Direct Acid-Catalyzed Hydration of Camphene as a Route to Isoborneol

Heterogeneous homophase hydration of camphene in the presence of H₄SiW_{1 2}O_{4 0}, H₃PW_{1 2}O_{4 0}, or HClO₄ was studied as a selective procedure for production of isoborneol.     

Acid-Catalyzed Diels-Alder Reactions of Cycloalkenones

Whereas the Diels-Alder reaction has been in vogue as a facile method of six-membered ring construction for many years, its application to a one-step cis-octalone synthesis was precluded by discouraging, early reports on the conditions required for the interaction of dienes with 2-cyclohexenones and on the yields of the resultant adducts.^1,2 It is quite conceivable that these observations were responsible in part for the minimization of use of an otherwise attractive route toward angularly alkylated six-membered polycycles in the field of steroid total synthesis. The discovery of Lewis acid catalysis of the cycloaddition of α, β-unsaturated carbonyl compounds³ has rekindled interest in the above reactions.⁴ The following discussion illustrates octalone     

Acid-Catalyzed Amine-Borane Reduction of Nitrite

The rate of reduction of nitrite by trimethylamine-borane was followed by observing the decrease in nitrite absorbance under pseudo-first-order conditions. The reaction is acid-catalyzed and exhibits a first-order dependence on both amine-borane and total nitrite concentration. The molar equivalence of NaNO(2) to (CH(3))(3)NBH(3) = 2:1. Equimolar amounts of hydrogen and nitrous oxide are formed, and the molar ratio of nitrite reacted to N(2)O produced is 2:1. In concentrated HCl or H(2)SO(4), a correlation of rate with the Hammett acidity function, h(o), is observed. The reaction is subject to a pronounced inversesolvent isotope effect (k(D)()2(O)/k(H)()2(O) approximately 2.7) and a modest normal substrate effect (k((CH)()3())()3(N.BH)()3/k((CH)()3())()3(N.BD)()3 approximately 1.4). The reaction is first-order in H(3)O(+) in the region pH 0.7-2.7, but a second-order dependence is observed above pH 4 with the transition occurring at pH approximately pK(a) for HNO(2). Results are consistent with a mechanistic model involving preequilibration protonation of molecular nitrous acid followed by rate-limiting hydride attack on H(2)ONO(+) or free NO(+) to produce nitrosyl hydride as a reactive intermediate.     

Acid-Catalyzed Cyclodehydration of Quinazolone Prepolymers

The effect of p-toluenesulfonic acid and phosphoric acid on the cyclodehydration of quinazolone prepolymers has been investigated. Isothermal weight loss during the curing process was monitored closely to follow the extent of cyclization. The optimum weight loss corresponding to 2 mol water is used as an indication of the formation of a fully cyclized quinazolone structure without concurrent decarboxylation. This can be achieved by incorporating the catalyst at 2 wt% concentration, which permits a lower curing temperature than those previously reported. The polymers formed in the presence of catalyst have thermal stability superior to those prepared without catalyst. The glass transition temperature is also raised. This we conclude to be a direct result of the decrease in the amount of incomplete quinazolone rings along the polymer backbone as a result of more efficient cyclization catalyzed by the acids.     

Energetics of the Acid-Catalyzed o-Cresol-Formaldehyde Reaction

The acid-catalyzed reaction of o-cresol with formaldehyde follows second-order kinetics. The reaction was carried out at 65, 70, 75, and 80°C and at pH values of 1.30, 1.80, 2.00, 2.50, and 3.00, using hydrochloric acid as a catalyst. The rate was found to increase with decreasing pH. The overall rate constant (k) has been resolved into stepwise rate constants (k₁ and k₂) for the formation of monomethylol and methylene derivatives. Values of Arrhenius parameters and of the entropy of activation for the overall reaction were also calculated. A mechanism consistent with our kinetic data is given.     

