反应详情

20 g of commercially available 2-ethyl-1,3-cyclopentanedione 2 partially dissolved in tetrahydrofuran (THF) was treated with etheral diazomethane generated from 50 g of N-methyl-N-nitroso-p-toluenesulfonamide, sold under the trademark Diazald (Aldrich Chemical Company). After standing several hours the solvent was removed under reduced pressure to give ca. 21 g of crude enol ether 3. Complete conversion to the enol ether 3 is obtained with a particularly clean reaction. But since the use of diazomethane is extremely hazardous, particularly when used in large scale reactions (greater than 20 grams), methanol with p-toluenesulfonic acid as the catalyst is preferred. The conversion to enol ether 3 is less complete, but the unreacted dione 2 can be recovered from the reaction mixture and recycled. The crude products of four reactions (84.1 g from 78.8 g of dione 2) were combined, dissolved in chloroform (CHCl3) and extracted with 5% NaHCO3. Removal of the CHCl3 under reduced pressure gave 69.5 g of the enol ether 3 (79% from dione 2) which was free of unreacted dione 2 as indicated by an nmr analysis. Distillation under reduced pressure (bp 82°-84° at 0.03 nm) gave a light colored oil: uv max (95% ethanol) 253 nm; ir (neat) 1685 cm-1 (cyclopentenone C=O) and 1620 cm-1 (enolic double bond), 1360 and 1270 cm-1 (enol ether); nmr (CDCl3) δ4.0 (s, 3, --OCH3), 2.17 (q,2, --CH2CH3 and 1.0 (t, 3, --CH2CH3).

1. waitAfter standing several hours
2. customthe solvent was removed under reduced pressure