Crystal structure of the α-racemate of methohexital

N—H⋯O=C bonded molecules of the title compound are linked into a inversion dimer with an (8) motif.

Molecules of the title compound, C 14 H 18 N 2 O 3 [systematic name: 5-allyl-5-(hex-3-yn-2-yl)-1-methylpyrimidine-2,4,6(1H,3H,5H)-trione in the (R b S h )/(S b R h ) racemic form], are connected by mutual N-HÁ Á ÁO C hydrogen bonds in which the carbonyl group at the 2-position of the pyrimidinetrione ring is employed. These interactions result in an inversion dimer which displays a central R 2 2 (8) ring motif. This dimer is topologically distinct from that of the previously reported (S b R h ) form, which is, however, also based on an R 2 2 (8) motif. The methyl group at the 1-position of the pyrimidinetrione ring in the title structure is disordered over two sets of sites in a 0.57 (2):0.43 (2) ratio.

Chemical context
The title compound is a barbiturate derivative, the Na salt of which (trade name Brevimytal, Eli Lilly) is a widely used short-acting anaesthetic with a rapid onset of action. The molecule contains two asymmetric centres and can exist as two diastereomeric enantiomer pairs. Its stereoisomerism is known to affect the anaesthetic activity and possible side effects of the drug (Gibson et al., 1959). The crystal structure of the (S b R h ) form of methohexital was previously reported by Brunner et al. (2003), who also established that the commercial product (-racemate) consists of the (R b S h ) and (S b R h ) isomers.
The previously reported (S b R h ) form contains two independent molecules (denoted A and B), which differ from the molecule of the title structure in the conformation adopted by the terminal groups of both 5-substituents (Fig. 2). Specifically, in molecule A, the torsion angle analogous to C5-C7-C8-C9 in the present -racemate is 125.3 , and the pseudo-torsion angles analogous to C5-C10Á Á ÁC13-C14 of the title structure are À15.4 (A) and À26.3 (B).

Supramolecular features
Two molecules are linked to one another by two mutual antiparallel N-HÁ Á ÁO C bonds so that an inversion dimer is formed (Table 1, Fig. 3), which displays a central R 2 2 (8) ring motif (Etter et al., 1990;Bernstein et al., 1995). This interaction involves the carbonyl group at the 2-position of the ring. The R 2 2 (8) ring motif is also present in the (S b R h ) form (Brunner et al., 2003) where it connects the two crystallographically independent molecules. However, in this case the dimer is based on two topologically distinct N-HÁ Á ÁO C interactions which involve the carbonyl groups at the 4-position of the ring of molecule A and at the 2-position of molecule B.

Database survey
The Cambridge Structural Database (Groom & Allen, 2014; Version 3.35) contains 11 unique entries for derivatives of barbituric acid which are analogous to the title compound and substituted at the 1-position, but not at the 3-position of the six-membered ring. A common characteristic of these compounds is the presence of one hydrogen-bond donor group (NH) and three potential acceptor groups, viz. the carbonyl groups at the ring positions 2, 4 and 6. Thus, three topologically distinct hydrogen-bonding acceptor interactions are possible. Additionally, there is a competition between possible dimer and catemer motifs, which is similar to the competition between hydrogen-bonded dimer and catemer motifs between carboxyl groups (Beyer & Price;2000) or carboxamide groups (Arlin et al., 2010(Arlin et al., , 2011. The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level; hydrogen atoms are drawn as spheres of arbitrary size.

Figure 3
The N-HÁ Á ÁO C hydrogen-bonded inversion dimer displaying a central R 2 2 (8) ring. These interactions (dotted lines) involve the carbonyl group at the 2-position of the six-membered ring. O and H atoms engaged in hydrogen bonding are drawn as spheres.

Synthesis and crystallization
The crystals investigated in this study were obtained at room temperature, by slow evaporation from an aqueous solution of the -racemate of methohexital (Lilly Research Centre Ltd., Windlesham, England).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were identified in difference maps. The H atoms of the C14 methyl group and disordered C1 methyl group [occupancy ratio 0.57 (2):0.43 (2)] were idealized and included as rigid groups allowed to rotate but not tip (C-H = 0.96 Å ) and refined with U iso set to 1.5U eq (C) of the parent carbon atom. H atoms bonded to secondary CH 2 (C-H = 0.97 Å ), tertiary CH (C-H = 0.98 Å ) carbon and aromatic CH carbon atoms (C-H = 0.93 Å ) were positioned geometrically and refined with U iso set to 1.2U eq (C) of the parent carbon atom. The NH hydrogen atom was refined with a restrained distance [N-H = 0.86 (2) Å ] and its U iso parameter was freely refined.

Figure 4
The three fundamental connection modes for the formation of N-HÁ Á ÁO C bonds in 1-substituted derivatives of barbituric acid arising from the involvement of different carbonyl groups, and the corresponding numbers of observed dimer and catemer isomers. The (S b R h ) form of methohexithal contains a dimer with mixed N-HÁ Á ÁO C2/N-HÁ Á ÁO C4 connectivity and was therefore not included.  (Sheldrick, 2015); molecular graphics: XP in SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. The C1 methyl group is disordered over two positions.