Wider die Ringspannung: neue Wege zu Cyclopropanen

Abstract

Spannung im Ring: Zwei neue Cyclopropanierungsreaktionen (siehe Schema; LA=Lewis‐Säure, Nu=Nucleophil) verlaufen mit hoher Stereoselektivität unter erstaunlich milden Bedingungen. In der ersten sind Epoxide die Methylenquelle in der Synthese enantiomerenangereicherter Cyclopropane. In der zweiten wird der Anwendungsbereich der etablierten allylischen Alkylierungen hin zur Synthese dreigliedriger Ringe in exzellenter Enantioselektivität erweitert.

Enantioselective Cyclopropanation of 2‐Cyano‐3‐arylacrylates Using Carbohydrate‐Based Crown Ethers

“…Thus, during the past few decades, various asymmetric cyclopropanation reactions have been developed. [30][31][32][33][34][35][36][37] The Michael-initiated ring closure (MIRC) reaction is a powerful method for the preparation of cyclopropanes, as it only requires cheap and readily available reagents, along with mild reaction conditions. The MIRC reaction strategy may also be utilized through a one-pot multicomponent reaction, asymmetric approaches of which have gained interest among synthetic organic chemists in recent times.…”

“…The vast majority of cyclopropane‐containing compounds are chiral. Thus, during the past few decades, various asymmetric cyclopropanation reactions have been developed [30–37] …”

Organocatalyzed Cyclopropanation of α‐Substituted α,β‐Unsaturated Aldehydes: Enantioselective Synthesis of Cyclopropanes Bearing a Chiral Quaternary Center

“…Over the years, cyclopropanes have gained many synthetic groups interest and inspired new asymmetric routes allowing to their stereocontrolled construction. [3] Nevertheless, in the field of organocatalysis, [4,5] none of such studies have yet been able to establish the synthesis of cyclopropanes in which the stereogenic carbon at the a-position of the aldehydes involves a chiral quaternary center. [6] Although syntheses of these unique strained rings which are useful targets for further elaboration into more complex structures are well documented, [7] the a,b-difunctionalization of a-substituted a,b-unsaturated enals by chiral aminocatalysis is still unexplored.…”

“…C chemical shifts are reported in ppm relative to the central line of the triplet at 77.0 ppm for CDCl 3 . Splitting patterns are designated as s, singlet; d, doublet; t, triplet; q, quartet; h, heptet; m, multiplet; and br, broad and combinations thereof.…”