Pyran-annulated heterocycles

Nair et al. subsequently developed a three-component synthesis of the pyran-annulated heterocycle 115 (Scheme 5.34) [71]. Thus, heating to reflux a benzene solution of 4-hydroxycoumarin (116), DMAD, and cyclohexyl isocyanide gave the pyranocoumarin 115 in 93% yield. It was hypothesized that protonation of the zwitterionic intermediate 109 by the hydroxycoumarin 116 occurred to afford ion pair 117. Subsequent 1,4-addition of enolate to nitrilium would furnish the ketamine 118, which would then cyclize to provide the heterocycle 115. 

Keywords Aldehydes, malononitrile, C-H activated acids, urea, organocatalyst, ethanol-water (1 1), room temperature, tandem Knoevenagel-cyclocondensation, one-pot three-component reaction, chemoselectivity, functionalized 2-amino-3-cyano-4//-pyrans and pyran annulated heterocycles... 

Brahmachari, G., andBaneqee, B. (2014). Facile and one-pot access to diverse and densely functionalized 2-amino-3-cyano //-pyrans and pyran annulated heterocyclic scaffolds via an eco-friendly multicomponent reaction at room temperature using urea as a novel organo-catalyst. ACS Sustainable Chem. Eng., 2, 411=122. 

Green Synthetic Approaches for Biologically Relevant 2 aminO 4H pyrans and 2-aminO 4H pyran Annulated Heterocycles... 

SYNTHETIC APPROACHES FOR 2-AMINO-4H-PYRANS AND 2-AMINO-4H-PYRAN-ANNULATED HETEROCYCLES IN WATER AND ETHANOL-WATER MEDIA... 

Recently, Brahmachari and his group [133] have developed a simple, facile, and convenient practical method for one-pot syntiiesis of pharmaceutically interesting diverse kind of pyran-annulated heterocycles such as 4H-pyrano[3,2-c]coumarins (14), 4H-pyrano[3,2-c]-a-pyrones (12), 5,6,7,8-tetrahydro-5-oxo-4H-chromenes (9), and benzo[//]-4H-chromenes (8) in the presence of ammonium or sodium formate as catalyst via three-component... 

SCHEME 28 Synthesis of pyran-annulated heterocyclic scaffolds in ethanol-water media [133],... 

A. Shaabani, S. Samadi, A. Rahmati, One-pot, three-component condensation reaction in water an efficient and improved procedure for the synthesis of pyran annulated heterocyclic systems, Synth. Commun. 37 (2007) 491M99. 

---