exo-1,7-Dimethyl-4-phenyl-10-oxa-4-azatricyclo[5.2.1.02,6]dec-8-ene-3,5-dione

The title compound, C16H15NO3, consists of an oxabicycle fused to an N-phenyl-substituted pyrrolidine ring anti to the double bond, affording the exo isomer. In the oxabicycle system, the six-membered ring presents a boat conformation, while the heterocyclic rings show envelope conformations with the O atom projected out of the plane. In the crystal, adjacent molecules are linked via weak C—H⋯O hydrogen bonds, forming chains propagating along the a-axis direction. The chains are linked by C—H⋯π interactions, forming two-dimensional networks lying parallel to the ac plane.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BH2487).
However the Diels-Alder adduct is predominantly the endo isomer. In this regard, the use of furan derivatives instead of cyclopentadiene affords the exo isomer at very high yield (Li, 2010(Li, , 2011Jarosz et al., 2001).
X-ray crystallography confirmed the molecular structure and the atom connectivity for the title compound, as illustrated in Fig. 1. The oxabicycle moiety is bound exo with respect to the N-maleimide group. This five-membered imide cycle shows a planar geometry by the contribution of the two carbonyl groups and the sp 2 hybridized nitrogen atom N1. This is a consequence of the conjugative delocalization of the nitrogen lone pair, and is supported by the sum of C-N-C angles around N1 of 359.8 (18)°. The phenyl ring (C15···C20) presents a rotation with respect of the plane of the maleimide group, indicated by the torsion angle C20-C15-N1-C2, which is 118.53 (17)°.

Experimental
Diels-Alder adduct was obtained by the reaction between 2,5-dimethylfuran (0.5 g, 5.2 mmol) and N-phenylmaleimide (1.0 g, 5.7 mmol) in ethyl acetate (7.5 ml). The mixture was stirred for 16 h at 333 K. After cooling, the formed precipitate was removed by filtration under vacuum. The collected filtrate was recrystallized twice from ethyl acetate.
The title compound was obtained in 86% yield, m.p. 401-403 K. The single-crystal suitable for X-ray determination was obtained by evaporation of an ethyl acetate solution of the title compound over 5 days.

Refinement
All H atoms were placed in geometrical idealized positions and were refined as riding on their parent atoms, with C-H = 0.93-0.98 Å, and with U iso (H) = 1.2 U eq (CH) or 1.5 U eq (CH 3 ).