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

The title compound, C17H15NO2, was prepared from 1-hydroxycarbazole and 3,3-dimethylacrylic acid with a mixture of AlCl3 and POCl3 as the cyclization catalyst. Owing to the presence of the –CMe2– group, the molecule is not quite planar. In the crystal structre, strong N—H⋯O hydrogen bonds and weaker C—H⋯π interactions occur, and a slipped π–π stacking interaction [centroid–centroid separation = 3.8425 (8) Å] is also observed.

The title compound, C 17 H 15 NO 2 , was prepared from 1-hydroxycarbazole and 3,3-dimethylacrylic acid with a mixture of AlCl 3 and POCl 3 as the cyclization catalyst. Owing to the presence of the -CMe 2 -group, the molecule is not quite planar. In the crystal structre, strong N-HÁ Á ÁO hydrogen bonds and weaker C-HÁ Á Á interactions occur, and a slipped stacking interaction [centroid-centroid separation = 3.8425 (8) Å ] is also observed. Knö lker & Reddy (2002) report on the isolation of pyranocarbazoles from various plant species, and Shanazarov et al. (1989) on their potential beneficial properties. Kavitha & Prasad (2003) 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 & Zeller (2008) report the structure of the 10-methyl derivative of the title compound.  Table 1 Hydrogen-bond geometry (Å , ).

Comment
Pyranocarbazoles such as grinimbine, mupamine, mahanimbine, murrayanol and mahanine have been isolated from plant species of the Rutaceae family (Knölker & Reddy, 2002, and references therein) and these alkaloids possess mosquitocidal, antimicrobial, anti-inflammatory and antioxidant activities (Shanazarov et al., 1989). In general pyranocarbazole alkaloids have a C-13, C-18 or C-23 framework with a C-12 carbazole nucleus as the basic unit in which one carbon is attached as a methyl, formyl, carboxylic or ester group. Another observation is that in many of the pyranocarbazole derivatives isolated so far, the oxygen atom of the pyran ring is attached to carbon atom 2 of the carbazole nucleus to form pyrano[3,2-a]carbazoles as in grinimbine. Of the 10 simple carbazole alkaloids isolated so far, five have the oxygen function on carbon 1 (or its equivalent position C 8). The C-18 pyrano[3,2-a] alkaloid mupamine posseses a methoxy group at position 8, hence there exists a substrate on which a pyrano[2,3-a]carbazole could be built upon in the plant body; however, none of the pyranocarbazoles isolated so far has a pyran ring with oxygen on carbon 1 or its equivalent position C 8.
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 /n with four largely planar molecules per unit cell. The plane defined by the sp 2 hybridized carbon atoms, the CH 2 group and the N and O atoms has an r.m.s. deviation from planarity of only 0.036 Å. Of all the ring C atoms only C15 of the pyran C(Me) 2 unit is significately out of plane with the atoms of the four fused rings, its deviation being 0.611 (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 (C17 with a deviation of 0.264 (1) Å). The other, C16, is however located 2.121 (1) 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 (Table 1) and a weaker C-H···C (Table 1) interaction. The unusual C-H···C bond could also be described as a C-H···π interaction [C14-H14b···Cg1 iii = 2.58 Å with Cg1 being the centroid of the C1/C6/C7/C12/N1 pyrrole ring and iii = 1 + x, y, z)]. The only noticeable π···π stacking interaction observed is a slipped one between Cg3 and Cg4 iv with a centroid to centroid distance of 3.8425 (8) Å (Cg3 and Cg4 are C1 to C6 and C7 to C12, respectively, iv = -1 + x, y, z).
supplementary materials sup-2 The N-H···O hydrogen bonds that dominate the packing of the title compound tie molecules together to infinite chains that extend along the crystallographic b axis as shown in Figure 3.
The structures of the 9-and 10-methyl derivatives of (I) are described in Sridharan, Rajendra Prasad & Zeller (2008) and Sridharan, Rajendra Prasad, Ngendahimana et al. (2008). For a more detailed comparison of structures and packing of these three compounds, see Sridharan, Rajendra Prasad & Zeller (2008).

Refinement
The amine H atom was located in a difference map and was refined with an N-H distance restraint of 0.88 (2) Å and U iso = 1.2 U eq (N). All other hydrogen atoms were added in calculated positions with C-H bond distances of 0.99 (methylene), 0.95 (aromatic) and 0.98 Å (methyl). They were refined as riding with U iso (H) = 1.2U eq (C) 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.