Ketenes, diacyl 5,5-acetals

Acylation. Reaction conditions employed to acylate an aminophenol (using acetic anhydride in alkaU or pyridine, acetyl chloride and pyridine in toluene, or ketene in ethanol) usually lead to involvement of the amino function. If an excess of reagent is used, however, especially with 2-aminophenol, 0,A/-diacylated products are formed. Aminophenol carboxylates (0-acylated aminophenols) normally are prepared by the reduction of the corresponding nitrophenyl carboxylates, which is of particular importance with the 4-aminophenol derivatives. A migration of the acyl group from the O to the N position is known to occur for some 2- and 4-aminophenol acylated products. Whereas ethyl 4-aminophenyl carbonate is relatively stable in dilute acid, the 2-derivative has been shown to rearrange slowly to give ethyl 2-hydroxyphenyl carbamate [35580-89-3] (26). 

Acylation of malonic esters. Monoacylation of sodiomalonic esters is usually accompanied by diacylation. This disadvantage can be eliminated by reaction of the sodiomalonate with trimethylchlorosilane to give a ketene acetal (1). This reacts readily with an acid chloride or anhydride to give an acylmalonic ester (2).11 Yields are in the range 50-80%. 

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