Highly diastereoselective annulation of 2-substituted 3-nitro-2H-chromenes with hemicurcuminoids and curcuminoids via a double and triple Michael reaction cascade

The K2CO3-catalysed double Michael addition of (E)-1,5-diaryl- and 1-alkyl-5-arylpent-4-ene-1,3-diones to 2-trifluoromethyl- and 2-phenyl-substituted 3-nitro-2H-chromenes in dichloromethane at room temperature for 48 h results in 10-aroyl(acyl)-7-aryl-6a-nitro-6,6a,7,8,10,10a-hexahydro-9H-benzo[c]chromen-9-ones in 75-98% yields as individual diastereomers with cis-arrangement of substituents at the C-6, C-6a and C-7 atoms and the H-9b atom relative to the fused tricyclic system. A similar stereoselective process involving curcuminoids under reflux in chloroform for 72 h in the presence of Cs2CO3 proceeds by way of a triple Michael/Michael/oxa-Michael cascade leading to 3,6-diaryl-6a-nitro-2,3,5,6a,7,12b-hexahydro-1H,6H-chromeno[6,5-c]chromen-1-ones in 71-97% yields. 2-Phenyl-2-(trifluoromethyl)-3-nitro-2H-chromenes under the analogous conditions react with enediones to form 2-(2-phenyl-2-(trifluoromethyl)-2H-chromen-4-yl)-1,5-diarylpent-4-ene-1,3-diones as a result of cine-substitution of the H-4 chromene atom.