No abstract

No abstract

The base-catalyzed condensation of a,b-unsaturated carbonyl compounds with 2-hydroxybenzaldehydes yielding tetrahydroxanthones and dihydrobenzopyrans has been investigated. A novel access to highly functionalized dihydrobenzopyrans via a mild generation of the dienol of senecialdehyde and subsequent conjugated aldol reaction has been reported.

Keywords: Coumarins / Ionic liquids / N-heterocyclic carbene (NHC) / Domino reaction / UmpolungStarting from salicylaldehydes and α,β-unsaturated aldehydes, a new coumarin synthesis in ionic liquids is presented. The key feature is the generation of N-heterocyclic carbenes (NHC) and an Umpolung reaction.During the last decade, N-heterocyclic carbenes (NHCs) have been successfully introduced as ligands for transitionmetal catalysis [1] and recently NHCs have been effectively applied as organocatalysts [2] in a large number of chemical transformations, including transesterification/acylation, [3] benzoin [4] and Stetter [5] reactions. The generation of homoenolate equivalents, by NHC-catalyzed Umpolung of α,β-unsaturated aldehydes, was independently described by Glorius [6] and Bode [7] in the synthesis of γ-butyrolactones. Subsequently, this methodology has been extended towards the synthesis of β-lactones, [8] spiro γ-butyrolactones, [9] γ-lactams [10] and to redox esterification of α,β-unsaturated alScheme 1. Reaction path a: synthesis of 2,2-dimethyl-2H-chromene-3-carbaldehydes 3 and 9-methyl-8,12-dioxatricyclo[7.3.1.0 2,7 ]trideca-2,4,6-trien-11-ols 4.[15] Reaction path b: preliminary synthesis of coumarin 6d-H.[a]

The combination of experimental with theoretical CD spectroscopy allows the observation of stereoselective helical folding in a self-assembled capsule.