tert-Butyl 4-isopropyl-2-oxo-6-phenyl-3,4-dihydro-2H-pyran-3-carboxylate

In the title compound, C19H24O4, the six-membered lactone ring adopts an envelope conformation with the tert-butoxycarbonyl and isopropyl substituents in axial positions, and the phenyl group in an equatorial position. In the crystal structure, weak intermolecular C—H⋯O hydrogen bonds link the molecules into centrosymmetric dimers.


Comment
Endocyclic enol lactones are important structural elements of biologically active natural products (Zhao et al., 1997) and useful synthetic intermediates for organic synthesis (Evans et al., 2005, Davies et al., 2004. The cyclization of alkynoic acids under acidic conditions (Krafft et al., 1981) , employing transition-metal complexes as catalysts (Zeni et al., 2004, Valerga et al., 2001,and the carbonylation coupling of alkynes and 1,3-dicarbonyl compounds are main synthetic pathways for the preparation of Enol lactones (Li et al., 2007) In the title compound as shown in Fig. 1, the six-membered lactone ring adopts an envelope conformation with the tert-butoxycarbonyl, isopropyl and phenyl groups attached to it. The tert-butoxycarbonyl and isopropyl groups occupy axial positions, and the phenyl group occupies equatorial position. The crystal packing (Fig. 2) is stabilized by weak intermolecular C-H···O hydrogen bonds between the pyran H atom and the oxygen of the C═O unit in pyran ring, with a C2-H2···O1 i (Table 1).
The resulting mixture was refluxed for 10 h monitored by TLC. After evaporation, the residue was carefully purified by flash chromatography on silica gel. The title compound was obtained as a by-product (25% yield), which was crystallized from n-hexane-ethyl acetate.

Refinement
All H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 Å for aryl, 0.98 Å for methyne and 0.96 Å for methyl H atoms. U iso (H) = 1.2U eq (C) for aryl and methyne H atoms, and 1.5U eq (C) for methyl H atoms.
supplementary materials sup-2 Figures Fig. 1. The molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms are presented as a small spheres of arbitrary radius.

Special details
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.