A Novel Bromine-Induced Ring Expansion of the Spiroimine Moiety of the Shellfish Toxin Gymnodimine

Abstract

Treatment of the shellfish toxin gymnodimine with one equivalent of molecular bromine at -78 °C resulted in stereoselective formation of a novel ring-expansion product. Participation of the spiromine unit in this novel bromine-induced ring expansion reaction provides insight into how the reactivity of spiroimine moiety may contribute to the toxicity of this marine biotoxin when activated by the presence of an appropriate environmental electrophile.

Key words

gymnodimine - shellfish toxin - ring expansion

References and Notes

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Isolation and Purification of Gymnodimine ( 1).
Gymnodimine was isolated from a laboratory culture of Karenia selliformis (syn Gymnodinium selliforme CAW D79) at the Cawthron Institute (Nelson, New Zealand). The extraction and purification procedure was based on the reported method, see ref. 14. Prior to passage through an HP-20 resin the culture was acidified with 50% HCl to pH 4.5 and filtered (Toyo GC 90, 124 mm). The toxin was eluted off the HP-20 resin with MeOH, concentrated to low volume and clean-up achieved by solvent partitioning. The purity of the isolated toxin was checked by 600 MHz ¹H NMR spectroscopy and LCMS. Two impurities, unknown isomers of gymnodimine, were detected at ca. 4%. The crude extract was further purified by flash column chromatography using CH₂Cl₂-MeOH-Et₃N (10:0.2:0.2) as eluent to obtain a white solid.

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Synthesis of Ring-Expansion Product 2. To a solution of gymnodimine (1, 10 mg, 19.7 µmol) in CH₂Cl₂ (2 mL) was added dropwise a solution of bromine in CH₂Cl₂ (0.5 M, 44 µL, 22 µmol) at -78 °C and stirring was continued for 5 min. The colourless mixture was warmed to r.t. and directly subjected to flash chromatography (CH₂Cl₂-MeOH, 85:15). The ring-expansion product 2 (9.7 mg, 81%) was obtained as a colourless wax. R _f = 0.44 (CH₂Cl₂-MeOH, 85:15); [α]_D ²⁰ 11.3 (CHCl₃, c 1.24 g/100 mL). ¹H NMR (600 MHz, CDCl₃): δ = 1.04 (m, 1 H, 17-H_a), 1.21 (d, J = 7.3 Hz, 3 H, 14-H₃), 1.72 (s, 1 H, 21-H₃), 1.75 (s, 3 H, 34-H₃), 1.76-2.20 (m, 12 H, 4-H₂, 5-H₂, 15-H₂, 17-H_b, 18-H_2, 26-H₂, 27-H_a), 1.93 (s, 3 H, 32-H₃), 2.23 (s, 3 H, 11-H₃), 2.51-2.60 (m, 2 H, 13-H, 27-H_b), 2.71 (m, 2 H, 8-H₂), 2.87 (m, 1 H, 7-H_a), 3.32 (d, J = 10.4 Hz, 23-H), 3.46 (m, 1 H, 7-H_b), 3.96-4.03 (m, 2 H, 6-H_a, 16-H), 4.19 (m, 2 H, 6-H_b, 19-H), 4.40 (d, J = 6.2 Hz, 1 H, 12-H), 4.93 (dd, J = 8.9, 8.9 Hz, 1 H, 9-H), 5.48 (d, J = 10.6 Hz, 1 H, 22-H), 5.81 (s, 1 H, 28-H), 6.94 (s, 1 H, 33-H). ¹³C NMR (150 MHz, CDCl₃): δ = 10.6 (q, C-32), 11.5 (q, C-21), 17.2 (q, C-34), 18.5 (t, C-26), 20.4 (t, C-4), 21.1 (q, C-14), 22.1 (t, C-18), 25.5 (q, C-11), 26.9 (t, C-8), 30.4 (t, C-27), 31.8 (t, C-5), 32.8 (t, C-7), 34.3 (t, C-17), 35.9 (d, C-13), 40.2 (t, C-15), 45.9 (s, C-3), 48.8 (d, C-23), 51.1 (t, C-6), 66.2 (s, C-10), 77.8 (d, C-16), 79.1 (d, C-19), 79.8 (2 × d, C-9, C-28), 95.6 (d, C-12), 123.8 (d, C-22), 126.4 (s, C-24), 130.7 (s, C-31), 131.7 (s, C-25), 144.9 (s, C-20), 146.7 (d, C-33), 174.3 (s, C-30), 198.5 (s, C-2). IR (film): ν = 3019, 2400, 1755, 1220, 770 cm^-1. HRMS: m/z calcd for C₃₂H₄₅ ⁷⁹BrNO₄ [MH⁺ - H₂O]: 586.2532; found: 586.2527.