Enantioselective organocatalytic sequential Michael-cyclization of functionalized nitroalkanes to 2-hydroxycinnamaldehydes: synthesis of benzofused dioxa[3.3.1] and oxa[4.3.1] methylene-bridged compounds

An organocatalytic enantioselective conjugate addition-initiated reaction sequence of 2-hydroxycinnamaldehydes with various functionalized nitroalkanes has been described. The combination of iminium catalysis and thiourea hydrogen-bonding catalysis was found to be crucial for this enantioselective conversion. Diverse chiral chromanes embedded within a dioxa[3.3.1] or oxa[4.3.1] methylene-bridged ring system were obtained with high enantiomeric purity. Owing to the properties of the nitro group, the developed protocol is very promising for the construction of various functionalities with complete diastereoselectivity.