Steviamine, a new class of indolizidine alkaloid [(1R,2S,3R,5R,8aR)-3-hydroxy­meth­yl-5-methyl­octa­hydro­indolizine-1,2-diol hydro­bromide]

Amber L Thompson; Agnieszka Michalik; Robert J Nash; Francis X Wilson; Renate van Well; Peter Johnson; George W J Fleet; Chu-Yi Yu; Xiang-Guo Hu; Richard I Cooper; David J Watkin

doi:10.1107/S1600536809043827

. 2009 Oct 28;65(Pt 11):o2904–o2905. doi: 10.1107/S1600536809043827

Steviamine, a new class of indolizidine alkaloid [(1R,2S,3R,5R,8aR)-3-hydroxymethyl-5-methyloctahydroindolizine-1,2-diol hydrobromide]

Amber L Thompson ^a,^*, Agnieszka Michalik ^b, Robert J Nash ^b, Francis X Wilson ^c, Renate van Well ^c, Peter Johnson ^c, George W J Fleet ^d, Chu-Yi Yu ^e, Xiang-Guo Hu ^e, Richard I Cooper ^a, David J Watkin ^a

PMCID: PMC2971327 PMID: 21578486

Abstract

X-ray crystallographic analysis of the title hydrobromide salt, C₁₀H₂₀N⁺·Br⁻, of (1R,2S,3R,5R,8aR)-3-hydroxymethyl-5-methyloctahydroindolizine-1,2-diol defines the absolute and relative stereochemistry at the five chiral centres in steviamine, a new class of polyhydroxylated indolizidine alkaloid isolated from Stevia rebaudiana (Asteraceae) leaves. In the crystal structure, molecules are linked by intermolecular O—H⋯Br and N—H⋯Br hydrogen bonds, forming double chains around the twofold screw axes along the b-axis direction. Intramolecular O—H⋯O interactions occur.

Related literature

For background to the biological activity of indolizidines, see: Asano et al. (2000a ▶,2000b ▶); Colegate et al. (1979 ▶); Davis et al. (1996 ▶); Donohoe et al. (2008 ▶); Durantel (2009 ▶); Hakansson et al. (2008 ▶); Hohenschutz et al. (1981 ▶); Kato et al. (1999 ▶, 2007 ▶); Klein et al. (1999 ▶); Lagana et al. (2006 ▶); Sengoku et al. (2009 ▶); Watson et al. (2001 ▶); Whitby et al. (2005 ▶); Yamashita et al. (2002 ▶). For the Hooft parameter, see: Hooft et al. (2008 ▶). For the extinction correction, see: Larson (1970 ▶).

Experimental

Crystal data

C₁₀H₂₀N⁺·Br⁻
M _r = 282.18
Orthorhombic,
a = 8.4616 (1) Å
b = 8.8762 (1) Å
c = 15.8270 (2) Å
V = 1188.72 (2) Å³
Z = 4
Mo Kα radiation
μ = 3.45 mm⁻¹
T = 150 K
0.46 × 0.46 × 0.26 mm

Data collection

Nonius KappaCCD area-detector diffractometer
Absorption correction: multi-scan (DENZO/SCALEPACK; Otwinowski & Minor, 1997 ▶) T _min = 0.20, T _max = 0.41
2675 measured reflections
2658 independent reflections
2484 reflections with I > 2σ(I)
R _int = 0.042

Refinement

R[F ² > 2σ(F ²)] = 0.025
wR(F ²) = 0.052
S = 1.00
2658 reflections
138 parameters
H-atom parameters constrained
Δρ_max = 0.37 e Å⁻³
Δρ_min = −0.52 e Å⁻³
Absolute structure: Flack (1983 ▶), 1102 Friedel pairs
Flack parameter: 0.002 (10)

