Organocatalytic Synthesis of Benzopyrans

Significance

An organocatalytic oxa-Michael-aldol condensation of salicylic aldehyde derivatives with α,β-unsaturated aldehydes to give chromene-3-carbaldehyde derivatives with high chemo- and enantioselectivities but variable yields is reported. An easily accessible proline derivative A, derived from the grandfather of organocatalysts, is used as catalyst. The addition of molecular sieves to the reaction improves the yields significantly without adverse influence on the enantioselectivity. A mechanism for the stereochemical outcome of the reaction is given which is based on the X-ray crystal structure analysis of an enantiopure chromene-3-carboxylic acid, obtained by oxidation of the corresponding aldehyde. The scope of the reaction was modestly investigated.