induced ring opening of some mesoionic xanthines

The ring-opening reactions of seven mesoionic thiazolo[3,2-a]pyrimidine-5,7-diones by a series of primary and secondary amines have been investigated. The rates of the ring fission of five N(8)-substituted mesoionic xanthines with benzylamine were measured and found to follow second order kinetics. The Hammett relationship is followed with ? value of + 0.48 in p-dioxane as solvent. The dependence of rates on temperatures has been studied for the N(8)-ethyl derivative; the energy of activation (Δ E* ) is 25.3 kcals mol^?1, the enthalpy of activation (Δ H* ) is 24.7 kcals mol^?1 and the entropy of activation (Δ S* ) is — 4.9 e.u. A slight increase in rate of reaction was observed when the solvent was changed from p-dioxane to dimethyl sulfoxide. In p-dioxane at constant mesoionic xanthine concentration, the rate constant for ring opening decreased with increasing benzylamine concentration. These results are consistent with a bimolecular nucleophilic mechanism proceeding by the rate-determining formation of a charged tetrahedral transition state.     

Regio- and stereospecific ring opening of 1,1-dialkyl-2-(aryloxymethyl)aziridinium salts by bromide

Enantiomerically pure 2-(aryloxymethyl)aziridines are efficiently transformed into chiral N-(2-bromo-3-aryloxypropyl)amines via a regio- and stereospecific ring opening of the intermediate aziridinium salts, and the experimental results are rationalized on the basis of some high level ab initio calculations.     

Regioselective ring opening and isomerization reactions of 3,4-epoxyesters catalyzed by boron trifluoride

Efficient ring opening of 3,4-epoxyesters with alcohols to produce 4-alkoxy-3-hydroxyesters and their isomerization into 4-ketoesters using boron trifluoride as the catalyst are presented. Both transformations are simple and efficient methods for the synthesis of the above named synthetically useful compounds.     

Alkylative ring opening of N-methylaziridinium ions and a formal synthesis of tyroscherin

Alkylative ring-opening reactions of stable 2-substituted N-methylaziridinium ions proceeded with various alkyl- or arylmagnesium bromides in the presence of CuI to yield synthetically valuable and optically pure alkylated acyclic amines in a completely regio- and stereoselective manner. This was applied to a formal synthesis of the cytotoxic natural product tyroscherin.     

Regio- and stereoselectivity of 1,3-dipolar cycloaddition of cyclic aldonitrones of the 3-imidazoline 3-oxide series to monosubstituted alkenes

Regio- and stereoselectivity of 1,3-dipolar cycloaddition of cyclic aldonitrones of the 3-imidazoline 3-oxide series mainly depends on the type of the substituent in the dipolarophile. The configuration of the main cycloadduct has been determined, and a method has been suggested to establish the stereochemistry of the cycloaddition products by¹H NMR spectroscopy. An increase in electron-acceptor properties of the substituent in the alkene molecule results in a decrease in the regio- and stereoselectivity of the cycloaddition.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 894–898, May, 1994.     

Regio- and stereoselective ring opening of enantiomerically enriched 2-aryl oxetanes and 2-aryl azetidines with aryl borates

The regioselective ring opening of 2-aryl-substituted four-membered heterocyclic rings with phenols, including catechol, was achieved by the use of aryl borates in mild and neutral reaction conditions without the aid of any transition metal catalysts. While N-alkyl azetidines were found not to be reactive, optically active N-tosyl azetidines gave the corresponding beta-aryloxy amines in a racemic form, thus indicating the considerable carbocationic character of the transition state. The introduction of a hydroxyl group in the azetidine ring (i.e., an azetidinol), able to anchor the aryl borate and to direct the subsequent nucleophilic delivery, was shown to determine the ring-opening process with predominant inversion of configuration. When enantiomerically enriched 2-aryl oxetanes were used, the reduced extent of racemization observed (up to 93:7 er) was rationalized by an intramolecular delivery through a six-membered transition state, giving beta-aryloxy alcohols with a predominant retention of configuration (i.e., a syn-stereoselective ring opening). The aryloxy alcohols obtained, endowed with suitable functionalities, can be cyclized to give access to enantiomerically enriched 2-aryl-1,5-benzodioxepins.