Data collection: COLLECT (Nonius, 2001 ▶).; cell refinement: DENZO/SCALEPACK (Otwinowski & Minor, 1997 ▶); data reduction: DENZO/SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1994 ▶); program(s) used to refine structure: CRYSTALS (Betteridge et al., 2003 ▶); molecular graphics: CAMERON (Watkin et al., 1996 ▶); software used to prepare material for publication: CRYSTALS.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536809043827/lh2918sup1.cif

e-65-o2904-sup1.cif^{(16.5KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536809043827/lh2918Isup2.hkl

e-65-o2904-Isup2.hkl^{(133KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5—H51⋯O2	0.84	2.34	2.684 (3)	105
O5—H51⋯O15	0.84	2.53	3.018 (3)	118
N7—H71⋯Br1	0.98	2.29	3.268 (2)	172
O2—H21⋯Br1ⁱ	0.82	2.55	3.364 (2)	177
O15—H151⋯Br1ⁱⁱ	0.84	2.39	3.211 (2)	169

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Symmetry codes: (i) Inline graphic ; (ii) .

supplementary crystallographic information

Comment

Well over 100 iminosugars - analogues of sugars in which the ring oxygen is replaced by nitrogen - have been isolated as natural products (Asano et al., 2000a; Watson et al., 2001). This paper establishes both the relative and absolute stereochemistry of the five chiral centres in steviamine (1), recently isolated from the leaves of Stevia rebaudiana (Asteraceae); (1) is the first example of a new class of indolizidine alkaloid with an alkyl group attached to the piperidine ring. Swainsonine (2, see Fig. 1), a trihydroxyindolizidine isolated from Swainsona canescens (Colegate et al., 1979), is a powerful inhibitor of α-mannosidases and has potential as a chemotherapeutic agent for the treatment of cancer (Lagana et al., 2006; Klein et al., 1999). l-Swainsonine 3, the enantiomer of 2, is a very powerful α-rhamnosidase inhibitor (Davis et al., 1996); 4 in which a methyl group is introduced into the piperidine ring is nearly 100 times more potent an inhibitor than 2 (Hakansson et al., 2008). Castanospermine 5, isolated from Castanospermum australe (Hohenschutz et al., 1981), is an inhibitor of some α-glucosidases and a potent inhibitor of dengue virus infection in vivo (Whitby et al., 2005); Celgosivir, a simple derivative of 5, is in development for the treatment of HCV infection (Durantel, 2009). Hyacinthacine A4 6, isolated from Scilla sibirica (Asano et al., 2000b; Yamashita et al., 2002), is the pyrrolizidine equivalent of steviamine 1. Many hyacinthacines have been isolated from a range of plants (Kato et al., 1999; Kato et al., 2007) and have attracted considerable attention from synthetic organic chemists (Sengoku et al., 2009; Donohoe et al., 2008). Steviamine 1 is unlikely to be the only naturally occurring indolizidine with a methyl branch which will provide similarly challenging synthetic targets.

As a natural product, the crystal was expected to be enantiopure and the Flack x parameter refined to 0.002 (10) (Flack, 1983). Analysis of the Bijvoet differences using within CRYSTALS (Betteridge et al., 2003) gives the Hooft y parameter as 0.023 (6), indicating that the probability that the configuration is incorrect allowing for the posibility of racemic twinning is less than 0.000001% (Hooft et al., 2008).

On examination of hydrogen bonding interactions in 1, the position of H51 initially seemed incorrect, lying between atoms O2 and O15. However, examination of the difference map indicates the presence of a peak believed to be a hydrogen atom which moves little on refinement suggesting the hydrogen bond is bifurcated (Fig. 2, Table 1). The molecules are linked together by three hydrogen bonds (two O—H···Br and one N—H···Br, Table 1) to form double chains around the twofold screw axes along the b direction (Fig. 3).

Experimental

Steviamine was isolated by a combination of strongly acidic cation, and strongly basic anion, exchange chromatography. The compound was retained on cation exchange resin (IR120) and was chromatographed on the anion exchange resin (CG400) from which it was eluted with water. Isolation was monitored using GC-MS of the trimethylsilyl-derivative (distinctive major ion at 314 amu). Steviamine was crystallized as its hydrobromide salt from ethanol.