2-Substituted 3-nitro-2H-chromenes in reaction with azomethine ylide derived from ninhydrin and proline: regio- and stereoselective synthesis of spiro[chromeno[3,4-a]pyrrolizidine-11,2'-indene]-1',3'-diones

Regio- and stereoselective 1,3-dipolar cycloaddition of stabilized azomethine ylides, generated in situ from ninhydrin and proline, occurred at the double bond activated by nitro group in 3-nitro-2-(trifluoromethyl)- and 3-nitro-2-phenyl-2 H -chromenes upon heating in ethanol. This reaction produced high yields of spiro[chromeno[3,4- a ]pyrrolizidine-11,2'-indene]-1',3'-diones. The structure of the obtained products was confirmed by the method of X-ray crystallography.