Diethyl 6H,12H-5,11-methanodibenzo[b,f][1,5]diazocine-1,7-dicarboxylate

In the molecule of the title compound, C21H22N2O4, the 1,7-diethyl ester analogue of Tröger’s base, the dihedral angle between the two benzene rings is 93.16 (3)°; the molecule is C 2 symmetric.

In the molecule of the title compound, C 21 H 22 N 2 O 4 , the 1,7diethyl ester analogue of Trö ger's base, the dihedral angle between the two benzene rings is 93.16 (3) ; the molecule is C 2 symmetric.

Related literature
For background to the synthesis of Trö ger's base products, see: Hansson et al. (2003); Solano et al. (2005); Bhuiyan et al. (2007); Didier & Sergeyev (2007); Zhu et al. (2008); Vande Velde et al. (2008). For related structures, see: Faroughi et al. (2006); Bhuiyan et al. (2006). Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT and XPREP (Siemens, 1995); program(s) used to solve structure: SIR97 (Altomare et al. 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and WinGX32 (Farrugia, 1999); software used to prepare material for publication: enCIFer (Allen et al., 2004 Comment Dibenzo Tröger's base analogues are formed from the acid catalysed condensation of an aniline with either formaldehyde or formaldehyde equivalents. It was a long-held belief that a para-substituent was required on the aniline to prevent polymerization during the Tröger's base reaction and that the presence of an electron-withdrawing group would result in neglible yields of Tröger's base products. These beliefs have been proved to be incorrect, with the synthesis of tetranitro- (Bhuiyan et al., 2007) and octafluoro- (Vande Velde et al., 2008) analogues (in yields of 11% and 37%, respectively), and the synthesis of Tröger's base analogues from 2-and 3-substituted anilines lacking a substitutent in the para-position (Hansson et al., 2003), and even from aniline itself (Didier & Sergeyev, 2007). The title compound is another example of a Tröger's base analogue unsubstituted in the 2,8-positions. An important feature of all Tröger's base analogues is the V-shaped structure of the compounds. The dihedral angle between the aromatic rings has been measured for over 25 simple dibenzo Tröger's base analogues and has been found to lie between 82° (Solano et al., 2005) and 110° (Zhu et al., 2008). The X-ray structures of two related Tröger's base esters have also been reported (Faroughi et al., 2006;Bhuiyan et al., 2006). It is noteworthy that the title compound was the sole Tröger's base analogue isolated from the reaction and results from carbon-carbon bond formation at the more hindered ortho-site, relative to the aniline amino group.

Experimental
The title compound, Fig. 1, crystallizes in space group C2/c and it was prepared as outlined in Fig. 2.

Experimental
Ethyl 3-aminobenzoate (2.0 g, 12.1 mmol) and paraformaldehyde (582 mg, 19.38 mmol) were dissolved in trifluoroacetic acid (75 ml) and the mixture was stirred under an argon atmosphere in the dark 7 days. The reaction mixture was then basified with a solution of concentrated ammonia (80 ml) in water (120 ml). A saturated sodium hydrogen carbonate solution (100 ml) was added and the crude material was extracted into ethyl acetate (3 × 75 ml). The combined organic layers were washed with brine (100 ml), dried over anhydrous sodium sulfate, filtered and evaporated to dryness to yield an orange solid. The crude material was purified by recrystallization from hexane to afford the title compound (760 mg, 34%) as a white solid and a racemic mixture, m.p. 441-443 K.
Single crystals of the title compound were produced by slow evaporation of a dichloromethane solution.

Refinement
C-bound H atoms were included in idealized positions and refined using a riding model. Methylene, aromatic and methyl C-H bond lengths were fixed at 0.99, 0.95 and 0.98 Å, respectively. U iso (H) values were fixed at 1.2U eq (C) for methylene and aromatic H atoms, and at 1.5U eq (C) for methyl H atoms.
supplementary materials sup-2 Figures   Fig. 1. View of the title compound, showing the atomic numbering scheme. Displacement ellipsoids are shown at the 50% probability level. Symmetry code used to generate equivalent atoms: 2-x, y, 1.5-z. Diethyl 6H, 12H-5,11-methanodibenzo[b,f] Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (