By treating 4-aryl-2,4-dioxobutanoic acids or their alkyl esters with diazomethane alkyl 4-aryl-2-methoxy-4-oxo-2-butenoates and 1-R-3-aroyl-4-methoxy-4-methoxycarbonyl-4,5-dihydropyrazoles were prepared. O-Alkyl derivatives and substituted pyrazoles were also obtained by reaction of 4-aryl-2,4-oxobutanoic acids aryl amides with diazomethane and diazoethane.