Crystal structure of methyl 2-(2H-1,3-benzodioxol-5-yl)-7,9-dibromo-8-oxo-1-oxaspiro[4.5]deca-2,6,9-triene-3-carboxylate

The title compound, C18H12Br2O6, was synthesized from Morita–Baylis–Hillman adducts. It incorporates the brominated spiro-hexadienone moiety typically exhibited by compounds of this class that exhibit biological activity. Both the brominated cyclohexadienone and the central five-membered rings are nearly planar (r.m.s. deviations of 0.044 and 0.016 Å, respectively), being almost perpendicularly oriented [interplanar angle = 89.47 (5)°]. With respect to the central five-membered ring, the brominated cyclohexadienone ring, the benzodioxol ring and the carboxylate fragment make C—O—C—C, O—C—C—C and C—C—C—O dihedral angles of −122.11 (8), −27.20 (11) and −8.40 (12)°, respectively. An intramolecular C—H⋯O hydrogen bond occurs. In the crystal, molecules are linked by non-classical C—H⋯O and C—H⋯Br hydrogen bonds resulting in a molecular packing in which the brominated rings are in a head-to-head orientation, forming well marked planes parallel to the b axis.


S1. Introduction
Compounds containing a spiro-hexadienone moiety typically exhibit biological activity, also sharing a structural architecture observed in some natural products (König & Wright, 1993;Sorek et al., 2009). In view of the importance of this class of compounds, recent efforts resulted in a new synthetic strategy, which starts from Morita-Baylis-Hillman adducts as building blocks for organic synthesis (Martins et al., 2014). Based on this methodology, it was possible to obtain a brominated spiro-hexadienone which resulted, to our knowledge, in the first report of a halogenated spirohexadienone crystal structure.
After chromatographic separation, the dibrominated compounds were easily transformed into halogenated spirohexadienones, in three steps procedure, starting with the Morita-Baylis-Hillman adducts.

S2.2. Refinement
The C-bound H atoms were positioned with idealized geometry and treated as riding atoms: phenyl, methyl and methylene C-H bond lengths were 0.95, 0.98 and 0.99 Å, respectively. The isotropic displacement parameters values (U iso (H)) were fixed at 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for all other attached H atoms.

S3. Results and discussion
The title compound ( In the hexadienone ring of the title compound, the C2-C3, C17-C18 and C1-O1 bond lengths are 1.3351 (13), 1.3354 (13) and 1.2139 (11) Å, respectively, and the bond length between the spiro-carbon (C4) and the oxygen atom C4 -O2 is 1.4684 (11) Å, similar to those reported for a related oxaspiro structure (Lou, 2012). In the latter, the plane-plane angle between the mean planes of the six-membered and the central five-membered rings in the spiro-carbon is 96.06°, slightly different from that observed in the title compound (90.53°). Further comparison also reveals different orientations of the carboxylate moiety, which makes a dihedral angle of 170.6 (1)° (C10-C11-C15-O5) in the title compound, with a corresponding angle equal to 11.3 (3)° in the related structure (Lou, 2012). This large difference is consistent with an observed C7-H7-O5 intramolecular hydrogen bond (Table 1) in the title compound, which results in a favourable conformation of the carboxylate group.

Figure 1
The molecular structure of the title compound with atom labels and 50% probability displacement ellipsoids. Crystal packing of the title compound, showing hydrogen bonding interactions.