Synthesis of Polycyclic Frameworks through Iron-Catalyzed Intramolecular [5+2] Cycloaddition

Abstract

A concise and efficient approach to the core of the C₁₈/C₁₉ diterpenoid alkaloids and phomopsterone B is reported. Both syntheses share the same iron-catalyzed intramolecular [5+2] cycloaddition to assemble the tricyclo[6.3.1.0^1,6]]dodecane skeleton. The following ­approach to the 6/5/6/7 tetracyclic core scaffold of C₁₈/C₁₉ diterpenoid alkaloids features a regioselective Grignard addition/thermal Claisen rearrangement/RCM cyclization. Meanwhile the synthetic steps to access the spiro 6/5/6 tricyclic subunits of phomopsterone B were characterized as intramolecular aldol reaction, Wacker oxidation, and Criegee ­reaction.

Key words

C₁₈/C₁₉-diterpenoid alkaloids - phomopsterone B - Iron-catalyzed - [5+2] cycloaddition

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• 20 Reduction of compound 10 under LiAlH₄ gave two diastereo­isomers, 20-1 and 20-2 in 49% yield and 24% yield, respectively. Compound 20-1 was further transformed into compound 20, the structure of which was confirmed by X-ray crystallography. The stereochemistry of hydroxyl group at C1 in target compound 11 could be identified from X-ray crystallographic analysis of compound 20 (Scheme 6).
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• 22 Experimental Procedure and Characteristic Data for Ketone (24): Compound 21 (22.3 mg, 0.08 mmol) was added to toluene (0.5 mL) in a sealed tube and stirred at 170 °C overnight. The solvent was then removed in vacuo to afford the crude product, which was used in the next step without further purification. To a solution of Grubbs II (17.5 mg, 0.02 mmol) in CH₂Cl₂ (8.0 mL) at room temperature was added the above product in CH₂Cl₂ (2.5 mL) and the mixture was stirred at room temperature for 10 h. The solvent was then removed in vacuo and the crude product was purified by column chromatography (EtOAc/petroleum ether = 1:5) to afford the product 24 (12.1 mg, 60% for two steps) as a white solid; mp 119–121 °C. ¹H NMR (400 MHz, CDCl₃): δ = 5.65–5.59 (m, 2 H), 3.87 (s, 1 H), 2.88 (t, J = 5.2 Hz, 1 H), 2.73–2.60 (m, 2 H), 2.46 (dd, J = 5.2, 7.2 Hz, 1 H), 2.04–1.98 (m, 2 H), 1.90–1.81 (m, 2 H), 1.70 (dd, J = 5.2, 13.2 Hz, 1 H), 1.66–1.59 (m, 1 H), 1.52–1.47 (m, 1 H), 1.42 (ddd, J = 2.8, 7.6, 14.0 Hz, 1 H), 1.15 (dddd, J = 2.8, 4.0, 12.4, 25.2 Hz, 1 H), 1.03 (ddt, J = 2.8, 12.8, 25.6 Hz, 1 H), 0.72 (ddd, J = 3.2, 13.2, 25.6 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 211.8, 211.0, 132.7, 127.9, 78.9, 60.5, 50.6, 48.5, 35.2, 34.8, 31.6, 30.6, 24.0, 22.7, 20.6. IR (neat): 2925, 2850, 1735, 1448, 1369, 1239, 1141, 1030 cm^–1. HRMS (ESI): m/z [M+Na]⁺ calcd for C₁₅H₁₈NaO₃: 269.1154; found: 269.1151.