Absolute structure of (3aS,5S,7aS,7bS,9aR,10R,12aR,12bS)-7b-hydroxy-4,4,7a,9a,12a-pentamethyl-10-[(2′R)-6-methylheptan-2-yl]-2,8,9-trioxooctadecahydrobenzo[d]indeno[4,5-b]azepin-5-yl acetate from 62-year-old crystals

The structure of the title compound was determined using single crystals obtained more than 60 years ago at the Facultad de Química, Universidad de la República. The chemical structure of the compound, now confirmed by X-ray diffraction, was determined spectroscopically and was relevant in the determination of the structure of lanosterol and other triterpenoids in the early 50′s.


Chemical context
Crystals of the title compound were obtained by Professor M. R. Falco (1922Falco ( -2015 in 1952 after a spectroscopic structure determination (Falco et al., 1952) that was relevant for the correct determination of the structure of lanosterol (Eschenmoser et al., 1955) and were handed in the glass vial shown in Fig. 1 to Professor R. Mariezcurrena (1940-2016 in the late 80 0 s for structure determination by X-ray diffraction. Structure determination was elusive for many years (see the Supramolecular features section for reasons) since the very thin needles available produced no measurable diffraction intensities with the available Weissenberg or Bü rger cameras or a sealed-tube Mo K source diffractometer with a scintillator detector available at the laboratory over that period. The availability of a diffractometer with a Cu K source (acquired and installed at our institution in 2014 during the IYCr) allowed for the determination of the structure at room temperature where significant positional disorder of the terminal aliphatic chain was observed. Data collection at 100 K allowed for the structure refinement reported herein, which confirms the structure determined spectroscopically in the 50 0 s. Professor Mariezcurrena had the chance to see the final structural model of the RT structure determination before passing away. We dedicate this manuscript to his memory, teachings and patience in keeping the glass vial in a ISSN 2056-9890 safe place allowing for this report of the successful structure determination.

Structural commentary
The title compound, shown in Fig. 2 with the numbering scheme, is a tetracyclic triterpenoid with six-, seven-, six and five-membered fused rings, with no insaturations except for three exocyclic carbonyl moieties at C2, C8 and C9. The first two rings define a hydrogenated benzazepine unit while the last two define a hydrogenated indene group. The presence of fused rings of different sizes, one heteroatom and different Catom hybridization states, together with a large number of exocyclic substituents, leads to a very strained bonding arrangement within the ring system. A full geometrical analysis performed with Mogul 1.8.2 (Build 248885) running on the May 2019 update of the CSD (Groom et al., 2016) shows that all bridgehead atoms in the molecule show atypical bond distances or angles. Table 1 shows all the bond distances and bond angles that were unexpected according to the z-score criterion in Mogul. In this table we find that C3A and C7A (bridgehead atoms in the benzazepine bicycle) C7B and C12B (bridgehead atoms of the fused azepine and indene groups) and C9A (bridgehead atom in the indene bicycle) display unusual bond distances [long C3A-C7A = 1.584 (4), C7B-C8 = 1.580 (4), C7B-C12B = 1.578 (4) Å and short C9-C9A = 1.500 (4) Å ] and C7B, C9A and C12A show unusual bond angles [low O7-C7B-C8 = 97.7 (2), C12A-C9A-C9 = 102.5 (2), C12-C12A-C9A = 101.1 (2) ] in addition to other unusual features.
Another significant contribution to the strain in this region of the molecule is the diketone group C7B-C8( O8)-C9( O9)-C9A that also shows an elongated Csp 2 -Csp 2 bond [C8-C9 of 1.549 (5) Å ] and a large O8-C8-C9-O9 torsion angle of À43.7 (5) . Repulsion between O8 and O9 leads to the increase of the torsion angle in the cis diketone group and lengthening of the C8-C9 bond distance, contributing to the unusual conformation of the C7B/C8/C9/C9A/ C12A/C12B ring. Puckering parameters for this ring are = 149 (or 31 considering the inverted order of atoms) and È = 13.5 , which fall far from all usual parameters for frequently observed geometries of six-membered rings, between a chair and a half-chair conformation, confirming the effects of the observed bond distances and angles. Considering a distorted research communications Acta Cryst. (2019). E75, 1348-1351 Suescun and Heinzen C 32 H 51 NO 6 1349 Table 1 Unusual bond distances and bond angles (') (Å , ) extracted from Mogul. z-score = |d À d mean |/SD, where d mean and SD are the mean and standard deviation of N observed values in the Mogul database. The z-score for bond angles is calculated replacing d by '. A bond distance or angle is considered unusual if the z-score > 2. Note: (a) This value is lower than 2 but this bond is still unusually long and relevant for the discussion.

