Crystal structure of (−)-(5R,7R,8S,9R,10S)-8-methyl-7-[(5R)-3-methyl-2-oxooxolan-3-en-5-yl]-1-aza-6-oxatricyclo[8.3.0.05,9]tridecan-13-one monohydrate

The title compound is an epimer of natural tetracyclic isosaxorumamide. The dihydrofuranone, oxolane, azepane and pyrrolidine rings adopt planar, twist, twist-chair and envelope forms, respectively. In the crystal, O—H⋯O hydrogen bonds connect the water and main molecules into a tape structure, which is further expanded into a three-dimensional network by C—H⋯O interactions.


Chemical context
Saxorumamide and isosaxorumamide are natural Stemona alkaloids isolated from the root of Stemona Saxorum (Wang et al., 2007). They are a pair of diastereomer (12-epimer of each other) which consist of a fused octahydrofuro[3,2-c]pyrrolo-[1,2-a]azepane nucleus with a dihydrofuranone substituent (Fig. 1). The Stemona alkaloids have been isolated from various Stemonaceae species, and over 150 metabolites have been elucidated (Pilli et al., 2000(Pilli et al., , 2010. Extracts of Stemonaceae plants have been traditionally used for folk medicines as antitussive and anthelmintic agents in the wide regions of East Asia and Southeast Asia (Greger, 2006). Stemona Saxorum has also been utilized for endemic disease in Vietnam. The title compound is an 11-epimer of isosaxorumamide afforded in a synthetic study of stemoamide-type alkaloids (Yoritate et al., 2017).

Figure 3
A partial packing diagram viewed down the a axis, showing the tape structure running along the b-axis direction.

Figure 2
The asymmetric unit of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Only H atoms connected to O and chiral C atoms are shown for clarity. Sridhar, 2011) and YAHMIF (Zhang et al., 2011), and three structures of its 7-(3-methyl-2-oxooxolan-3-en-5-ylidene) derivatives, (c), with refcodes PROTMI (Ishizuka et al., 1972), PROTOS10 (Irie et al., 1973) and OJIRII (Kaltenegger et al., 2003). For the former four structures, the stereochemical and conformational properties, as trans-fused furoazepane, rela-tive stereochemistry and conformation of nitrogen-containing rings, are almost coincident with those of the title compound. On the other hand, the oxolane ring shows an envelope form in these four structures rather than a twist form as in the title compound. KEGYIF (space group P2 1 ) is the natural alkaloid (-)-stemoamide, which is a -7,13-dione derivative of (a), and XATFOP (space group P2 1 /n) is its racemate.

Synthesis and crystallization
The title compound was afforded in a synthetic study of saxorumamide and isosaxorumamide, from ethyl 4-bromobutanoate and a siloxypyrrole analogue (Yoritate et al., 2017). The stereochemistry was controlled at the first step of the synthesis by enantioselective alkynylation according to the reported conditions (Trost et al., 2006(Trost et al., , 2012, and confirmed with HPLC analysis (>98% ee). The (-)-stemoamide was provided as a tricyclic core intermediate, and its structure and relative and absolute configurations were identical with those reported (Lin et al., 1992). Purification was carried out by silica gel column chromatography, and pale-yellow crystals were obtained from an EtOAc/hexane mixed solvent (9:1) under a hexane-saturated atmosphere by slow evaporation at ambient temperature, m.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2   ride on their parent atoms with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). Water H atoms were located in a difference-Fourier map, and then refined freely with U iso (H) = 1.5U eq (O), and with distance restraints of O-H = 0.84 (2) Å and HÁ Á ÁH = 1.33 (4) Å .
Acta Cryst. (2018). E74, 555-558 research communications  program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010) and PLATON (Spek, 2009).  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. 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 > 2σ(F 2 ) is used only for calculating R-factors(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 N1  (2)