[CH3:1][CH2:2][O:3][C:4]([CH:6](P(OCC)(OCC)=O)[CH3:7])=[O:5].CC(C)([O-])C.[K+].[O:22]1[CH2:25][C:24](=O)[CH2:23]1>C1COCC1>[O:22]1[CH2:25][C:24](=[C:6]([CH3:7])[C:4]([O:3][CH2:2][CH3:1])=[O:5])[CH2:23]1To a solution of triethyl 2-phosphonopropionate (800 mg, 3.4 mmol) in anhydrous THF (3 mL) was added potassium tert-butoxide (340 mg, 3.1 mmol). The reaction was stirred for 1 h at RT then oxetan-3-one (CAS-RN 6704-31-0, available from commercial suppliers, 220 mg, 3.1 mmol) was added in a single portion. The reaction was heated at reflux for 20 h then cooled to RT and adsorbed onto silica gel. Purification by automated column chromatography (Biotage SP4) on silica gel (12 g Grace cartridges) eluting with EtOAc: n-heptane (gradient elution, 0% (equil., 5 CV), 0% (5 CV) 0-100% (25CV), 100% (10 CV)) afforded (i) (151 mg, 31%).
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