Unusual thermolysis of azacyclic allene under microwave conditions: crystal structure of (3RS,3aSR,8RS,8aRS)-methyl 5,6-dimethoxy-3a,10-dimethyl-1-phenyl-3,3a,8,8a-tetrahydro-3,8-(epiminomethano)cyclopenta[a]indene-2-carboxylate from synchrotron X-ray diffraction

The structure of methyl 5,6-dimethoxy-3a,10-dimethyl-1-phenyl-3,3a,8,8a-tetrahydro-3,8-(epiminomethano)cyclopenta[a]indene-2-carboxylate, the product of unusual thermolysis of azacyclic allene under microvawe conditions, was studied by synchrotron X-ray diffraction.

The title compound, C 25 H 27 NO 4 (I), the product of the unusual thermolysis of azacyclic allene methyl 10,11-dimethoxy-3,8-dimethyl-6-phenyl-3-azabenzo- [d]cyclodeca-4,6,7-triene-5-carboxylate, represents a bicyclic heterosystem and crystallizes in the monoclinic space group P2 1 /c with three crystallographically independent molecules in the unit cell. These independent molecules adopt very similar geometries and differ only in the conformations of the two methoxy substituents on the benzene ring. In two of the three independent molecules, both methoxy groups are almost coplanar with the benzene ring [the C-C-O-Me torsion angles are 10.8 (2), 12.3 (2), 9.1 (2) and 13.6 (3) ], whereas in the third molecule, one of the methoxy groups is practically coplanar to and the other methoxy group is roughly perpendicular to the benzene ring, the C-C-O-Me torsion angles being 14.1 (2) and 76.5 (2) . The molecule of (I) comprises a fused tetracyclic system containing two five-membered rings (cyclopentenes) and two six-membered rings (piperidine and benzene). The five-membered rings have the usual envelope conformation, with the methylsubsituted C atom as the flap in each molecule, and the six-membered piperidine ring has a chair conformation. The methyl substituent at the N atom occupies the sterically favourable equatorial position. The carboxylate group lies almost within the basal plane of the parent cyclopentene ring [making dihedral angle of 11.68 (8), 18.94 (9) and 15.16 (9) in the three independent molecules], while the phenyl substituent is twisted by 48.26 (6), 42.04 (6) and 41.28 (6) (for the three independent molecules) relative to this plane. In the crystal, molecules of (I) form stacks along the b-axis direction. The molecules are arranged at van der Waals distances.
Recently, we proposed a relatively simple synthesis of benzoazadecatrienes-4,6,7 from 1R-1-phenylethynyl tetrahydroisoquinolines and activated terminal alkynes in trifluoroethanol at 256 K (Voskressensky et al., 2017). However, the thermal transformations of such strained systems have not yet been studied.
In this work, the thermolysis of an azacyclic allene under microwave conditions was carried out by our research group. The structure of the product of this unusual reaction was established unambiguously by synchrotron X-ray diffraction study.

Structural commentary
The title compound crystallizes with three crystallographically independent molecules (A, B and C, containing atoms N10, N32 and N54, respectively) in the unit cell (Fig. 1). These independent molecules adopt very similar geometries and differ only in the conformations of the two methoxy substituents at the benzene ring. In two of the three independent molecules, both methoxy groups are almost coplanar to the benzene ring [the C-C-O-Me torsion angles are 10.8 (2) and 12.3 (2) in molecule A and 9.1 (2) and 13.6 (3) in B], whereas in the third molecule, C, one of the methoxy groups is practically coplanar with and the other methoxy group is roughly perpendicular to the benzene ring, the C-C-O-Me torsion angles being 14.1 (2) and 76.5 (2) ].
The molecule of (I) comprises a fused tetracyclic system containing two five-membered rings (cyclopentenes) and two six-membered rings (piperidine and benzene) (Fig. 1). The five-membered rings have the usual envelope conformation, with the methyl-subsituted C atom as the flap in each molecule, and the six-membered piperidine ring has a chair conformation. The methyl substituent at the nitrogen atom occupies the sterically favourable equatorial position. The carboxylate group lies almost within the basal plane of the parent cyclopentene ring, making dihedral angles of 11.68 (8), 18.94 (9) and 15.16 (9) , respectively, in molecules A, B and C, while the phenyl substituent is twisted by 48.26 (6), 42.04 (6) and 41.28 (6) relative to this plane in molecules A, B and C, respectively.
The title molecule possesses four asymmetric centers at the C3, C3A, C8 and C8A carbon atoms and can have potentially numerous diastereomers. The crystal of (I) is racemic and consists of enantiomeric pairs with the following relative configuration of the centers: rac-3R*,3aS*,8R*,8aR*.

Supramolecular features
The crystal packing motif of molecules of (I) is stacking along the crystallographic b axis (Fig. 2). The molecules are arranged at van der Waals distances.

Synthesis and crystallization
A stirred solution of cyclic allene (0.13 g, 0.32 mmol) in toluene (5 mL) was heated at 453 K for 1 h in a microwave reactor (Anton Paar Monowave 300) (Fig. 3). The solvent was evaporated in vacuo, and the residue recrystallized from ether to give 60 mg of colourless crystals of (I) in a yield of 50%, m.p. = 422-424 K (ether).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The X-ray diffraction study was carried out on the 'Belok' beamline of the National Research Center 'Kurchatov Institute' (Moscow, Russian Federation) using a Rayonix SX165 CCD detector. A total of 720 images were collected using an oscillation range of 1.0 (' scan mode, two different crystal orientations) and corrected for absorption using the Scala program (Evans, 2006). The data were indexed, integrated and scaled using the utility iMOSFLM in CCP4 (Battye et al., 2011).
Hydrogen atoms were placed in calculated positions with C-H = 0.95-1.00 Å and refined in the riding model with fixed isotropic displacement parameters [U iso (H) = 1.5U eq (C) for the CH 3 -groups and 1.2U eq (C) for the other groups].
A rather large number of reflections have been omitted for the following reasons: (i) In order to achieve better I statistics for high-angle reflections, we selected exposure times so as to admit a minor fraction of intensity overloads in the low-angle part of the detector. These low-angle reflections with imprecisely measured intensities were excluded from the final steps of the refinement. (ii) In the present setup of the synchrotron diffractometer, the low-temperature device eclipses a small region of the 2D detector near the high-angle limit. This small shadowed region was not masked during integration of the diffraction frames, which erroneously resulted in zero intensity for some reflections. Thermolysis of azacyclic allene methyl 10,11-dimethoxy-3,8-dimethyl-6phenyl-3-azabenzo[d]cyclodeca-4,6,7-triene-5-carboxylate under microwave conditions.

Figure 2
Crystal structure of (I) illustrating the stacks along [010]. For clarity, H atoms have been omitted.

Funding information
The publication was prepared with the support of the RUDN University Program '5-100' and by the Russian Foundation for Basic   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.46 e Å −3 Δρ min = −0.38 e Å −3 Extinction correction: SHELXL2014 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00120 (18) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.