Aryl Acetates Research Articles

Sulfurated Borohydride Exchange Resin: Reductive Cleavage of Aryl Acetates under Mild Conditions

A variety of aryl acetates are selectively cleaved to the corresponding phenols using sulfurated borohydride exchange resin in methanol at 25°C. Other sensitive groups present elsewhere in the molecule remain unaltered under these reaction conditions.

ChemInform Abstract: Selective Deprotection of Aryl Acetates, Benzoates, Pivalates, and Tosylates under Nonhydrolytic and Virtually Neutral Conditions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Selective Deprotection of Aryl Acetates, Benzoates, Pivalates, and Tosylates under Nonhydrolytic and Virtually Neutral Conditions

Selective Deprotection of Aryl Acetates, Benzoates, Pivalates, and Tosylates under Nonhydrolytic and Virtually Neutral Conditions

On the Mechanism of Ester Hydrolysis: Trifluoroacetate Derivatives

The hydrolysis rate of trifluoacetates of ArX (X = p-CH3, H, p-F, p-Cl, and m-Cl) was measured as function of pH and buffer concentration. All the reactions were catalyzed by the buffer bases, and the Bronsted plot showed a small upward curvature. The kinetic isotope effect is ca. 2.5−2.3 for the reactions of water. The Hammett plots as well as the plots of log kB vs pKlg are linear. The slopes of these plots are remarkably similar to those corresponding to the hydrolysis of aryl acetates and aryl formates. From these results, we conclude that the reaction corresponds to general base-catalyzed addition of water and break of the leaving group with no intermediate with finite lifetime despite the fact that the trifluoromethyl group stabilized the intermediate considerably.

Concerted Mechanisms of the Reactions of Ethyl S-Aryl Thiocarbonates with Substituted Phenoxide Ions

The reactions of 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl O-ethyl thiolcarbonates with substituted phenoxide ions are subjected to a kinetic study in water, 25.0 °C, ionic strength 0.2 M (KCl). By following the reactions spectrophotometrically, pseudo-first-order rate coefficients (kobsd) are found under excess of the nucleophile. Plots of kobsd vs phenoxide anion concentration at constant pH are linear, with the slope (kN) independent of pH. The Brönsted-type plots (log kN vs pKa of the phenols) are linear with slopes β = 0.92, 0.77, and 0.61 for the reactions of the 4-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl derivatives, respectively. For these reactions, a concerted mechanism is proposed since the slope values are similar to those found in the concerted phenolysis of aryl acetates; the slope magnitudes are not consistent with a stepwise mechanism where the formation of a tetrahedral intermediate is rate limiting. Our results are in line with the finding that the reactions of the 2,4-dinitrophenyl and 2,4,6-trinitrophenyl derivatives with secondary alicyclic amines in water are concerted. In contrast, the reactions of the same substrates with pyridines are stepwise, which means that substitution of a pyridine moiety in a tetrahedral intermediate with a benzenethio group by a phenoxy group destabilizes the intermediate.

Correlation analysis of carbonyl carbon 13C NMR chemical shifts, IR absorption frequencies and rate coefficients of nucleophilic acyl substitutions. A novel explanation for the substituent dependence of reactivity

Rate coefficients of nucleophilic acyl substitutions, carboxylate carbon 13C NMR chemical shift values and ν(CO) frequencies of several series of aryl and acyl substituted aryl acetates or alkyl benzoates have been investigated. An increasing electron-withdrawal by the acyl or aryl substituents results in higher reaction rates, upfield 13C NMR chemical shifts and higher frequencies of the CO stretching. Good correlations are observed for the log kversus δC(CO) plots. The increase of the reaction rate with increased electron density at the CO carbon (as proved by 13C NMR shifts) contradicts the previous concept of increased electrophilicity of the carbonyl carbon by electron-withdrawing substituents. The rate increase is now attributed to the decrease of the ester ground state resonance stabilization caused by electron-withdrawing substituents. The use of log kversus δC(CO) correlations is presented as a practical method to evaluate rate coefficients especially for compounds for which Hammett type correlations cannot be used.

