Two-Carbon Ring-Enlargement of Cyclic 1,3-Diketones to Cyclic 1,5-Diketones

Abstract

3-Hydroxy-3-vinylcycloalkanones, which were prepared from the corresponding cyclic 1,3-diketones by mono-vinylation, were rearranged to two-carbon ring enlarged cyclic 1,5-diketones by treatment with a catalytic amount of potassium tert-butoxide.

Key words

ring enlargement - cyclic 1,3-diketone - cyclic 1,5-diketone - mono-vinylation - potassium tert-butoxide

• References and Notes

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• 14 At this stage, the reasons for improving yields of the desired products under dilute conditions are not clear. Products formed by intermolecular reactions were not isolated in the case of undilute conditions due to the appearance of many products. In ring-enlargement of 2-monoalkyl- or 2-nonsubstituted 3-vinyl-3-hydroxycycloalkanones, basic treatment under thermodynamic conditions might form regioisomeric enolates from the intermediate 2′, and they could induce production of complex mixtures. The dilute conditions might suppress isomerization of 2′.
• 15 Potassium tert-Butoxide-Catalyzed Ring Enlargement of 3-Hydroxy-3-vinylcycloalkanones; Typical Procedure (Table [1], entry 3): To a stirred solution of 3-hydroxy-3-vinylcyclohexanone 2a (42.8 mg, 0.25 mmol) in THF (2.5 mL) was added potassium tert-butoxide (5.6 mg, 0.05 mmol) at room temperature, and the mixture was stirred at room temperature for 20 min. The reaction was quenched by adding a saturated aqueous solution of ammonium chloride. The resulting mixture was extracted with ethyl acetate (three times). The combined organic extracts were washed with brine, dried over anhydrous sodium sulfate, filtered, and concentrated. The crude product was purified by column chromatography on silica gel (hexane/ethyl acetate) to afford 3a (41.0 mg, 0.24 mmol, 96%) as colorless crystals. Compound 3a: mp 79.0–80.5 °C (recryst. from CH₂Cl₂), ¹H NMR (CDCl₃, 600 MHz): δ = 2.57–2.55 (m, 2 H), 2.51–2.49 (m, 2 H), 2.47–2.45 (m, 2 H), 2.21–2.19 (m, 2 H), 2.02–1.98 (m, 2 H), 1.12 (s, 6 H), ¹³C NMR (CDCl₃, 150 MHz): δ = 216.0, 212.5, 47.9, 42.0, 39.7, 36.1, 32.7, 26.0, 23.5, IR (CHCl₃): 1701 cm^–1, HRMS (DART): m/z [M + H]⁺ calcd for C₁₀H₁₇O₂: 169.12285; found: 169.12293.