Reactions of 2-Methyl- and 1,2-Dimethyl-1,4,5,6-tetrahydropyrimidine with Aroyl Chlorides: Diverse Reactivities of Cyclic Ketene N,N′-Acetals Generated in situ

Abstract

2-Methyl-1,4,5,6-tetrahydropyrimidine reacted with two equivalents of aroyl chloride in tetrahydrofuran-triethylamine to give N,N¢-diaroyl cyclic ketene N,N-acetals that are inert to excess aroyl chlorides. No carbon-carbon bond was formed in these reactions. Conversely, 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine reacted with three equivalents of aroyl chloride under the same conditions to give N-aroyl N¢-methyl b,b-dioxo cyclic ketene N,N¢-acetals, with formation of two carbon-carbon bonds. Various reactions of cyclic amidines with aroyl chlorides were observed and are discussed in terms of the effects of the ring size and substituents on the reactivities of the cyclic ketene N,N¢-acetals generated in situ.