Observations on the generation of organozinc carbenoids from acetals and ketals

The use of acetals and ketals as direct precursors for organozinc carbenoid chemistry is illustrated by their direct conversion to alkenes in a one pot operation. Aryl acetals can also be used as precursors for the cyclopropanation of cyclohexene.

A novel arylesterase active toward 7-aminocephalosporanic acid from Agrobacterium radiobacter IFO 12607: Purification and characterization

Through the screening of yeasts and bacteria, some strains of Agrobacterium radiobacter have been found to efficiently convert 7-aminocephalosporanic acid (7-ACA) to deacetyl 7-ACA. The enzyme responsible for the conversion was constitutively formed by the bacterium. The 7-ACA deacetylating enzyme was purified from a cell-free extract of A. radiobacter IFO 12607 to homogeneity. The purified enzyme was a tetramer, composed of identical subunits with a molecular mass of 26 kDa, and showed activity toward some derivatives of 7-ACA, aryl acetates, monoacetin and triacetin, but was not active towards alkyl acetates and cephalosporin C. The K m and k cat values of the enzyme for 7-ACA were 8.24 mM and 81 s −1, respectively. Serine inhibitors and sulfhydryl reagents were both found to inhibit the activity. An amino acid sequence, GDSLT, which is the active center motif of an arylesterase, was identified in the amino terminal region. These results indicate that the enzyme belongs to the family of arylesterase (EC 3.1.1.2), and is a new 7-ACA-deacetylating enzyme.

Bi(III) as New Catalyst for the Selective Hydrolysis of Esters

The selective hydrolysis of esters has been carried out in a reaction catalyzed by Bi(III)-mandelate in DMSO. The reaction is specific of aryl esters. Competition tests resulted in the quantitative hydrolysis of p-nitrophenyl acetate in the presence of ethyl benzoate or other aryl acetates. © 1997 Published by Elsevier Science Ltd.

Stereoelectronic effects on reactivity. Crystal-structure–reactivity correlations for acetals with synperiplanar lone pairs

The synthesis and five crystal structures are reported for a series of tetrahydropyranyl acetals 3, in which the tetrahydropyranyl ring is fixed in the symmetrical boat conformation. The spontaneous hydrolysis of four aryl acetals 3 is a few times faster than that of corresponding compounds in which the tetrahydropyran ring is conformationally mobile and thus free to adopt the chair conformation. There appears to be no stereoelectronic barrier to participation by synperiplanar (sp) as compared with antiperiplanar (ap) lone pairs in the acetal cleavage reaction. However, at least part of this increased reactivity towards hydrolysis must derive from the higher ground state energy of the boat conformation, and a careful examination of a series of compounds 3 reveals intriguing and systematic differences between the two systems. The spontaneous hydrolysis of the synperiplanar series shows extraordinarily high sensitivity to the leaving group (βLG = 1.4), compared with the already high value known for axial tetrahydropyranyl acetals with antiperiplanar lone pairs. In contrast, bond length correlations show a reduced sensitivity. The results suggest that the n–σ*C–O interaction (endocyclic anomeric effect) is weaker in the ground state in the sp geometry.

Reactions of Lignin Model ß-Aryl Ethers with Acetyl Bromide

The reactivities of several softwood lignin model compounds with acetyl bromide have been examined in a number of good lignin solvents. Primary (y-)alcohols acetylate more rapidly than phenolic hydroxyls, whereas benzylic (α-)hydroxyls are converted to bromides via their acetates. Models treated under mild conditions with acetyl bromide cleanly produced acetylated α-bromo derivatives. Benzyl aryl ethers are cleaved rapidly and quantitatively to yield aryl acetates and acetylated α-bromo products. The rate order for the observed reactions is : primary (γ-)OH acetylation > benzyl aryl ether cleavage/bromination > benzylic secondary (α-)OH acetylation > benzylic acetate to benzylic bromide interconversion >> phenolic OH acetylation. These reactions are synthetically useful and have further potential in analytical methodologies.

