Tunable Zinc-Mediated Reductive Cyclization of Diastereomeric 3-Nitro-4-phenacyl-2-(trihalomethyl)chromanes to Fused Pyrroline -Oxides, Pyrrolines, and Pyrrolidines.

Reductive cyclization of trans,trans- and trans,cis-isomers of the 2-CF-substituted 3-nitro-4-phenacylchromanes with Zn-based reductive systems, depending on the conditions, affords 4-CF-substituted 1,3a,4,9b-tetrahydrochromeno[3,4-]pyrrole 3-oxides, 1,3a,4,9b-tetrahydrochromeno[3,4-]pyrroles, or 1,2,3,3a,4,9b-hexahydrochromeno[3,4-]pyrroles in good yields without changing the relative configuration of the pyran ring. A similar process involving the 2-CCl-substituted 3-nitro-4-phenacylchromanes is accompanied by reductive dehalogenation to form the corresponding 4-dichloromethyl-substituted fused chromanes along with the 3-(2-hydroxyaryl)-2-(2,2-dichlorovinyl)-5-phenyl-2-pyrroline 1-oxides as pyran ring opening products. The structure and relative configuration of the obtained products was reliably confirmed by X-ray diffraction analysis and 2D NMR spectroscopy.