rac-Methyl (3aR*,4S*,5R*,7aR*)-5,7a-bis(acetyloxy)-3-oxo-2-phenyloctahydro-1H-isoindole-4-carboxylate

The title molecule, C20H23NO7, the product of nucleophilic cleavage of the 3a,6-epoxy bridge in 1-oxo-2-phenyloctahydro-3a,6-epoxyisoindole-7-carboxylate, comprises a cis-fused bicyclic system containing a 2-pyrrolidinone ring in an envelope conformation (with the C atom bearing the carboxylate substituent as the flap) and a cyclohexane ring in a chair conformation. The carboxylate substituent occupies the equatorial position, whereas the two acetyloxy substituents are in axial positions. The N atom has a trigonal-planar geometry, the sum of the bond angles being 359.3 (3)°. The dihedral angle between the mean plane of the four planar atoms of the pyrrolidinone ring and the phenyl ring is 25.98 (6)°. In the crystal, molecules are linked into zigzag chains along the c-axis direction by C—H⋯O hydrogen bonds.

The title molecule, C 20 H 23 NO 7 , the product of nucleophilic cleavage of the 3a,6-epoxy bridge in 1-oxo-2-phenyloctahydro-3a,6-epoxyisoindole-7-carboxylate, comprises a cisfused bicyclic system containing a 2-pyrrolidinone ring in an envelope conformation (with the C atom bearing the carboxylate substituent as the flap) and a cyclohexane ring in a chair conformation. The carboxylate substituent occupies the equatorial position, whereas the two acetyloxy substituents are in axial positions. The N atom has a trigonal-planar geometry, the sum of the bond angles being 359.3 (3) . The dihedral angle between the mean plane of the four planar atoms of the pyrrolidinone ring and the phenyl ring is 25.98 (6) . In the crystal, molecules are linked into zigzag chains along the c-axis direction by C-HÁ Á ÁO hydrogen bonds.

Comment
3a,6-Epoxyisoindoles, which are very easy prepared by intramolecular Diels-Alder reaction of furan (IMDAF) (Vogel et al., 1999;Zubkov et al., 2005), find a wide application for synthesis of various complicated natural-like molecules (Balthaser et al., 2011;Zubkov et al., 2011). Most of these transformations proceed via electrophilic or nucleophilic opening of the epoxy bridge. As a rule, the first leads to aromatic compounds, whereas the latter gives rise to perhydroisoindoles with several (three or four) asymmetric centers in mild conditions (Zubkov et al., 2009(Zubkov et al., , 2012Claeys et al., 2010). Stereochemistry of the nucleophilic process is hardly predictable, because it depends on mechanism of the reaction (S N 1 or S N 2).
Molecule of the title compound comprises a cis-fused bicyclic system containing one five-membered (2-pyrrolidinone) and one six-membered (cyclohexane) rings (Fig. 2). The five-membered ring has envelope conformation (the C7A carbon atom is out of the plane through the other atoms of the ring by 0.540 (2) Å), and the six-membered ring adopts chair conformation. The carboxylate substituent at the C4 carbon atom occupies the equatorial position, whereas the two acetyloxy substituents at the C5 and C7A carbon atoms are in the sterically unfavorable axial positions. Such disposition is explained by the direction of the nucleophilic cleavage of 3a,6-epoxy bridge in the initial 1-oxo-2-phenyloctahydro-3a,6-epoxyisoindole-7-carboxylate. The nitrogen N2 atom has a trigonal-planar geometry (sum of the bond angles is 359.3 (3)°). The dihedral angle between the planar part of the pyrrolidinone ring and phenyl ring plane is 25.98 (6)°.
The molecule of the title compound> possesses four asymmetric centers at the C3A, C4, C5 and C7A carbon atoms and can have potentially numerous diastereomers. The crystal of the title compound is racemic and consists of enantiomeric pairs with the following relative configuration of the centers: rac-3aR*,4S*,5R*,7aR*.
In the crystal, the molecules of the title compound are bound into the zigzag chains along the c axis by the intermolecular C-H···O hydrogen bonds ( Figure 3, Table 1).

Refinement
The hydrogen atoms were placed in calculated positions with C-H = 0.95-1.00 Å and refined in the riding model with fixed isotropic displacement parameters [U iso (H) = 1.5U eq (C) for CH 3 -groups and U iso (H) = 1.2U eq (C) for the other groups].    Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.