Syntheses of 3-methylpyrrole via methyl 4-methylpyrrole-2-carboxylate. A thermal oxazolone–pyrone rearrangement

Abstract

3-Ethoxy-2-methylpropenal with hippuric acid and acetic anhydride gave 4-(3-ethoxy-2-methylallyl-idene)-2-phenyloxazol-5(4H)-one. By successive treatment with methanolic potassium hydroxide, acetic–hydrochloric acid, and methanolic sodium methoxide, methyl 4-methylpyrrole-2-carboxylate was formed in high overall yield, and was converted into 3-methylpyrrole by hydrolysis and decarboxylation. The oxazolone with sodium hydroxide in acetone or dioxane gave 4-(3-hydroxy-2-methylallylidene)-2-phenyloxazol-5(4H)-one, isomerized in boiling acetone, or on melting, to 3-benzoylamino-5-methylpyran-2-one. 3-Ethoxy-2-methylpropenal condensed with glycine methyl ester to give an enaminal, cyclized in moderate yield to methyl 4-methylpyrrole-2-carboxylate.