Transfer of a positively charged acyl group between substituted phenolate ion nucleophiles: the brønsted β for the calibrating equilibrium for N-methylisonicotinyl (4-carbonyl-N-methylpyridinium) transfer

Rate constants have been measured for the reaction of substituted phenolate ions with aryl acetate esters and with aryl N-methylisonicotinate esters in aqueous solution. A new method is demonstrated for determining βeq for group transfer from 4-nitrophenyl esters; it employs the rate constant for the reaction of 2,6-difluorophenolate ion with substituted phenyl ester as a surrogate for the reactivity of the 4-nitrophenolate ion and yields βeq= 1.55 for the N-methylisonicotinyl transfer reaction. The Bronsted-type plot of the rate constant for phenolate ion attack on 4-nitrophenyl N-methylisonicotinate is linear over a range of pKa values from 5.5 to 10 and provides good evidence for a concerted displacement mechanism for this reaction. The reactivity of the N-methylisonicotinate esters to phenolate ions is some 300 times larger than that of the corresponding acetate esters but the larger βnuc value (0.90 compared with 0.74) suggests a ‘later’ transition structure. Calibration of the β values with the corresponding βeq gives a Leffler αnuc= 0.58 and 0.42 for N-methylisonicotinate and acetate respectively, which contrasts with the order expected from reactivity–selectivity. The tighter transition structure indicated by comparison of these α values is explained by a less favourable acylium ion in the case of the N-methylisonicotinyl transfer reaction.

Kinetic resolution of sulfoxides with pendant acetoxy groups using cholesterol esterase: substrate mapping and an empirical rule for chiral phenols

Sulfoxides are valuable chiral auxiliaries because they direct the formation of carbon–carbon bonds. As a route to enantiomerically pure sulfoxides, we examined hydrolase-catalyzed kinetic resolution by hydrolysis of a pendant acetoxy group. Screening hydrolases for the enantioselective hydrolysis of the acetate in 2-(methylsulfinyl)phenyl acetate, 1b, identified cholesterol esterase (CE) as the most enantioselective enzyme. The enantiomeric ratio, E, ranged from 10 to 25, favoring the (R) configuration at sulfur. Competing chemical hydrolysis of 1b caused the large range in the measured values of E. A small-scale (250 mg) resolution of (±)-1b yielded (S)-1b with >99% ee (11% yield) after recrystallization. Changing the methyl substituent to phenyl or n-butyl did not significantly change the enantioselectivity (E = 10 and 15, respectively), but changing it to a chloromethyl substituent lowered the enantioselectivity slightly (E = 5). Changing the phenyl acetate to a naphthyl acetate (2-(phenylsulfinyl)phenyl acetate vs. 1-(phenylsulfinyl)-2-naphthyl acetate) increased the enantioselectivity from 10 to 19. In all cases CE favored the (R)-sulfoxide. To aid the design of new resolutions with CE, we propose an empirical rule that accounts for the observed enantiopreference of CE toward these 5 sulfoxides and 15 other chiral aryl acetates. This empirical rule uses both the size of the substituents and their conformational preferences to predict which enantiomer reacts faster. Keywords: kinetic resolution, lipase, cholesterol esterase, sulfoxides, empirical rule.

Determination of the absolute configuration of alcohols by low temperature 1H NMR of aryl(methoxy)acetates

Aromatic shielding effects in esters of ( R)- and ( S)- aryl(methoxy)acetic acids (AMAAs) differ by a factor Δδ RS , which, due to changes in rotamer populations, was seen to increase in 1H NMR spectra of esters of AMAAs 6 – 10 as NMR probe temperature ( T) was decreased. For example, Δδ RS for esters of 6 increased by more than 100% between T = 298 and T = 220−175 K (depending on the alcohol). By comparing Δδ RS values obtained from low temperature 1H NMR spectra of their esters with ( R)- and ( S)- AMAAs 6 – 10, the absolute configuration of chiral alcohols can be reliably assigned.

ChemInform Abstract: Selective ortho‐Arylation of Polyfluorinated Hydroxyaromatic Compounds with Lead Aryl Acetates.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Borohydride exchange resin; selective removal of the acetyl groups from aryl acetates

Borohydride exchange resin exhibits high selectivity in the removal of acetyl groups only from aryl acetates, however, no deacylation being observed for alkyl acetates.

