(5S)-3-Chloro-4-(2,5-dihydro-1H-pyrrol-1-yl)-5-[(1R,2S,5R)-2-isopropyl-5-methylcyclohexyloxy]furan-2(5H)-one

The title compound, C18H26ClNO3, was obtained via a tandem asymmetric Michael addition–elimination reaction of 3,4-dichloro-5-(S)-(l-menthyloxy)furan-2(5H)-one and 2,5-dihydro-1H-pyrrole in the presence of potassium fluoride. In the molecule, the nearly planar dihydropyrrole ring [maximum atomic deviation = 0.019 (3) Å] is oriented at a dihedral angle of 10.73 (8)° to the the nearly planar furanone ring [maximum atomic deviation = 0.011 (2) Å]; the cyclohexane ring adopts a chair conformation. In the crystal, molecules are linked via weak intermolecular C—H⋯O hydrogen bonds, forming supramolecular chains running along the b axis.

The structure of the title compound (I) is illustrated in Fig. 1. The crystal structure of the title compound, which has four chiral centers (C4(S), C5(R), C6(S), C9(R)), contains a five-membered furanone ring and a six-membered ring connected each other via C4-O3-C5 ether bond. The furanone ring of C4-O1-C1-C2-C3 is approximately planar, whereas the six-membered ring displays a chair conformation.

Experimental
The precursor 3,4-dichloro-5-(S)-(l-menthyloxy)-2(5H)-furanone was prepared according to the literature procedure (Song et al., 2009). After the mixture of 3,4-dichloro-5-(S)-(l-menthyloxy)-2(5H)-furanone (2.0 mmol) and potassium fluoride (6.0 mmol) was dissolved in absolute tetrahydrofuran (2.0 mL) under nitrogen atmosphere, tetrahydrofuran solution of 2,5-dihydro-1H-pyrrole (2.0 mmol) was added. The reaction was carried out under the stirring at room temperature for 6 h. Once the reaction was complete, the solvents were removed under reduced pressure. The residual solid was dissolved in dichloromethane. Then the combined organic layers from extraction were concentrated under reduced pressure, and the crude product was purified by silica gel column chromatography with the gradient mixture of petroleum ether and ethyl acetate to give the product yielding (I) 0.561 g (82.4%).

Refinement
H atoms were positioned in calculated positions with C-H = 0.93-0.98 Å and were refined using a riding model, with U iso (H) = 1.5U eq (C) for methyl and 1.2U eq (C) for the others.   Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > 2sigma(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq