Organocatalytic conjugate addition of nitroalkanes to 2H-chromene-3- carbaldehydes: Synthesis of highly functionalized chroman derivatives

Conjugate addition of nitroalkanes to 2H-chromene-3-carbaldehydes has been studied using a number of amines as the catalysts. Prolinol triethylsilyl ether was found to be the best catalyst for the reaction. The use of protonic solvents was also important. The reaction of variously substituted 2H-chromene-3-carbaldehydes with nitromethane afforded the products with excellent yields and diastereoselectivities. Nitroethane and 1-nitropropane were also applicable, however in decreased yields. The enantioselectivities of the reaction were very low and almost racemic products (3 similar to 5% ee) were obtained in all cases. The reaction provides a new method for the preparation of highly functionalized chroman derivatives.