Die katalytische asymmetrische Acetalisierung

Abstract

Beengte Verhältnisse: Eine asymmetrische Version der säurekatalysierten Acetalisierung von Aldehyden wird vorgestellt. Ein neues Mitglied der beengten chiralen Brønsted‐Säuren zeigte eine deutlich bessere Leistungsfähigkeit als bisher bekannte Katalysatoren und ergab cyclische Acetale mit ausgezeichneter Enantioselektivität (siehe Schema; Ar=2‐iPr‐5‐MeC6H3).

Chiral Phosphoric Acid Directed Regioselective Acetalization of Carbohydrate‐Derived 1,2‐Diols

“…Thus, no isomerization of the product acetals was observed under the reaction conditions, and the observed selectivity for the formation of 5 c catalyzed by ( R )‐ 2 d remained constant throughout the reaction progression at −55 °C (see the Supporting Information). To rationalize the observations, both a concerted mechanism4j, 16, 17 or a more traditional stepwise mechanism proceeding through a carbocationic intermediate might be proposed. Thus, chiral phosphate is involved in the formation of a hydrogen bond with the diol, and the observed regioselectivities likely result from a more favored coordination of the catalyst to the substrate (see the Supporting Information).…”

Synthesis of Heterocycles through Transition‐Metal‐Catalyzed Isomerization Reactions

“…Chiral phosphoric acids have been widely explored for the induction of enantioselectivity during nucleophilic additions to iminium species, but only few examples of the analogous additions to oxocarbenium ions have been reported 38. Enantioselective versions of the methodology were investigated by using a range of different chiral phosphoric acids, with the best results obtained for product 67 b , 75 % yield and 47 % ee using an imidophosphoric acid previously reported by List and co‐workers 38f,h. Notably, besides indole nucleophiles, the scope of the reaction was demonstrated to include O‐ nucleophiles and other heteroaromatic nucleophiles (Scheme and Scheme ).Mono‐, di‐, and tri‐substituted benzenes afforded the products 69 a – c in good to excellent yields (65–78 %), and also benzothiophene‐ and thiophene nucleophiles worked well ( 69 d and 69 e ), although C3‐appended allyl ethers afforded the products in significantly better yields than their C2 counterparts ( 69 e vs. 69 f ).…”

One Hundred Years of the Max‐Planck‐Institut für Kohlenforschung

“…Confined Brønsted acids based on a C 2 ‐symmetric imidodiphosphoric acid motif and possessing an extremely sterically demanding chiral microenvironment with a single catalytically relevant and geometrically constrained bifunctional active site enabled a catalytic enantioselective spiroacetalization reaction with excellent enantiomeric ratios (Scheme ) 100a. This catalyst class was also used in asymmetric sulfoxidation and in asymmetric acetalization of aldehydes 100b,c…”