Highly stereospecific protonation of stereoregular living PMMA anions with several alcohols

Living and highly isotactic poly(methyl methacrylate) (PMMA) anion (M̄n = 2.5 × 10³) prepared with t-C₄H₉MgBr as an initiator was protonated with phenol in toluene at −78°C. The reaction was stereospecific toward meso addition, and the meso/racemo ratio at the chain-end of the resultant polymer was 89/11. Addition of 1,4-dioxane to the living isotactic PMMA anion in toluene at −78°C remarkably reduced the viscosity of the system, and protonation of the PMMA anion with phenol in the presence of 1,4-dioxane enhanced the meso-specificity to 94%. On the other hand, the protonation reaction of the living syndiotactic PMMA anion (M̄n = 2.5 × 10³), which was generated by t-C₄H₉Li/(n-C₄H₉)₃Al in toluene at −93°C, with t-butanol was found to be 97% racemo-specific. These highly stereospecific protonation reactions of the stereoregular PMMA anions were in contrast to the protonation of the anions with methanol or benzyl alcohol which was almost non-stereospecific.     

First reversible protonation of the all-nitrogen 1-aryl pentazole ring

At −40 °C in chlorosulfonic acid 1-aryl pentazoles were protonated at N-3. The N-3 atom showed a ¹⁵N NMR shielding shift of −60 to −62 ppm. The -N₅ ring is the highest member of the azole series.     

Acetylation and methylation of flavins and the effects on isoalloxazinc ring protonation

The acetylation of N¹⁰-(poly)hydroxy side chains of 7,8-dimethylisoalloxazines has been accomplished in conventional ways using perchloric acid as catalyst with excess acetic anhydride in acetic acid for flavins or in pyridine for the coenzyme, D -riboflavin-5′-phosphate (FMN). Although complete acetylation of the primary and secondary alcoholic functions of flavins with various hydroxyalkyl chains occurs, the primary product purified by DEAE-cellulose chromatotraphy of the reaction with FMN, followed by neutralization, is the 2′,3′-diacetyl derivative. Diacetyl FMN and other acetylated flavins exhibit the usual absorption spectra of prolonated isoalloxazinium compounds, with λ max values of 390, 264, and 222 nm, only upon greater acidification than necessary for their hydroxy or especially deoxy counterparts. A similar though less marked effect is found with flavins methylated by treatment with methyl iodide and silver oxide in dimethylformamide. Protonation of the isoalloxazine ring, which occurs on the most basic nitrogen 1, is most effectively impaired by acetylation or methylation of the proximal 2′-hydroxyl group.     

Lewis acid-catalyzed stereospecific ring expansion of aziridine-2-carboxylates to imidazolidin-2-ones

Lewis acid-catalyzed ring expansion reaction of chiral aziridine-2-carboxylate proceeds regio- and stereospecifically to yield enantiomerically pure 4-functionalized imidazolidin-2-ones in high yields.     

The protonation of three-membered ring molecules Theab initio SCF versus the electrostatic picture of the proton approach

The protonation processes for eight three-membered ring molecules have been investigated using the SCF LCAO MO method with Gaussian basis sets and the results are compared with those obtained at the first order approximation, i.e. the electrostatic approximation. The electrostatic results are linearly connected with the SCF ones and are sufficient to get an ordering of the protonation energies in different chemical sites and to obtain reliable representations of the proton approaching paths.     

The stereospecific synthesis of spectinomycin

The first synthesis of spectinomycin, a complex and sensitive antibiotic, is described. The synthesis, which requires only four steps from known materials, has flexibility for modifying either half of the molecule.     

The protonation of gaseous cyclopropane

The protonation of cyclopropane by gaseous Br?nsted acids of varying strength in radiolytic experiments at atmospheric pressure leads to two distinct C3H7- isomers that have been sampled by their reaction with benzene. The neutral end products, nC3H7-C6H5 and iC3H7-C6H5, arise from the electrophilic aromatic substitution reaction with the cC3H7+ and iC3H7+ ions, respectively. Their relative abundance was studied as a function of pressure, temperature, and the presence of additives in the gaseous systems; the results indicate a large extent of isomerization to the thermodynamically favored iC3H7+ from the protonation by strong acids. The presence of a kinetic barrier prevents any thermal isomerization from taking place in the time frame of 10(-8) s. In the peculiar case in which protonated benzene is the Br?nsted acid, C3H7+ ions are formed in the presence of neutral benzene within the same ion - molecule complex. The ensuing reaction shows that cC3H7+ ions are formed exclusively and react in the 10(-10) s(-1) estimated lifetime of the complex. Still, such cC3H7+ ions undergo complete randomization of their hydrogen atoms; this points to a low kinetic barrier for the process. Agreement is found between the reported experimental results and updated computations of the relevant species in the C3H7+ potential energy surface.     

Cyclisation of optically active dihydromyrcenes (2,6-dimethyl-2,7-octadiene): A stereospecific ring contraction

The isomeric (+) and (?)-dihydromyrcene cyclise in organic acids through a stereospecific ring contraction process to give the esters of α-(S)-(+)-1-(α-hydroxyethyl)-3,3-dimethylcyclohexane and α(R)-(?)-1-(α-hydroxyethyl)-3,3-dimethylcyclohexane respectively. Small amounts of enantiomeric cis- and trans-tetrahydroeucarvyl esters are also formed. The absolute configuration of the chiral carbinol centres has been determined using Horeau's method. Also, the synthesis of both optical isomers of 3,3-dimethylcyclohexanol has been achieved for the first time.     

The protonation of chlorins (dihydroporphyrins)

Octaethylchlorin forms a violet mono-cation and a deep blue di-cation in two discrete stages on spectrophotometric titration with trifluoroacetic acid in chloroform solution, and similar results are obtained with chlorophyll derivatives although the colour changes are much less pronounced.     

The site of protonation in amino-substituted pteridines

SCF -LCAO -MO -CI calculations made on 2- and 4-amino-, and 2,4-diaminopteridines indicate that the most favorable site where the protonation can take place is N-1.     

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.     

The stereospecific epoxidation of olefins catalysed by ruthenium

Epoxidation of olefins by sodium periodate is effected by the catalysis of RuCl₃, (H₂O)_n associated with bipyridyl. The reaction is stereospecific for both cis and trans alkenes.     

Regio and stereospecific ring expansion of optically active cyclopropylvinylcarbinols. Diastereoselective synthesis of cis Quercus- lactone b.

(-)-(S) dimethyl α-methylsuccinate, from enzymatic resolution, underwent acyloin cyclization followed by stereospecific C4→ C3 ring contraction to provide chiral cyclopropylvinylcarbinols. Then, BF₃-Et₂O induced regio- and stereospecific C3 → C4 ring expansion led to optically active cyclobutanones precursors of cis γ-lactones, with high enantiomeric excesses.