1600-5368 rac-5-Acetyl-6-( 4-nitrophenyl )-6 , 7-dihydro-5 H-1 , 3-dioxolo [ 4 , 5g ] quinoline-8-one : chains of rings built from CÐH O and CÐH p ( arene ) hydrogen bonds

Compound (I) (Fig. 1) crystallizes in the polar space group Pna21. The molecule contains a stereogenic centre at C6, and the selected reference molecule is of S con®guration; however, the space group accommodates equal numbers of R and S enantiomers. For the heterocyclic ring (N5/C4A/C8A/C8/C7/C6), the ring-puckering parameters (Cremer & Pople, 1975) for this atom sequence, = 124.7 (3) and ' = 120.3 (4) , indicate an almost pure envelope form (Evans & Boeyens, 1989). The inter-bond angles at N5 (Table 1) indicate that this atom has Received 17 May 2004 Accepted 19 May 2004 Online 22 May 2004

Compound (I) (Fig. 1) crystallizes in the polar space group Pna2 1 . The molecule contains a stereogenic centre at C6, and the selected reference molecule is of S con®guration; however, the space group accommodates equal numbers of R and S enantiomers.
For the heterocyclic ring (N5/C4A/C8A/C8/C7/C6), the ring-puckering parameters (Cremer & Pople, 1975) for this atom sequence, = 124.7 (3) and 9 = 120.3 (4) , indicate an almost pure envelope form (Evans & Boeyens, 1989). The inter-bond angles at N5 (Table 1) indicate that this atom has effectively planar coordination, as expected for amidic N, but, unexpectedly, the nitrophenyl substituent at C6 occupies an axial site. Within the fused-ring system, the bond lengths (Table 1) show several unexpected features. In particular, the bonds C3AÐC4 and C9ÐC9A are signi®cantly shorter than the other bonds in the carbocyclic aromatic ring, and the bond C4AÐN5 is very long for its type: the mean value for bonds of this type (Allen et al., 1987) is 1.371 A Ê . It may also be noted that the two carbonyl C O distances are identical, despite their different local environments.
The molecules of (I) are linked into chains by a combination of CÐHÁ Á ÁO and CÐHÁ Á Á%(arene) hydrogen bonds, augmented by a dipolar carbonyl±carbonyl interaction. Atoms C62 and C63 in the molecule at (x, y, z) act as hydrogen-bond donors, respectively, to amidic atom O51 and to the ring C61Ð C66, both in the molecule at ( 1 2 + x, 3 2 À y, z), and propagation of these interactions produces a chain of rings running parallel to the [100] direction and generated by the a glide plane at y = 0.75 (Fig. 2). In addition, carbonyl atom O51 in the molecule at (x, y, z) forms a short contact with the carbonyl atom C51 in the molecule at (x À 1 2 , 3 2 À y, z): the OÁ Á ÁC distance is 3.001 (3) A Ê , the CÐOÁ Á ÁC angle is 150 (4) and the OÁ Á ÁCÐO angle is 87.4 (2) , indicative of a type I interaction (Allen et al., 1998), which reinforces the [100] chain.
Two antiparallel chains of this type pass through each unit cell, generated by the a glide planes at y = 0.25 and 0.75, but there are no direction-speci®c interactions between adjacent chains. In particular, there are no %±% stacking interaction and no CÐHÁ Á ÁO hydrogen bonds involving the nitro O atoms.
Symmetry code: (i) 1 2 xY 3 2 À yY z. Cg1 is the centroid of ring C61±C66 All H atoms were located in difference maps and subsequently treated as riding atoms, with distances CÐH = 0.95 (aromatic), 0.98 (methyl), 0.99 (CH 2 ) or 1.00 A Ê (aliphatic CH), and with U iso (H) = 1.2U eq (C), or 1.5U eq (C) for the methyl group. The value of the Flack parameter [0.2 (13); Flack, 1983] was indeterminate (Flack & Bernardinelli, 2000), and hence the correct orientation of the structure relative to the polar axis direction could not be established (Jones, 1986). Accordingly, the Friedel-equivalent re¯ections were merged prior to the ®nal re®nement.