Crystal structure of 3-(3,4,5-trimethoxyphenyl)-1,2,3,4-tetrahydrocyclopenta[b]indole-2-carboxylic acid

In the title compound, C21H21NO5, obtained from a Morita–Baylis–Hillman adduct, the hydrogenated five-membered ring adopts a shallow envelope conformation, with the C atom bearing the carboxylic acid substituent deviating by 0.237 (1) Å from the mean plane of the other four atoms (r.m.s. deviation = 0.007 Å). The dihedral angle between the fused ring system (all atoms; r.m.s. deviation = 0.057 Å) and the pendant trimethoxy benzene ring is 66.65 (3)°. The C atoms of the meta-methoxy groups lie close to the plane of the benzene ring [deviations = 0.052 (1) and −0.083 (1) Å], whereas the C atom of the para-methoxy group is significantly displaced [deviation = −1.289 (1) Å]. In the crystal, carboxylic acid inversion dimers generate R 2 2(8) loops. The dimers are connected by N—H⋯O hydrogen bonds, forming [011] chains. A C—H⋯O interaction is also observed.


S1. Introduction
Indole skeleton is an aromatic heterocycle possessing a benzene ring fused to a pyrrole ring which exhibits a wide range of biological and pharmacological activities. Compounds presenting this moiety have been successfully synthesized and used in medicinal chemistry (Xu et al., 2012). In spite of the number of developed methods for the preparation of indoles (Xu et al., 2012;Humphrey and Kuethe, 2006), our interest to use Morita-Baylis-Hillman adducts as building blocks for organic synthesis resulted in a successful stereoselective strategy to obtain compounds of this class.

S2.2. Refinement
The positions of hydrogen atoms bound to carbon atoms were idealized and calculated by riding model, with C-H bond lengths of 0.95, 0.98 and 0.99 Å for phenyl, methyl and methylene, respectively. The isotropic displacement parameters values (Uiso(H)) were fixed at 1.5Ueq(C) for methyl H atoms and 1.2Ueq(C) for all other attached H atoms.

S3. Results and discussion
The molecules of the title compound present a trimethoxyphenyl ring bonded to a system of three rings, in which an indole skeleton unit is fused to a five-membered ring possessing a carboxylic unit (Fig. 1). It crystallized in P1 space group and has a conformational structure determined by intra and intermolecular nonclassical (C-H-O) and inter- All of the rings in the structure are almost planar, with r.m.s. of 0.010, 0.066, 0.006 and 0.006 Å for trimethoxyphenyl, five-membered, pyrrole and benzene rings, respectively. The three rings fused system is essentially planar (r.m.s. deviation of 0.057 Å) and make a plane-plane angle of 113.35° with the trimethoxyphenyl ring.

Figure 1
The molecular structure of the title compound with 50% probability displacement ellipsoids.

Figure 2
Crystal packing of the title compound, showing hydrogen-bonding interactions.