Comment

As part of a program for the synthesis of fused pyrazole derivatives (Quiroga et al., 1998, 2001; Cannon et al., 2001a,b; Low et al., 2001), we have been investigating three-component cyclocondensation reactions induced by microwave irradiation. We report here the molecular and supramolecular structures of two compounds, (I) and (II), obtained from condensation reactions between a substituted aminopyrazole, 5,5-dimethylcyclohexane-1,3-dione (dimedone) and a simple carbonyl compound or its equivalent. Thus, from the reaction involving 5-amino-3-tert-butyl-1-phenylpyrazole and formaldehyde, we have now obtained 3-tert-butyl-7,7-dimethyl-1-phenyl-5,6,7,8-tetrahydroimidazo[3,4-b]quinolin-5-one, (I), in which a single formaldehyde unit has been utilized in the construction of the fused ring system. When two such units are incorporated, spiro compound (III) results (Low et al., 2004). When 5-amino-3-methyl-1H-pyrazole is used in combination with orthobenzoic acid trimethyl ester, the product is (II), analogous to the compound, (IV), formed from this pyrazole in the presence of formaldehyde (Low et al., 2004).

In both (I) (Fig. 1) and (II) (Fig. 2), the heterobicyclic portions of the fused ring systems are planar, but the carbocyclic rings are puckered. The ring-puckering parameters (Cremer & Pople, 1975) for (I) [ = 127.4 (3)° and = 353.8 (3)° for the atom sequence C4a-C5-C6-C7-C8-C8a] and (II) [ = 65.2 (2)° and = 174.3 (3)° for the atom sequence C5a-C6-C7-C8-C9-C9a] indicate envelope conformations for both these rings (Evans & Boeyens, 1989), consistent with the enforced coplanarity of atoms C5, C4a, C8a and C8 in (I), and of atoms C6, C5a, C9a and C9 in (II).

In (I), the C3a-C4 and C4-C4a bonds are of very similar length (Table 1), as are the C8a-N9 and N9-C9a bonds, consistent with aromatic delocalization within the central ring of (I). The formally single C3a-N4 and C9a-N9b bonds in (II) (Table 3) are only slightly longer than the formal double bond N1=C2, although each is significantly longer than the cross-ring C3a-N9b bond, also formally a single bond. The lengths of the C2-C3 and C3=C3a bonds, formally single and double, respectively, differ by less than 0.03 Å. These observations suggest that this heterocyclic system exhibits a degree of naphthalene-type delocalization, involving a peripheral system of ten electrons but with only modest participation by the cross-ring bond (Glidewell & Lloyd, 1984).

In each of (I) and (II), the molecules are linked weakly into chains by means of a single C-HN hydrogen bond (Tables 2 and 4); the structure of neither compound exhibits any C-H(arene) hydrogen bonds or aromatic - stacking interactions. In (I), atom C6 in the molecule at (x, y, z) acts as a hydrogen-bond donor, via atom H6B, to pyridine ring atom N9 in the molecule at (1 + x, y, z), so generating by translation a C(6) chain (Bernstein et al., 1995) running parallel to the [100] direction (Fig. 3). In (II), aryl atom C54 in the molecule at (x, y, z) acts as a hydrogen-bond donor to pyrazole-ring atom N1 in the molecule at (1 + x,  - y, - + z), so producing a zigzag C(10) chain running parallel to the [20] direction and generated by the c-glide plane at y = (Fig. 4).

The constitutions of (II) and (IV) differ only by the presence of the phenyl substituent in (II); however, this difference profoundly influences the differences in the supramolecular structures of these compounds. In (IV), the C-H bond that is replaced by the C-phenyl bond in (II) acts as the sole hydrogen-bond donor, forming, by means of paired C-HN hydrogen bonds, a centrosymmetric R₂²(6) dimer. Dimers of this type are then linked into chains by a single - stacking interaction (Low et al., 2004).

Experimental

For the synthesis of (I), a mixture of 5-amino-3-tert-butyl-1-phenylpyrazole (1 mmol), dimedone (1 mmol) and formaldehyde (3 mmol) was placed in Pyrex-glass open vessels and irradiated in a domestic microwave oven for 4 min (at 600 W). The reaction mixture was extracted with ethanol and the product, (I), was isolated by column chromatography on silica gel, using CHCl₃ as eluant, and crystallized from ethanol, yielding crystals suitable for single-crystal X-ray diffraction (m.p. 413 K; yield 41%). Analysis found: C 75.5, H 7.3, N 12.1%; C₂₂H₂₅N₃O requires: C 76.0, H 7.3, N 12.1%. For the synthesis of (II), an equimolar mixture of 5-amino-3-methyl-1H-pyrazole, dimedone and orthobenzoic acid trimethyl ester (1 mmol of each) was placed in Pyrex-glass open vessels and irradiated in a domestic microwave oven for 2 min (at 600 W). The reaction mixture was extracted with ethanol and the product, (II), was crystallized from ethanol, producing crystals suitable for single-crystal X-ray diffraction (m.p. 533 K; yield 55%). MS EI (70 eV) m/z (%): 306 (23), 305 (100, M⁺), 304 (60), 291 (13), 290 (54), 250 (14), 249 (73), 248 (14), 220 (13), 153 (11), 127 (16), 126 (10), 77 (29), 66 (10), 55 (10), 53 (16), 52 (13), 351 (17), 42 (20), 41 (34), 39 (35).

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: GG1220 ). Services for accessing these data are described at the back of the journal.

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