Figure 1
Crystals of the title compound in the original tube where they were saved for more than 60 years.

Supramolecular features
In the crystal, molecules of the title compound pack in an elongated conformation laying parallel to the [102] direction. The packing is directed by O7-H7Á Á ÁO2 i hydrogen bonds (see Table 2, Fig. 3), forming zigzag chains running along the [010] direction (determined using PLATON software; Spek, 2009). These chains are connected in the [001] direction through weak C-HÁ Á ÁO interactions: C71-H71AÁ Á ÁO8 ii and C121-H12EÁ Á ÁO52 iii with H71AÁ Á ÁO8 ii and H12EÁ Á ÁO52 iii distances of 2.55 and 2.53 Å , respectively [symmetry codes: (ii) Àx + 3 2 , y À 1 2 , Àz + 1; (iii) Àx + 3 2 , y À 1 2 , Àz]. These interactions define double planes of molecules with the polar regions of the molecules in contact, leaving the terminal aliphatic chains pointing outwards. Parallel planes are only weakly bound by dispersion forces: indeed, voids of ca 39 Å 3 are found between non-polar residues from parallel planes (Fig. 4). These strong interactions along [010], weak along [001] and very weak along [100] nicely explain the flat needle crystal shape observed where face indexing suggests the needle length and longer dimension of the largest planes is [010], the shorter dimension of the planes is [001] and the very narrow width of the crystals is [100]. The large unit cell, the presence of positional disorder (aggravated at room temperature) and voids in the crystal structure, combined with the C, H, N and O composition of the crystals explain the poor scattering power that prevented structure determination with older instruments.

Figure 4
View of the packing of the title compound along [010] showing the formation of C-HÁ Á ÁO hydrogen-bonded planes and the voids left by symmetry-related methylheptane chains. type) with an N atom in the seven-membered ring. Three pentacyclic compounds with a tetrazole ring at N1-C2 have been reported [LEXVOB (Alam et al., 2013), TZANDT (Husain et al., 1981) and VEVLAK (Rajnikant et al., 2006)], the former and latter showing very similar molecular conformation and interactions that lead to very similar unit-cell dimensions ($35Â6Â12 Å ). There is only one entry with the same six-, seven-, six-and five-membered ring combination containing O instead of N (HIXSAI; Morales et al., 1999) but the configuration of C5 is inverted and therefore the dihedral angle between mean planes of the six-and seven-membered rings differ significantly and thus also the molecular conformation. There are also three tetracyclic compounds with no heteroatom in the ring arrangement [OQIVIU (Kranz et al., 2011), UBEDIO (Wang et al., 2000) and WECQAY (Kranz et al., 2012)] all showing very different stereochemistry; therefore, the molecular conformations are not comparable. This is, therefore, the first report of this 6-7-6-5 ring system containing the azepine ring.

Synthesis and crystallization
Synthesis and crystallization were reported by Falco et al. (1952). Crystals were not recrystallized after the initial preparation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-and N-bound H atoms were placed in calculated positions (C-H = 0.93-0.99, N-H = 0.87 Å ) and included as riding contributions. The OH H atom was found in a difference-Fourier map and refined as riding with a rotating torsion angle and O-H distance restraint. All H atoms were refined with isotropic displacement parameters set at 1.2-1.5 times the U eq value of the parent atom.  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010). Rms deviation of fitted atoms = 0.3855 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.