2,2,10-Trimethyl-2,3-dihydropyrano[2,3-a]carbazol-4(11H)-one

The title compound, C18H17NO2, was prepared from 1-hydroxy-8-methylcarbazole and 3,3-dimethylacrylic acid with trifluoroacetic acid as the cyclization catalyst. Due to the –CMe2– group, the molecule is not quite planar. The packing is dominated by the strong N—H⋯O hydrogen bonds and some weaker C—H⋯O and C—H⋯π interactions. π–π Stacking interactions [centroid–centroid separation = 3.806 (2) Å] join neighboring molecules into loosely connected inversion dimers.

The title compound, C 18 H 17 NO 2 , was prepared from 1hydroxy-8-methylcarbazole and 3,3-dimethylacrylic acid with trifluoroacetic acid as the cyclization catalyst. Due to the -CMe 2 -group, the molecule is not quite planar. The packing is dominated by the strong N-HÁ Á ÁO hydrogen bonds and some weaker C-HÁ Á ÁO and C-HÁ Á Á interactions. -Stacking interactions [centroid-centroid separation = 3.806 (2) Å ] join neighboring molecules into loosely connected inversion dimers. Knö lker & Reddy (2002) report on the isolation of pyranocarbazoles from various plant species. Sridharan et al. (2007) describe the synthesis of compounds related to the title compound. Sridharan, Rajendra Prasad & Zeller (2008) report the structure of the 9-methyl derivative of the title compound. Sridharan, Rajendra Prasad, Ngendahimana et al. (2008) report the structure of the 10-H derivative of the title compound.  Table 1 Hydrogen-bond geometry (Å , ). 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 in a simple and efficient route.
The single-crystal structure confirmed the formation of the dihydropyrano-[2,3-a]carbazol-4(11H)-one framework as shown in Figure 2. Data collection and structure refinement were unproblematic and all structural parameters (bond lengths, angles, etc) are in the expected ranges. The molecules crystallize in a monoclinic setting in P2 1 /c with four largely planar molecules per unit cell. The plane defined by the sp 2 hybridized carbon atoms, the C1 methyl and C15 methylene carbon atoms, and the N and O atoms has an r.m.s. deviation from planarity of only 0.0754 Å. Of all the ring C atoms only C14 of the pyran C(Me) 2 unit is significately out of plane with the atoms of the four fused rings, its deviation being 0.534 (1) Å. The pyran ring thus exhibits a half chair conformation.
One of the methyl groups of the C(Me) 2 unit is also located close to the average plane of the molecule (C18 with a deviation of 0.125 (2) Å). The other, C17, is however located 2.039 (2) Å away from this plane and thus makes the molecule as a whole not planar and prevents it form forming extensive π-π stacked entities in the solid state. The packing is thus indeed dominated by strong N-H···O hydrogen bonds ( Figure 3, Table 1) and some weaker C-H···O (Table 1, Figure 4) and C-H···π interactions (e.g. C18-H18b···Cg1 ii = 2.94 Å with Cg1 being the ring C8 to C13 and ii = -x, -1/2 + y, 1/2 -z). The only significant π···π stacking interaction with a centroid to centroid distance of 3.806 (2) Å is found between the pyrrole ring and the the aromatic ring made up of C2 to C7 (Figure 4). Two neighboring molecules related by an inversion center are forming loosly connected dimers via two sets of these π-π interactions (symmetry operator 1 -x, 2 -y, 1 -z).
supplementary materials sup-2 The structures of the 2,2-dimethyl and the 2,2,10-methyl derivatives of the title compound are described in Sridharan, Rajendra Prasad, Ngendahimana et al. (2008) and Sridharan, Rajendra Prasad & Zeller (2008), the two preceeding articles in this journal. For a more detailed comparison of structures and packing of the three two derivatives please see in Sridharan,

Rajendra Prasad & Zeller (2008).
Experimental 1-hydroxy-8-methylcarbazole (0.001 mol) dissolved in 10 ml of trifluroaceticacid and 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 trifluroacetic acid 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 eluant to yield yellow plates of (I) (0.239 g, 86%), m.p. 475-477 K.

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.30161 (12