2,2,9-Trimethyl-2,3-dihydropyrano[2,3-a]carbazol-4-(11H)-one

The title compound, C18H17NO2, was prepared from 1-hydroxy-7-methylcarbazole and 3,3-dimethylacrylic acid with trifluoroacetic acid as the cyclization catalyst. The molecules contain an essentially planar 6-methylindole unit. The second aromatic ring is significantly bent away from the plane of this unit, with maximum deviations of 0.171 (1) and 0.185 (1) Å for two of the C atoms. In the crystal structure, there are neither N—H⋯O hydrogen bonds nor π–π stacking between the aromatic sections of neighboring molecules. There is only one weak C—H⋯O hydrogen bond and a number of weak C—H⋯π interactions.

In this class girinimbine was the first member of the pyrano[3,2-a]carbazole alkaloid family to be isolated from M. Koenigii Spreng (Knölker & Reddy, 2002, and references therein). The isolation of these classes of compounds became an active area of study since these compounds possess high levels of biological and pharmacological activity. Hence we attempted to synthesize pyranocarbazoles by a simple and efficient route.
Using trifluoroacetic acid as the acylating agent we had been able to synthezize in high yields a range of pyranocarbazolones and we recently reported (Sridharan et al., 2007) the synthesis and crystallographic behaviour of 2,3-dihydro-2,2,8-trimethylpyrano[2,3-a]carbazol-4-(11H)-one. As an extension of this reasearch, and to further proof the credibility of trifluoroacetic acid as a good acylating agent, we further extended this synthetic route with a series of substituted 1-hydroxycarbazoles. The components thus synthesized were used as starting synthons to develop routes towards substituted pyranocarbazole derivatives. Herein we report the crystal structures of the title compound, (I).
The molecules in (I) consist of an essential planar 6-methyl-indole unit made up of the atoms C1 to C7, C12 and N1.
Also in plane with this unit are the atoms C8, C11 and O1, and the overall r.m.s. deviation from planarity for these atoms is 0.0319 Å. In the structures of two related compounds (Sridharan et al., 2008a and2008b) differentiated from the title compound only by the presence and/or position of one methyl group, the molecules were essentially planar with the only exception being the atoms of the C(CH 3 ) 2 unit of the pyranone rings. For the title compound, however, the remainder of the molecule, including the atoms C9 and C10 of the second aromatic six membered ring, are all deviating from the plane formed by the indole subunit. The aromatic ring is signifcantely bent away from this plane with C9 and C10 being displaced by 0.171 (1) and 0.185 (1) Å, respectively.
Differences between (I) and the two related structures (Sridharan et al., 2008a and2008b) extend into the crystal packing as well. The other compounds are dominated by strong N-H···O hydrogen bonds and also exhibit a range of π-π stacking as well as weak C-H···O interactions. The title compound, however, while only being differentiated from the other two molecules by the presence and/or position of one methyl group, does not exhibit any strong N-H···O hydrogen bonds and it also does not exhibit any π-π stacking between the aromatic sections of neighboring molecules. There is only one weak C-H···O hydrogen bond (Table 1). Other intermolecular interactions are limited to even weaker C-H···π interactions (Table 1, Figure 3). Centroids given in Table 1 are defined as follows: Cg1: the pyrrol ring (N1, C1, C6, C7, C12); Cg2: In the absence of stronger interactions the large number of these contacts dominates the packing forces. Molecules within the unit cell are rougly aligned along the long axis of the unit cell (the c axis) and neighboring molecules are twisted against each other so that their planes are approximately perpendicular to each other, thus allowing the C-H···π interactions to establish themselves as shown in Figure 3. Each molecule is both C-H donor towards two neighboring molecules and supplementary materials sup-2 C-H acceptor for two other neighbors. In combination this creates layers of molecules connected by C-H···π interactions. Each layer is in turn connected with parallel layers by weak C-H···O hydrogen bonds as shown in Figure 4.
Experimental 1-hydroxy-7-methylcarbazole (0.001 mol) dissolved in 10 ml of trifluoroaceticacid was heated with 3,3-dimethylacrylicacid (0.001 mol) at 323 K for 5 h. The reaction was monitored by TLC. After completion of the reaction, the excess trifluroaceticacid was removed using rotary evaporation. The solid that precipitated out was poured onto ice water, then extracted using ethyl acetate and dried over anhydrous sodium sulfate and filtered. Then the solvent was removed under vacuum and the residue was purified by column chromatography on silica gel using petroleum ether/ ethyl acetate (95:5 v/v) as the eluant.

Refinement
All hydrogen atoms were added in calculated positions with C-H bond distances of 0.99 (methylene), 0.95 (aromatic) and 0.98 Å (methyl) and an N-H distance of 0.88 Å. They were refined as riding with U iso (H) = 1.2U eq (C,N) or 1.5U eq (methyl C).   Table 1.