Abstract
2-Dimethoxymethyl-3-methoxypropionitrile (I), an important compound for thiamine production, was obtained in good yield by the hydrogen chloride-catalyzed acetalation of 2-formyl-3-methoxypropionitrile (II) in methanol. The reaction was proved not to proceed through direct acetalation of the formyl group but via the pathway of II→allyl hydroxy-(V) and allyl ether-(VI) cations→2-dimethoxymethylacrylonitrile (III)⇌2-methoxymethylene-3-methoxypropionitrile (IV)⇌I. The reaction gave 2-oxopiperidines (VII, VIII) and pyrans (IX, X) as minor products which should be formed by the attack of the allyl cations to II or IV. Kinetic studies on the reaction pathway of III⇌IV⇌I revealed that the cis isomer of IV was more reactive than the trans counterpart. The same acetalation of methyl 2-formyl-3-methoxypropionate (XVI) was similarly investigated.
