Crystal structure of sepaconitine, a C19-diterpenoid alkaloid from the roots of Aconitum sinomontanum Nakai

The title compound [systematic name: [(1α,14α,16β)-20-ethyl-8,9,10-trihydroxy-1,14,16-trimethoxyaconitan-4-yl 2-aminobenzoate], C30H42N2O8, a natural C19-diterpenoid alkaloid, possesses an aconitane carbon skeleton with four six-membered rings and two five-membered rings. The fused ring system contains two chair, one boat, one twist-boat and two envelope conformations. Intramolecular N—H⋯O hydrogen bonds are observed between the amino and carbonyl groups. The molecules are linked together via O—H⋯O hydrogen bonds, forming a three-dimensional framework.

The molecular structure is shown in Fig. 1. The molecule has a rigid structure consisting of six main rings (A-F), which is identical with that of lappaconitine and mesaconitine (Wang et al., 2007;He et al., 2008 Fig. 2).

S2. Experimental
The title compound was isolated from the roots of Aconitum sinomontanum, using a method described previously (Wei et al., 1996). Colourless crystals were grown from methanol at room temperature by slow evaporation.

S3. Refinement
The hydrogen atoms were placed in calculated positions and refined as riding with U iso (H) = 1.2 Ueq (C) or 1.5Ueq(C, O). The positions of methyl and hydroxy hydrogens were rotationally optimized. The absolute configuration of the title compound, sepaconitine, has been assigned to be the same as that reported for typical natural C 19 -diterpenoid alkaloids (Wang et al., 2007;He et al., 2008).  ORTEPII drawing of sepaconitine (I) with the atomic numbering scheme. Displacement ellipsoids are plotted at the 50% probability level.

Figure 2
The packing of molecules in the crystal structure of sepaconitine (I), viewed along the c direction (Hydrogen bonds are shown as dashed lines).
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.46 e Å −3 Δρ min = −0.19 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.