Methyl 9H-xanthene-9-carboxylate

The title compound, C15H12O3, was obtained unintentionally as the by-product of an attempted recrystallization from methanol of propantheline bromide, an antimuscarinic drug. The xanthone unit is folded, with a dihedral angle of 24.81 (9)° between the benzene rings. The ester substituent adopts a trans staggered conformation, with a C—C—O—C torsion angle of 178.4 (1)°. The molecules pack in distinct layers, facilitated by C—H⋯π and weak π–π ring interactions. A weak C—H⋯O interaction also occurs; however, no classical hydrogen bonding is observed.

The title compound, C 15 H 12 O 3 , was obtained unintentionally as the by-product of an attempted recrystallization from methanol of propantheline bromide, an antimuscarinic drug. The xanthone unit is folded, with a dihedral angle of 24.81 (9) between the benzene rings. The ester substituent adopts a trans staggered conformation, with a C-C-O-C torsion angle of 178.4 (1) . The molecules pack in distinct layers, facilitated by C-HÁ Á Á and weakring interactions. A weak C-HÁ Á ÁO interaction also occurs; however, no classical hydrogen bonding is observed.

Comment
It was found that propantheline bromide (George et al., 2007) undergoes facile transesterification by methanol to produce the by-product 9H-xanthene-9-carboxylic acid methyl ester (Avdovich et al., 1986). Surprisingly, the structural elucidation of this analogue ( Fig. 1) has not been reported in the literature until now. Now the structural determination and analysis is briefly described.
The overall packing is shown in Fig. 2. Molecules are related by centres of symmetry, resulting in a head-to-head arrangement, that packs in aromatic and non-aromatic layers lying parallel to the (100) plane. Fig. 2 displays the orientation of the molecules, facilitating the weak C-H···O hydrogen bonding between the methyl and carbonyl groups (distance: Table 1) and the C-H···π and weak π···π ring interactions (Table 2). A short range contact, 2.683 (2) Å, also occurs between the aromatic C4-H4 and the carbonyl oxygen O2 (distance: C4-H4···O2 ii (ii = x,1 -y,-1/2 + z).

Experimental
The title compound was obtained unintentionally as the product of an attempted recrystallization of propantheline bromide (50 mg) in methanol (2 ml) at room temperature. Crystals resulted after 6 days; these were coated with Paratone N oil (Exxon Chemical Co., TX, USA) immediately after isolation and cooled in a stream of nitrogen vapour on the diffractometer.

Refinement
All H atoms were observed in difference syntheses and were then placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.95-1.00 Å. U iso (H) = xU eq (C), where x = 1.5 for methyl and 1.2 for all other C atoms.
supplementary materials sup-2 Figures   Fig. 1. The molecular structure of the title compound, with displacement ellipsoids drawn at the 50% probability level and hydrogen atoms as spheres of arbitrary radius.  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 Rfactors(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.