(5SR,10SR,15SR)-Trimethyl 5H,10H,15H-diindeno[1,2-a:1′,2′-c]fluorene-5,10,15-tricarboxylate 0.167-hydrate

The title compound, C33H24O6·0.17H2O, which is commonly known as (SR,SR,SR)-trimethyl 1,10,19-truxentricarboxylate, crystallizes as a hydrate with the water molecule encapsulated between three ester groups by O—H⋯O hydrogen bonding to two of them. The water molecule site is not fully occupied in the crystal studied, with a refined site occupancy of 0.167 (5). The 27-atom ring system is approximately planar, with a maximum deviation of 0.148 (1) Å, and the three ester substituents are all on the same side of this plane.

The title compound, C 33 H 24 O 6 Á0.17H 2 O, which is commonly known as (SR,SR,SR)-trimethyl 1,10,19-truxentricarboxylate, crystallizes as a hydrate with the water molecule encapsulated between three ester groups by O-HÁ Á ÁO hydrogen bonding to two of them. The water molecule site is not fully occupied in the crystal studied, with a refined site occupancy of 0.167 (5). The 27-atom ring system is approximately planar, with a maximum deviation of 0.148 (1) Å , and the three ester substituents are all on the same side of this plane.

D-HÁ
Bucky bowls can be generated by opening the C 60 cage along different symmetry pathways with the appropriate attachment of hydrogen atoms. These high symmetry bucky ball intermediates could be involved in the evolution of flat graphite to the spherical C 60 . Bucky bowls have been recognized as an important intermediate in the synthesis of C 60 . In addition to their relation to bucky ball, bucky bowls also exhibit many interesting properties including surface selective chemistry (Mehta & Sarma, 2002;Rao, 1998).
As part of an investigation of bucky bowls, C 33 H 24 O 6 . 0.17H 2 O was prepared. Synthesis by aldol condensation of indanone with NaOH yielded the trisannulated benzene (Amick & Scott, 2007). This was accomplished in the one-pot, acid catalyzed, head-to-tail cyclotrimerization synthesis method detailed in a previous report (Amick & Scott, 2007). The star ester was synthesized by treating the trisannulated benzene with n-butyl lithium, followed by chloromethyl formate to introduce the three ester groups. The star esters were synthesized as possible bucky bowl precursors, which when sewed together make bucky balls.
The structure, shown in Figure 1, contains three ester groups on the same side of the 27-atom ring system, such that the configurations at the asymmetric C atoms C7, C16 and C25 are all the same, S in the asymmetric unit of the racemic crystal.
The ring system is only slightly nonplanar, with maximum deviation from its best plane 0.148 (1) Å for C16, and mean deviation 0.050 Å. The nature of the distortion from planarity is such that all three peripheral benzene rings form small dihedral angles with the central benzene ring. The ring containing C10 bends away from the central plane, forming a dihedral angle of 4.19 (7)° with the central ring, while the ring containing C19 bends in the same direction, forming a dihedral angle of 2.80 (7)° with the central ring. The third peripheral ring, containing C1, twists above and below the central ring, forming a dihedral angle of 1.98 (7)° with the central ring. Out-of-plane bending is necessary for buckybowls, and adoption of the bowl shape reduces the angle strain that would result in the planar form (Rao, 1998;Mehta & Sarma, 2002;Billups & Ciufolini, 1993). There is considerable flexibility in buckybowls, which undergo rapid inversion in solution. Corannulene, C 20 H 10 , inverts about 2×10 5 times a second at room temperature (Rao, 1998).
One of the ester groups has the carbonyl oxygen atom O4 pointed inward, while the other two have their carbonyl oxygen atoms outward. A partially occupied [16.7 (5)%] water molecule has potential hydrogen bond contacts to the three ester groups. This water comes from the second step in the synthesis, where water is the solvent. The hydrogen bond distances are consistent with previously reported values (Emsley, 1980). Difference maps were consulted to determine the hydrogen supplementary materials sup-2 positions. Although the disorder of the water molecule can be considered dynamic, the positional disorder of the water molecule was modeled with the two H atoms within hydrogen-bonding distance to the ester O atoms. A close contact, H29A···H7A, 2.03 Å, exists in this model, and it seems likely that when the water molecule is present, methyl group C29 probably rotates to alleviate the contact. We likely would not be able to detect the alternate positions of the methyl H atoms.
The same crystal, freshly prepared, had been used in a room-temperature structure determination 15.5 years earlier. At that time, the occupancy of the water molecule refined to 45.0 (6)%, but its U eq value was large, 0.215 (4) Å 2 , and the water H atoms could not be located.
Due to the observed stereochemistry (syn-SR,SR,SR)-trimethyl-1,10,19-truxentricarboxylate can be used as a bucky bowl precursor with all three ester groups on the same side of the trisannulated benzene plane. A bucky bowl could be produced by conversion of the ester groups to acid chloride groups followed by pyrolysis (Mehta & Sarma, 2002;Rao, 1998).

Experimental
The synthesis of the truxene backbone, a triply annulated benzene ring, was accomplished by a two-step aldol condensation.
Indanone, p-toluenesulfonic acid monohydrate, propionic acid, and o-dichlorobenzene were mixed in a single pot. After heating for 16 h at 378 K (105 °C), the reaction mixture was poured into methanol and slowly neutralized with NaOH. The precipitate was collected by filtration and washed. The resulting truxene, a pale yellow solid, was treated with n-butyl lithium to generate an anion by deprotonating the three pentlyl groups. The corresponding anion was treated with chloromethyl formate to introduce the three ester groups (Amick & Scott, 2007;De Frutos et al., 1999, 2002. Crystals were grown by slow evaporation from a mixture of CH 2 Cl 2 and methanol.