3-(2-Acetamidoethyl)-1H-indol-5-yl 4-nitrophenyl carbonate

In the title molecule, C19H17N3O6, the indole ring system is essentially planar (r.m.s. deviation = 0.009 Å) and forms a dihedral angle of 31.96 (9)° with the nitro-substituted benzene ring. In the crystal, molecules are linked by pairs of N—H⋯O hydrogen bonds, forming inversion dimers which are connected by further N—H⋯O hydrogen bonds into a two-dimensional network parallel to (102).


Comment
The title compound is one of the aromatic carbonates, which constitute an important class of esters which facilitate the synthesis of a carbamate bond in the nucleofilic substitution reaction of carbonate derivatives with amines (Freer & McKillop, 1996;Um et al., 2006;2008). It is also a derivative which has potential use in peptide synthesis (Gray et al., 1977). We have used the title compound in the synthesis of novel tacrine-melatonine heterodimers (Zawadzka, et al., 2012).
The title compound consists of three major planar fragments and a large flexible substituent having several degrees of freedom. The planar fragments are the p-nitrophenyl fragment, carbonate group and indole group. The r.m.s. deviations of in-plane atoms for the respective planes are 0.006, 0.009 and 0.009 Å, respectively. The carbonate group forms a dihedral angle of 80.54 (8)° with the nitro-substituted benzene ring and forms a dihedral angle of 73.23 (6)° with the indole ring system. The nitro group is slightly rotated from the plane of the attached benzene ring with a dihedral angle of 8.0 (4)°. In general the bond lengths and angles have expected values. The benzene ring may affected by anistropic displacement causing slightly shorter than expected bond lengths to be observed. In the crystal, molecules are linked by a pair of N-H···O hydrogen bonds to forms inversion dimers which are connected by further N-H···O hydrogen bonds to form a two-dimensional network parallel to (102) ( Table 1 and Fig. 2).

Experimental
To a solution of N- [2-(5-hydroxy-1H-indol-3-yl)ethyl]acetamide (0.9 g, 4 mmol) in N-methylomorfoline (0.92 ml, 8 mmol) 4-nitrophenyl chloroformate (1.61 g, 8 mmol) dissolved in 1 ml of tetrahydrofuran was added. The reaction mixture was stirred under argon for 1 h at room temperature. Evaporation of the solvents gave a residue that was purified by silica gel chromatography using a mixture of methylene chloride/methanol 95:5 as eluent to produce the title compound (0.92 g, 60%) as a yellow solid. Crystals suitable for X-ray analysis were obtained by slow evaporation of a solution of the title compound in a methylene chloride/methanol/diethyl ether mixture.

Refinement
H atoms bonded to C atoms were placed in calculated positions with distances in the range 0.93-0.97Å and inlcuded in the refinement with U iso (H) = 1.2U eq (C) or 1.5U eq (C methyl ). The positional parameters of the H atoms bonded to N atoms were refined independently with U iso (H) = 1.2U eq (N).  The molecular structure of the title compound with 30% dispalcement ellipsoids.

Figure 2
Part of the crystal structure with hydrogen bonds shown as dshed lines. 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 R-factors(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.