Hydrogen-bonded sheets in 4-amino-8,8-dimethyl-2-(methylsulfanyl)-8,9-dihydropyrimidino[4,5-b]quinolin-6(7H)-one

We report here the structure of the title compound, (I) (Fig. 1), which was prepared by microwave irradiation of a twocomponent mixture of a 6-aminopyrimidine and the condensation product formed from dimedone and formaldehyde. By contrast a similar reaction between 6-aminopyrimidines, 5,5dimethylcyclohexane-1,3-dione and a large excess of formaldehyde yielded spiranopyridopyrimidines (Quiroga et al., 2006).

The title molecule, C 14 H 16 N 4 OS, shows strong bond fixation within the fused heterocyclic rings. In the crystal structure, molecules are linked into sheets by a combination of N-HÁ Á ÁN and N-HÁ Á ÁO hydrogen bonds.

Comment
We report here the structure of the title compound, (I) (Fig. 1), which was prepared by microwave irradiation of a twocomponent mixture of a 6-aminopyrimidine and the condensation product formed from dimedone and formaldehyde. By contrast a similar reaction between 6-aminopyrimidines, 5,5dimethylcyclohexane-1,3-dione and a large excess of formaldehyde yielded spiranopyridopyrimidines (Quiroga et al., 2006).
The bond distances within the fused heterocyclic system (Table 1) provide evidence for significant bond fixation of the naphthalene type. Thus, for example, the bonds N1-C2, N3-C4 and C9A-N10 are all significantly shorter than the bonds C2-N3, N10-C10A and C10A-N1, while C5-C5A is the shortest of the C-C bonds. The carbocyclic ring adopts a The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. conformation best described as intermediate between an envelope form, with the fold across the vector C7Á Á ÁC9, and a half-chair form. The ring-puckering parameters (Cremer & Pople, 1975) for the atom sequence C5A-C6-C7-C8-C9-C9A are = 129.2 (2) and ' = 342.4 (4) ; the idealized values for the envelope and half-chair forms, respectively, are = 126.3 and 129.8 , and ' = (60k) and (60k + 30) , where k is zero or an integer. The two S-C distances are clearly different, and atom C21 lies almost in the plane of the adjacent pyrimidine ring.
The molecules are linked into sheets by two hydrogen bonds (Table 2), and the formation of the sheets is readily analysed in terms of two simple substructures, each formed by just one hydrogen bond. In the first substructure, amino atom N4 in the molecule at (x, y, z) acts as a hydrogen-bond donor, via H4A, to the pyrimidine ring atom N3 in the molecule at (1 À x, 1 À y, 2 À z), so generating by inversion an R 2 2 (8) (Bernstein et al., 1995) ring centred at ( 1 2 , 1 2 , 1) (Fig. 2). In the second substructure, amino atom N4 at (x, y, z) acts as a hydrogen-bond donor, via H4B, to atom O6 in the molecule at (x, 3 2 À y, 1 2 + z), so forming a simple C(8) chain running parallel to the [001] direction and generated by the c-glide plane at y = 0.75 (Fig. 3). The combination of these two substructures generates a sheet parallel to (100) (Fig. 4), but there are no direction-specific interactions between adjacent sheets; in particular C-HÁ Á Á hydrogen bonds andstacking interactions are absent.

Experimental
A mixture of 4,6-diamino-2-methylsulfanylpyrimidine (1.0 mmol), 2,2-methylenebis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one) (1.0 mmol) and triethylamine (0.5 mmol) was placed in an open Pyrex-glass vessel and irradiated in a domestic microwave oven for 80 s at 600 W. The resulting solid product was collected by filtration, washed with cold ethanol, dried and then recrystallized from ethanol to provide crystals of compound (I) suitable for single-crystal X-ray diffraction; yield 60%, m.p. 580 K.  Part of the crystal structure of compound (I), showing the formation of the R 2 2 (8) substructure. Hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) are at the symmetry position (1 À x, 1 À y, 2 À z).

Figure 3
Part of the crystal structure of compound (I), showing the formation of the C(8) substructure. The hydrogen bonds are shown as dashed lines and for the sake of clarity the H atoms bonded to C atoms have been omitted. The atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions (x, 3 2 À y, 1 2 + z) and (x, 3 2 À y, À 1 2 + z), respectively.
All H atoms were located in difference maps and then treated as riding atoms with distances C-H = 0.95 Å (aromatic), 0.98 Å (CH 3 ) or 0.99 Å (CH 2 ) and N-H = 0.90 Å , and with U iso (H) = kU eq (C,N), where k = 1.5 for methyl H atoms and 1.2 for all other H atoms.