Methyl 6-amino-6-oxohexanoate

The title compound, C7H13NO3, adopts an approximately planar conformation. The torsion angles in the aliphatic chain between the carbonyl group C atoms range from 172.97 (14) to 179.38 (14)° and the r.m.s. deviation of all non-H atoms is 0.059 Å. The crystal packing is dominated by two strong N—H⋯O hydrogen bonds involving the amide groups and forming R 2 2(8) rings and C(4) chains. Overall, a two-dimensional network parallel to (100) is formed. A weak intermolecular C—H⋯O interaction is also present.

The title compound, C 7 H 13 NO 3 , adopts an approximately planar conformation. The torsion angles in the aliphatic chain between the carbonyl group C atoms range from 172.97 (14) to 179.38 (14) and the r.m.s. deviation of all non-H atoms is 0.059 Å . The crystal packing is dominated by two strong N-HÁ Á ÁO hydrogen bonds involving the amide groups and forming R 2 2 (8) rings and C(4) chains. Overall, a twodimensional network parallel to (100) is formed. A weak intermolecular C-HÁ Á ÁO interaction is also present.

Comment
Adipic acid has importance in various industrial applications including the production of polyamides and polyurethanes.
Methyl 6-amino-6-oxohexanoate (I) crystallizes from methanol as colourless crystals in the monoclinic space group P2 1 /c (Fig. 1). The molecule is approximately planar; the largest deviation from the mean plane defined by the nonhydrogen atoms is 0.116 Å for carbonyl oxygen O1 and the aliphatic chain between the carbonyl carbons is only slightly twisted with torsion angles ranging from 172.97 (14) to 179.38 (14)°. The crystal packing is dominated by two strong N -H···O hydrogen bonds (see Table 1), similar to those seen in the two polymorphs of adipamide (monoclinic: Hospital & Housty, 1966;triclinic: Seaton & Tremayne, 2002). In (I), the the amide nitrogen in serves as a double intermolecular hydrogen donor: N3-H31···O1 i forms an R 2 2 (8) amide dimer around an inversion centre, while N3-H32···O1 ii connects pairs of dimers to form C(4) chains parallel to the c axis. The combination of the C(4) and R 2 2 (8) motifs generates a secondary network of R 10 6 (24) as described for related compounds including benzamide etc. (Bernstein et al. (1995) ; Fig.   2).
Notably, the methyl ester carbonyl group is not involved in hydrogen bonding, however, it is in a suitable position to engage in a weak C-H···O intermolecular interaction with an ester methyl group [d(H···O) = 2.614 (3) Å].
In conclusion, the structure of (I), together with those similar and previously reported, suggest that the variation in the carbonyl substituent at adipic acid does not cause substantial changes to the conformation of the molecule.

Experimental
The title compound was recovered as a side product in 0.5% yield from the cyclization reaction of amino pimelic acid methylester in p-cymene via a redox process (Nishitani et al., 1982). Crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of the compound in methanol.
Alternatively, the title compound can also preprared by reaction of the respective acid chloride with ammonia (Micovic et al., 1988) and the partial hydrolysis of the corresponding nitrile (Kulikova et al., 1960).

Refinement
The structure was refined by full-matrix least-squares. H atoms were treated in the usual manner: positioned geometrically (aliphatic) or located in the difference map (amide) and refined prior to inclusion in the model using riding constraints (Cooper et al., 2010).
Dihedral angles were calculated with PLATON (Spek, 2009); all other standard uncertainties calculated from the full variance co-variance matrix within CRYSTALS (Betteridge et al., 2003).

Figure 1
Molecular structure of (I) with displacement ellipsoids drawn at 50% probability.