3-Nitro-2-phenyl-2-(trifluoromethyl)-2H-chromenes in reaction with N-methylazomethine ylide: stereoselective synthesis of 3a,4,4-trisubstituted chromeno[3,4-c]pyrrolidines

1,3-Dipolar cycloaddition of unstabilized azomethine ylide obtained in situ from sarcosine and paraform occurred stereoselectively at the double bond of 3-nitro-2-phenyl-2-(trifluoromethyl)-2H-chromenes activated by the presence of nitro group. The synthetic procedure required refluxing in benzene for 2 h and gave chromeno[3,4-c]pyrrolidines with trans configuration of trifluoromethyl group and nitro group as the major products. The structure of the obtained products was confirmed by X-ray structural analysis.