159684-74-9Relevant academic research and scientific papers
Highly Acidic Conjugate-Base-Stabilized Carboxylic Acids Catalyze Enantioselective oxa-Pictet–Spengler Reactions with Ketals
Zhu, Zhengbo,Odagi, Minami,Zhao, Chenfei,Abboud, Khalil A.,Kirm, Helmi Ulrika,Saame, Jaan,L?kov, M?rt,Leito, Ivo,Seidel, Daniel
, p. 2028 - 2032 (2020)
Acyclic ketone-derived oxocarbenium ions are involved as intermediates in numerous reactions that provide valuable products, however, they have thus far eluded efforts aimed at asymmetric catalysis. We report that a readily accessible chiral carboxylic acid catalyst exerts control over asymmetric cyclizations of acyclic ketone-derived trisubstituted oxocarbenium ions, thereby providing access to highly enantioenriched dihydropyran products containing a tetrasubstituted stereogenic center. The high acidity of the carboxylic acid catalyst, which exceeds that of the well-known chiral phosphoric acid catalyst TRIP, is largely derived from stabilization of the carboxylate conjugate base through intramolecular anion-binding to a thiourea site.
Hemiacetals of Acetophenone. Aromatic Substituent Effects in the H+- and General-base-catalysed Decomposition in Aqueous Solution
McClelland, Robert A.,Engell, Karen M.,Larsen, Truels S.,Soerensen, Poul E.
, p. 2199 - 2206 (2007/10/02)
We describe a double-mixing stopped-flow technique for the study of the acid- and base-catalysed breakdown of the unstable methyl hemiacetals of aryl-substituted acetophenones in aqueous solution.The approach takes advantage of the build-up of the hemiacetal during the H+-catalysed decomposition of the corresponding dimethyl acetal.Thus, mixing a weakly basic solution of acetal with excess acid provides a solution containing unchanged acetal, the acetophenone product of the hydrolysis and the hemiacetal intermediate.After a short period of time ( +-catalysed acetal and hemiacetal breakdown are in good agreement with literature data, but our analysis suggests a larger resonance effect component than previously assumed.The corresponding set of Hammett plots for base catalysis of hemiacetal breakdown reveals a relatively weak dependence of the catalytic constants on aromatic substitution, which appears to be inconsistent with much stronger dependences on substitution in the leaving alcohol.We propose a case of non-perfect synchronization or imbalance in the transition state where, in the breakdown direction in a class n mechanism, the degree of C-O bond breakage as measured by β1g is considerable, but the change in hybridization of the central carbon (sp3 -> sp2), as measured by ρ, lags behind in the transition state so that there is less interaction with the aromatic substituents here.
