264.2 g (2.0 mol) of dimethyl malonate, 637 g (6 mol) of trimethyl orthoformate and 20.4 g (0.2 mol) of acetic anhydride containing 2 g of H2 SO4 -activated bentonite were initially introduced into a reaction flask provided with a stirrer, thermometer, dropping funnel, column and reflux condensor, and the mixture was heated to boiling. 40.8 g (0.4 mol) of acetic anhydride was continuously metered for6 hours. The low boiling component formed was distilled off at a head temperature of 62° to 67° C. The bottom temperature increased slowly from 109° to 133° C. After 8 hours; the excess orthoester was distilled off at reduced pressure. 40.8 g (0.4 mol) of acetic anhydride was added to the remaining reaction mixture, and the mixture was heated at 140° C. for 2 hours. The catalyst was filtered off, and the crude product was distilled in vacuo. 330.4 g of dimethyl methoxymethylenemalonate (98% pure), having a melting point of 44° to 46° C., were obtained at 0.5 mbar and a head temperature of 110° C.