钼酸催化简便合成D－葡萄庚酮糖

本文首次报道了在三氧化钼水溶液体系下催化碳链重排合成D-葡萄庚酮糖的简便方法。首先以D-甘露糖为原料在碘催化下进行异丙叉保护,产率90％;接着在碱催化下与甲醛水溶液亲核加成反应,在C-2上引入羟甲基侧链,产率40％;然后在酸性条件下异丙叉脱保护,产率92％;最后在三氧化钼水溶液催化下碳链重排合成葡萄庚酮糖,产率56％（总产率18％）。另外对2-C-羟甲基-D-甘露糖和葡萄庚酮糖的乙酰化产物进行了核磁氢谱表征。该方法原料易得,缩短了反应步骤,避免了昂贵金属试剂的使用,有较好的应用前景。     

A Novel Acid-Catalyzed Isomerization of Aib-Containing Thiodipeptides

The use of amino thioacids in the `azirine/oxazolone method' led to completely epimerized Aib-containing endothiodipeptides (Aib=2-aminoisobutyric acid). It could be established that the epimerization occurred during the acidic hydrolysis of the primarily formed dipeptide thioanilides in which the thiocarbonyl group was shifted from the last to the penultimate amino-acid residue. Several conditions for the hydrolysis were tested, and, in some of them, the degree of epimerization could be reduced. By treatment of the Aib-containing dipeptide thioanilides 21 with ZnCl₂ in AcOH followed by HCl in AcOH, the isomeric endothiodipeptide anilides 25 were formed, i.e., the thiocarbonyl group was again shifted from the last to the penultimate amino acid residue. Under optimized reaction conditions, this novel isomerization proceeded in high yields and without any epimerization. Two conceivable mechanisms are proposed in Scheme 12. X-Ray diffraction analyses were performed for Z−Gly−Aib-Ψ(CS)−N(Me)Ph ( 21f ) and the isomeric Z−Gly-Ψ(CS)−Aib−N(Me)Ph ( 25f ).     

The Acid-Catalyzed Hydrolysis of Anomeric Alkyl Fructofuranosides

Abstract

The rate constants for the hydrolysis of some al-kyl α- and β-d-fructofuranosides in aqueous perchloric acid have been determined at various temperatures. The effects of varying the aglycon structure on the hydrolysis rate suggest, together with the markedly positive entropies of activation, that the substrate, protonated on the glycosidic oxygen atom, undergoes a rate-limiting unimolecular heterolysis to form a glycosyl oxocar-benium ion. The rate variations in mixtures of aqueous perchloric acid and dimethyl sulfoxide are interpreted as lending further support for the proposed mechanism.     

Acid-Catalyzed Rearrangements of (-)-Thujopsene

The transformations of (-)-thujopsene in liquid (HSO₃F-SO₂FCl) and over solid (TiO₂/SO₄ ^{2 -}) superacids, as well as by the action of peroxy acids, were studied. New tricyclic hydrocarbons were isolated. The experimental results were compared with the data of computer analysis of the most probable transformation pathways using molecular-mechanics and quantum-chemical methods.     

Acid-Catalyzed Cyclodimerizations via Vinyl Cations as Intermediates

The electrophilic addition of hydrogen chloride and bromide to allene and of hydrogen bromide to methylacetylene at ?70 °C leads not only to the simple Markownikoff adducts but also to cis- and trans-1,3-dihalo-1,3-dimethylcyclobutanes. These cyclodimerizations, which evidently proceed via vinyl cations, have opened up new short routes for the synthesis of cyclobutane-, cyclobutene-, and bicyclobutane derivatives starting from substrates that are industrially readily accessible.     

Acid-Catalyzed Transacetalization from Glycol to Pinacol Acetals

A one-pot transacetalization of glycol acetals, frequently used as protecting groups of the aldehyde function, into the more stable pinacol acetals is given. A clean transformation of aromatic and aliphatic substrates is possible with trifluoroacetic acid within 30 min at 0 °C. Glycol acetals derived from ketones (ketals) cannot be converted with this protocol. Deprotection of the pinacol acetals is possible with trifluoromethanesulfonic acid in the presence of water.     

手性Lewis酸催化硝酮与烯烃的1,3-偶极环加成反应

综述了近年来利用手性Lewis酸催化剂对映选择催化硝酮与1,2-二取代烯烃的1,3-偶极环加成反应的研究进展.     

