Abstract

Background: 2-Amino-4H-chromenes are associated with diverse biological properties, its synthesis has gained significant consideration. Two enantiomers of same compounds have different chemical properties, the two enantiomers of a molecule interact differently with a living organism. In this consideration researchers have started investigation for single enantiomer of chromen molecules. Many asymmetric reactions have been recorded using chiral organocatalysts. Theses catalysts help to provide stereo controlled organic reactions. The field of asymmetric synthesis of chromenes molecules is not much studied. This report describes asymmetric synthesis of chromenes molecules using organocatalysts.

Methods: We have designed and synthesized various organocatalysts with different structural framework based on (S) -2-amino-N-[(1R, 2R) - 2 - (4-methylphenylsulfonamido) cyclohexyl] aryl / sulfon amide. The synthesized organocatalysts were studied in the model reaction between 5, 5- dimethylcyclohexane-1, 3-dione and 2-benzylidenemalononitrile to give 2-amino-5-oxo-4-phenyl-5, 6, 7, 8-tetrahydro-4H-chromene-3-carbonitrile as the product. The reaction was optimized and scope of the reaction was applied to various 2-arylidenemalononitriles to obtain enantiopure 2-Amino-4Hchromenes.

Result: The efficiency of synthesized chiral bifunctional organocatalysts was tested in model reaction between 5, 5-dimethylcyclohexane-1, 3-dione and 2-benzylidenemalononitrile. Out of all synthesized catalysts, the organocatalyst (S) -2-amino-N-[(1R, 2R) - 2 - (4-methylphenylsulfonamido) cyclohexyl] butanamide was proved to be an efficient for the reaction. The reaction was also optimized in terms of solvent and catalytic loading. Optimized reaction conditions were used for different substrates to give corresponding products in excellent yields and good to moderate enantioselectivities.

Conclusion: we have developed an efficient organocatlyzed route for asymmetric synthesis of 2- amino-4H-chromens using chiral organocatalyst and silica gel as an additive. We synthesized various organocatalysts with different structural framework based on (S) -2-amino-N-[(1R, 2R) - 2 - (4- methylphenylsulfonamido) cyclohexyl] aryl / sulfon amide and employed in the synthesis of 2-amino- 5-oxo-4-phenyl-5, 6, 7, 8-tetrahydro-4H-chromene-3-carbonitrile. The organocatalysts (S) -2-amino- N-[(1R, 2R) - 2 - (4-methylphenylsulfonamido) cyclohexyl] butanamide was proved to be ideal catalyst for the reaction and catalyzed broad range of substrates to give desired product at room temperature. In addition, some advantageous features of the method are mild reaction conditions, excellent yields with high enantioselectivities and easy work up of procedure.

Keywords: 2-amino-4H-chromenes, arylidenemalononitrile, asymmetric synthesis, enantioselective, hydrogen bonding, organocatalysts.

Graphical Abstract