Alkali Metal Selectivity of Calix[n]aryl Acetates as Estimated by Mass Spectrometry on the Relative Stability of the 2:1 Metal/Calix[n]arene Complex

Abstract It was found that the metal selectivity of calix[n]aryl acetates (n = 4, 6, and 8) can be conveniently estimated by mass spectrometry. The results have disclosed several new phenomena which cannot be found through conventional solvent extraction.

Selective ortho-arylation of polyfluorinated hydroxyaromatic compounds with lead aryl acetates

Reactions of salts of pentafluorophenol and heptafluoronaphthols with lead aryl acetates have led to the formation of arylcyclohexadienones with an ortho-quinoid structure. The arylation of sodium pentafluorophenoxide and heptafluoro-2-naphthoxide is accompanied by nucleophilic substitution of fluorine at a double bond by pentafluorophenoxy and heptafluoro-2-naphthoxy groups, respectively. Reduction of the resulting arylcyclohexadienones gave polyfluorinated phenols and naphthols with aryl and hydroxy groups in the 1,2-position.

Lipase YS-Catalyzed Enantioselective Transesterification of Alcohols of Bicarbocyclic Compounds

Abstract Lipase YS-catalyzed enantioselective acylations of (1RS, 2RS, 4RS, 5RS)-bicyclo[2.2.1]heptane-2,5-diol, (1RS, 2SR, 4RS, 5SR)-bicyclo[2.2.2]octane-2,5-diol, (1RS, 2RS, 4RS, 5RS)-bicyclo[2.2.2]octane-2,5-diol, and (1RS, 2SR, 5RS, 6SR)-bicyclo[3.3.1]nonane-2,6-diol with aryl acetates such as phenyl acetate, 1-naphthyl acetate, and 2-naphthyl acetate gave the monoacetates and the diols in optically active forms. The optical purities of (+)-(2R, 5R)-2-acetoxybicyclo[2.2.1]heptan-5-ol, (−)-(1S, 2R, 4S, 5R)-2-acetoxybicyclo[2.2.2]octan-5-ol, and (+)-(2R, 6R)-2-acetoxybicyclo[3.3.1]nonan-6-ol obtained using phenyl acetate were higher than those of the monoacetates which have been resolved by PLE-catalyzed hydrolysis of the corresponding racemic diacetates. Enzymic acylations of the diols using phenyl esters such as phenyl propanoate, phenyl butanoate, and phenyl octanoate as acylating agents gave the corresponding monoesters, which showed nearly the same enantiomeric excess value as the acetates. A simple model for predicting which enantiomer of racemic alcohols is acylated preferentially by lipase YS-catalyzed enantioselective transesterification is proposed on the basis of the results obtained here.

Alkaline earth metal ion catalysis of alcoholysis of crown ether aryl acetates. Effect of the base–solvent system

AbstractThe effect of strontium and barium ions on the acyl transfer reaction to ethoxide ion from 2‐acetoxy‐1,3‐xylyl‐15‐crown‐4, 2‐acetoxy‐1,3‐xylyl‐18‐crwon‐5, 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6 and the model compound phenyl acetate was investigated in ethanol at 25°C. Significant catalytic effects were observed in all of the studied reactions as a result of stronger metal ion associations with the transition states than with reactants. The rate enhancements observed with the crown ether substrates are much larger than those observed with phenyl acetate, the largest acceleration being displayed by 2‐acetoxy‐1,3‐xylyl‐21‐crown‐6, which reacts with EtOBaBr 5 × 105 times faster than with EtONMe4. These findings provide strong evidence that cooperation of electrostatic binding to the negative charge developing at the carbonyl oxygen and coordinative binding to the polyether chain is highly efficient in the metal‐bound transition state. Comparison of rate data reported here with analogous data obtained in methanol solution clearly shows that both modes of interaction contributing to the stability of the metal‐bound transition state are much more efficient in ethanol than in methanol solution.