Adamantylazoles: IV. Acid-Catalyzed Adamantylation of Pyrazoles

Pyrazoles with pK _{B H} ⁺ no more than 0.8 and having substituents in 3(5) position with effective van der Waals radii not exceeding 2 Å in a mixture of phosphoric and acetic acids at weight ratio 4:1 (H ₀ -1,8) react with 1-adamantanol to afford the corresponding 1-(1-adamantyl)- or 1,4-di(1-adamantyl)pyrazoles.     

Acid-Catalyzed [3,3]-Sigmatropic Rearrangements of N-Propargylanilines

The acid-catalyzed rearrangement of N-(1′,1′-dimethylprop-2′-ynyl)-, N-(1′-methylprop-2′-ynyl)-, and N-(1′-arylprop-2′-ynyl)-2,6-, 2,4,6-, 2,3,5,6-, and 2,3,4,5,6-substituted anilines in mixtures of 1N aqueous H₂SO₄ and ROH such as EtOH, PrOH, BuOH etc., or in CDCl₃ or CCl₄ in the presence of 4 to 9 mol-equiv. trifluoroacetic acid (TFA)has been investigated (cf. Scheme 12-25 and Tables 6 and 7). The rearrangement of N-(3′-X-1′,1′-dimethyl-prop-2′-ynyl)-2,6- and 2,4,6-trimethylanilines (X = Cl, Br, I) in CDCl₃/TFA occurs already at 20° with τ_1/2 of ca. 1 to 5 h to yield the corresponding 6-(1-X-3′-methylbuta-1,2′-dienyl)-2,6-dimethyl- or 2,4,6-trimethylcyclohexa-2,4-dien-1-iminium ions (cf. Scheme 13 and Footnotes 26 and 34) When the 4 position is not substituted, a consecutive [3,3]-sigmatropic rearrangement takes place to yield 2,6-dimethyl-4-(3′-X-1′,1′-dimethylprop-2′-ynyl)anilines (cf. Footnotes 26 and 34). A comparable behavior is exhibited by N-(3′-chloro-1′-phenylprop-2′-ynyl)-2,6-dimethylaniline ( 45 ., cf. Table 7). The acid-catalyzed rearrangement of the anilines with a Cl substituent at C(3′) in 1N aqueous H₂SO₄/ROH at 85-95°, in addition, leads to the formation of 7-chlorotricyclo[3.2.1.0^2,7]oct-3-en-8-ones as the result of an intramolecular Diels-Alder reaction of the primarily formed iminium ions followed by hydrolysis of the iminium function (or vice versa; cf. Schemes 13,23, and 25 as well as Table 7). When there is no X substituent at C(1′) of the iminium-ion intermediate, a [1,2]-sigmatropic shift of the allenyl moiety at C(6) occurs in competition to the [3,3]-sigmatropic rearrangement to yield the corresponding 3-allenyl-substituted anilines (cf. Schemes 12,14–18, and 20 as well as Tables 6 and 7). The rearrangement of (?)?(S)-N-(1′-phenylprop-2′-ynyl)-2,6-dimethylaniline ((?)- 38 ; cf. Table 7) in a mixture of 1N H₂SO₄/PrOH at 86° leads to the formation of (?)-(R)-3-(3′-phenylpropa-1′,2′-dienyl)-2,6-dimethylaniline ((?)- 91 ), (+)-(E)- and (?)-(Z)-6-benzylidene-1,5-dimethyltricyclo[3.2.1.0^2′7]oct-3-en-8-one ((+)-(E)- and (?)-(Z)- 92 , respectively), and (?)-(S)-2,6-dimethyl-4-( 1′-phenylprop-2′-ynyl)aniline((?)- 93 ). Recovered starting material (10%) showed a loss of 18% of its original optical purity. On the other hand, (+)-(E)- and (?)-(Z)- 92 showed the same optical purity as (minus;)- 38 , as expected for intramolecular concerted processes. The CD of (+)-(E)- and (?)-(Z)- 92 clearly showed that their tricyclic skeletons possess enantiomorphic structures (cf. Fig. 1). Similar results were obtained from the acid-catalyzed rearrangement of (?)-(S)-N-(3′-chloro-1′phenylprop-2′-ynyl)-2,6-dimethylaniline ((?)- 45 ; cf. Table 7). The recovered starting material exhibited in this case a loss of 48% of its original optical purity, showing that the Cl substituent favors the heterolytic cleavage of the N–C(1′) bond in (?)- 45. A still higher degree (78%) of loss of optical activity of the starting aniline was observed in the acid-catalyzed rearrangement of (?)-(S)-2,6-dimethyl-N-[1′-(p-tolyl)prop-2′-ynyl]aniline ((?)- 42 ; cf. Scheme 25). N-[1′-(p-anisyl)prop-2-ynyl]-2,4,6-trimethylaniline( 43 ; cf. Scheme 25) underwent no acid-catalyzed [3,3]-sigmatropic rearrangement at all. The acid-catalyzed rearrangement of N-(1′,1′-dimethylprop-2′-ynyl)aniline ( 25 ; cf. Scheme 10) in 1N H₂SO₄/BuOH at 100° led to no product formation due to the sensitivity of the expected product 53 against the reaction conditions. On the other hand, the acid-catalyzed rearrangement of the corresponding 3′-Cl derivative at 130° in aqueous H₂SO₄ in ethylene glycol led to the formation of 1,2,3,4-tetrahydro-2,2-dimethylquinolin-4-on ( 54 ; cf. Scheme 10), the hydrolysis product of the expected 4-chloro-1,2-dihydro-2,2-dimethylquinoline ( 56 ). Similarly, the acid-catalyzed rearrangement of N-(3′-bromo-1′-methylprop-2′-ynyl)-2,6-diisopropylaniline ( 37 ; cf. Scheme 21) yielded, by loss of one i-Pr group, 1,2,3,4-tetrahydro-8-isopropyl-2-methylquinolin-4-one ( 59 ).     

