Unmodified Nano-Powder Magnetite or Iron(III) Oxide Catalyze the Easy and Fast Synthesis of 4-Substituted-4H-Pyrans

Abstract

Inexpensive and commercially available nano-power magnetite or iron(III) oxide can be used as catalyst in a new, straightforward, and fast protocol for the construction of 4-substituted-4H-pyrans. The reaction implies a tandem process, involving an aldol condensation, a Michael-type addition, and a dehydrating annulation.

Key words

annulation - heterocycles - heterogeneous catalysis - iron - pyrans

References and Notes

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12

Representative Procedure for the Synthesis of Dimethyl 4-(4-Bromophenyl)-2,6-dimethyl-4 H -pyran-3,5-dicar-boxylate (4a) To a stirred solution of 4-bromobenzaldehyde (2a, 1 mmol) in toluene (3 mL) were added Fe₃O₄ (50 mg, 65 mol%), methyl 3-oxobutanoate (1a, 2.5 mmol), and AcCl (0.14 mL, 2 mmol). The resulting mixture was stirred at 25 ˚C during 3 h. The mixture was quenched with H₂O (5 mL) and extracted with EtOAc (3 × 5 mL). The organic phases were dried over MgSO₄, followed by evaporation under reduced pressure
to remove the solvent. The product was purified by chroma-tography on silica gel (hexane-EtOAc) to give the titled product; mp 96-100 ˚C(hexane). R _f = 0.5 (hexane-EtOAc = 4:1). IR (KBr) : ν = 1721, 1671, 1629, 1584, 1296 cm^-¹. ^¹H NMR (300 MHz, CDCl₃): δ = 2.36 (6 H, d, J = 0.6 Hz, 2  CCH₃), 3.64 (6 H, s, 2  CO₂CH₃), 4.72 (1 H, s, CH), 7.10 (2 H, d, J = 8.5 Hz, 2  BrCCHCH), 7.36 (2 H, d, J = 8.5 Hz, 2  BrCCH). ^¹³C NMR (75 MHz, CDCl₃): δ = 18.7 (2 C), 37.8, 51.4 (2 C), 107.7 (2 C), 120.4, 129.9 (2 C), 131.2 (2 C), 144.3, 158.7 (2 C), 166.9 (2 C). CG-MS: m/z (%) = 382 (11) [M⁺ + 2], 380 (11) [M⁺], 367 (14), 365 (13), 323 (14), 321 (14), 226 (11), 225 (100), 193 (11). HRMS:
m/z calcd for C₁₇H₁₇BrO₅: 382.0259; found: 380.0256.