A Novel Method for Preparing Chromones via 1,2-Migration of α-Diazo-Flavanones

Abstract

A novel method for preparing chromones from α-diazo-flavanones has been developed. Good yields and regioselectivity are obtained through HCl catalysis and Rh₂(Oct)₄ catalysis. The chromone obtained can undergo a one-pot Diels–Alder reaction to synthesize flavanone propellane in high yield. Mechanisms about 1,2-migration under both catalysts are proposed.

Key words

chromones - α-diazo-flavanones - propellanes - 1,2-migration - Diels–Alder reaction

• References and Notes

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• 10 General Procedure for the Preparation of Chromones 2 CH₂Cl₂ (2 mL), 1 (0.2 mol, 1 equiv), and HCl (60 μL) (or Rh₂(Oct)₄, 0.2 mg) were added to a round-bottomed flask with a magnetic stirrer. The reaction was monitored by TLC and quenched by water after its completion. Then solvent was transferred to a dispensing funnel. Saturated brine and EtOAc were added to the funnel to extract the crude product. The organic phase was dried by Na₂SO₄ for 20 min and concentrated by the rotary evaporator. Further purification was conducted by a silica gel column using petroleum ether/EtOAc (5:1) as the eluent to give the corresponding product 2. 2-Methyl-4H-chromen-4-one (2a₁) Pale yellow solid; mp 72–73 ℃. ¹H NMR (400 MHz, CDCl₃): δ = 8.17 (dd, J = 7.9, 1.7 Hz, 1 H), 7.63 (ddd, J = 8.7, 7.1, 1.7 Hz, 1 H), 7.43–7.34 (m, 2 H), 6.17 (s, 1 H), 2.38 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 176.18, 164.12, 154.43, 131.38, 123.59, 122.86, 121.52, 115.73, 108.53, 18.55. General Procedure for the Preparation of 3 or 4 CH₂Cl₂ (2 mL), 2 (0.2 mol, 1 equiv), and olefin (1 mmol, 5 equiv) were added to a quartz flask with a magnetic stirrer. Then the flask was irradiated by a lamp with a wavelength of 365 nm. The reaction was monitored by TLC and quenched by water after its completion. Then solvent was transferred to a dispensing funnel. Saturated brine and EtOAc were added to the funnel to extract the crude products. The organic phase was dried by Na₂SO₄ for 20 min and concentrated by the rotary evaporator. Further purification was conducted by a silica gel column using petroleum ether/EtOAc (30:1) as the eluent to give the corresponding product 3 or 4. (4aR,4bR,10aS,10bS)-1,2,3,4,4a,10b-hexahydro-10H-4b,10a-Butanobenzo[3,4]cyclobuta[1,2-b]chromen-10-one (3s) Pale yellow solid; mp 34–38℃. ¹H NMR (400 MHz, CDCl₃): δ = 7.89 (dd, J = 7.7, 1.8 Hz, 1 H), 7.44 (ddd, J = 8.0, 7.7, 1.8 Hz, 1 H), 6.99–6.93 (m, 2 H), 2.79–2.71 (m, 1 H), 2.36–2.20 (m, 2 H), 1.97–1.90 (m, 1 H), 1.78–1.55 (m, 8 H), 1.50–1.32 (m, 5 H), 0.99–0.88 (m, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 193.70, 160.56, 135.64, 127.60, 120.73, 119.04, 118.69, 88.54, 52.91, 47.09, 46.73, 30.85, 26.60, 26.10, 25.97, 25.13, 24.94, 23.02, 22.45. HRMS (ESI): m/z calcd for C₁₉H₂₂O₂ + H [M + H]⁺: 283.1620; found: 283.1616. Procedure for the One-Pot Preparation of 3s CH₂Cl₂ (2 mL), 1s (0.2 mol, 1 equiv) and HCl (60 μL) were added to a quartz flask with a magnetic stirrer. After stirring for 30 min, cyclohexene (0.1 mL, 5 equiv) was added. Then the mixture was irradiated by a lamp with a wavelength of 365 nm. After 6 h, the mixture was transferred to a dispensing funnel. Saturated brine and EtOAc were added to the funnel to extract the crude products. The organic phase was dried by Na₂SO₄ for 20 min and concentrated by the rotary evaporator. Further purification was conducted by a silica gel column using petroleum ether/EtOAc (30:1) as the eluent to give the corresponding product 3s. Analytical data are provided in the Supporting Information.