Kinetics and Mechanism of Acid-Catalyzed 2,5-Xylenol-Formaldehyde Reaction

This paper deals with kinetic studies of the 2,5-xylenol and formaldehyde reaction catalyzed by hydrochloric acid. The catalyst concentrations used were 0.008, 0.012, 0.02, and 0.04 N. The investigations were carried out at 65, 70, 75, and 80°C. It was observed that the reaction follows a second-order rate law. The rate of reaction was found to increase with an increase in acid concentration. The overall rate constant was resolved into stepwise rate constants. It is a two-step reaction, the second step of the reaction being a rapid follow-up of the first step. Activation parameters for the overall reaction have been calculated, and a mechanism conforming to the experimental observations is suggested.     

A Kinetic Model for the Acid-Catalyzed Decomposition of Delrin

Abstract

A kinetic model is derived for the acid-catalyzed decomposition of Delrin, an acetal-formaldehyde resin. The kinetic model proposed assumes the hydrolysis of Delrin to form polyformaldehyde and the subsequent decomposition of polyformaldehyde to formaldehyde.     

Phosphoric Acid-Catalyzed Enantioselective Synthesis of Axially Chiral Anthrone-based Compounds

Anthrones and analogues are structural cores shared by diverse pharmacologically active natural and synthetic compounds. The sp²-rich nature imposes inherent obstruction to introduce stereogenic element onto the tricyclic aromatic backbone. In our pursuit to expand the chemical space of axial chirality, a novel type of axially chiral anthrone-derived skeleton was discovered. This work establishes oxime ether as suitable functionality to furnish axial chirality on symmetric anthrone skeletons through stereoselective condensation of the carbonyl entity with long-range chirality control. The enantioenriched anthrones could be elaborated into dibenzo-fused seven-membered N-heterocycles containing well-defined stereogenic center via Beckmann rearrangement with axial-to-point chirality conversion.     

Acid-Catalyzed Heterocyclization of Trialkylnaphthazarin Thioglucosides in Angular Quinone-Carbohydrate Tetracycles

Russian Journal of Organic Chemistry - Tetracyclic naphthoquinone-carbohydrate conjugates with an angular-fused heterocycle were obtained for the first time by the intramolecular condensation of...