Crystal structure of 2-amino-5,6,7,8-tetrahydro-7,7-dimethyl-4-(naphthalen-2-yl)-5-oxo-4H-chromene-3-carbonitrile

Both six-membered rings of the fused heterocyclic system of the title compound display envelope conformations. Two hydrogen bonds involving the amino group lead to a double-layer structure.

In the title compound, C 22 H 20 N 2 O 2 , both six-membered rings of the fused heterocyclic system display envelope conformations; the two carbon atoms bearing the methyl groups and the naphthyl substituent both lie outside the planes of the other atoms of each ring. In the crystal, the amino group forms hydrogen bonds of the types N-HÁ Á ÁO C and N-HÁ Á ÁN C, leading to the formation of a double layer structure propagating parallel to the bc plane. Weak C-HÁ Á ÁO and C-HÁ Á Á interactions may reinforce the layers.

Chemical context
Six-membered heterocycles involving 4H-pyran units represent an important class of biologically active synthetic and natural products, many of which attract the interest of the drug industry (Lega et al., 2016). Pyrans possess antimicrobial (Dazmiri et al., 2020), antituberculosis (Kalaria et al., 2014) and antitumor (Wang et al., 2014) activities, whereby 4Hpyrans are moieties in a series of natural products (Singh et al., 1996). A number of 4H-pyrans are used, for example, as photoactive ingredients (Armesto et al., 1989) or agrochemicals (Kumar et al., 2009). Synthetically, they are intermediates for the synthesis of heterocyclic compounds such as pyranopyrimidines and pyranopyrazoles (Elgemeie et al., 1987(Elgemeie et al., , 1988 and consequently the synthesis of 4H-pyrans themselves is of interest to organic chemists. Some time ago, we reported the synthesis of pyridine-2(1H)-thiones and their condensed derivatives from the reactions of arylmethylenecyanothioacetamides with suitable active methylene compounds (Elgemeie et al., 2002).We also described the reaction of the dimedone 1 with naphthylmethylenecyanothioacetamide to produce a condensed pyridine-2(1H)-thione (Attia et al., 1997). The course of this reaction prompted us to investigate how 1 would react with naphthylmethylenecyanoacetamide [2-cyano-3-(naphthalen-2-yl)acrylamide, 2] in boiling ethanol containing triethylamine. The product was shown to be neither of the expected condensed pyridin-2(1H)-ones 3 or 4 but rather the condensed pyran nitrile 5 (Scheme 1). The latter structure was inferred on the basis of elemental analysis and spectroscopic data: thus, the mass spectrum of 5 was compatible with the molecular formula C 22 H 20 N 2 O 2 (M + , 344), and the 1 H NMR spectrum had signals at 4.37 (pyran-4H), 7.06 (br, NH 2 ) and 7.29-7.90 (m, ArH).
We assume that the formation of 5 proceeds via addition of the active methylene group of 1 to the double bond of 2 to give the intermediates 6, 7 and then 8, the latter finally losing one molecule of water to give the final product 5 (Scheme 2). In order to establish the structure of this compound unambiguously, its crystal structure was determined and is reported here.

Supramolecular features
In the crystal, the amino group acts as donor for two classical hydrogen bonds (Table 2). This leads to a double layer structure ( Fig. 2) propagating parallel to the bc plane. The HÁ Á ÁO separation of the weak hydrogen bond C6-H6BÁ Á ÁN2 (x, À1 + y, z) is rather long at 2.69 Å but acceptably linear (160 ) and presumably reinforces the layer structure, but is not shown in Fig. 2. The short contact C10-H10BÁ Á ÁCg (C12-16/ C21), with HÁ Á ÁCg 2.79 Å and a C-HÁ Á ÁCg angle of 139 , may represent a C-HÁ Á Á interaction between the double layers.
There are no shortstacking contacts.

Database survey
A search of the Cambridge Database (Version 2021.3.0; Groom et al., 2016) showed that the motif of a 4-substituted 2-amino-5,6,7,8-tetrahydro-7,7-dimethyl-5-oxo-4H-chromene-3-carbonitrile has been the subject of many structure determinations. A total of 54 hits with variously substituted phenyl groups was found, which reduces to 32 when duplicate structure determinations, various solvates and polymorphs are not considered. For all but one of these structures, the 4-position also bears a hydrogen atom, the exception being the 4-methyl, 4-nitrophenyl derivative (Cai et al., 2012;refcode TESNEM).
Additionally, the 4-(1-naphthyl) derivative was found (Nesterov et al., 2004;refcode ETOKIH), which is an isomer of the title compound 5. The packing of ETOKIH is quite different from that of 5; the hydrogen atom corresponding to H01 in 5 forms N-HÁ Á ÁN hydrogen bonds, leading to inversion dimers, whereas the other NH hydrogen atom is not involved in hydrogen bonding. A least-squares overlay of 5 and ETOKIH (excluding methyl groups and all naphthyl carbon atoms except the ipso C atom) gave an r.m.s. deviation of 0.15 Å ; Fig. 3 shows the slight differences in ring conformation.

Figure 2
Crystal packing of 5 viewed parallel to the a axis in the region x ' 0.5.
Dashed lines indicate classical hydrogen bonds. Naphthyl rings are reduced to the ipso carbon atoms for clarity. Hydrogen atoms not involved in classical hydrogen bonding are omitted. The figure is depthcoded; molecules of the lower layer are drawn with thinner bonds. Atom labels indicate the asymmetric unit (which lies in the lower layer).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms of the NH 2 group were refined freely, but with N-H distances restrained to be approximately equal using a SADI instruction in SHELXL. The methyl groups were included as idealised rigid groups allowed to rotate but not tip (C-H = 0.98 Å ; H-C-H = 109.5 ). The other hydrogen atoms were included using a riding model starting from calculated positions (C-H = 0.95, 0.98 and 1.00 Å for aromatic, methylene and methine H atoms, respectively). The U iso (H) values were fixed at 1.5 Â U eq of the parent carbon atoms for the methyl groups and 1.2 Â U eq for other hydrogen atoms.  CrysAlis PRO (Rigaku OD, 2021); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015b).