Alkylation of pyrazolones via the Mitsunobu reaction

Results and discussion

Several reactions of pyrazolones 1 and 4 in various solvents (tetrahydrofuran, dichloromethane, chloroform, acetonitrile, benzene) using methanol as the alkylating agent were performed. Additionally, educt 4a was also treated with benzyl alcohol, isopropyl alcohol and 2,3-epoxypropanol. The results of our reaction runs are given in Table 1. In many cases mixtures of both isomers were obtained with the O-alkylated species predominating. However, using benzene or THF as the solvent often led to selective O-alkylation (for instance 4a and methanol, isopropyl alcohol, benzyl alcohol and 2,3- epoxypropanol, respectively). In conclusion, this mild and versatile approach seems to be a useful access to N- and O-alkylation products of pyrazolones, particularly to special 5-alkoxypyrazoles being not (or not easily) amenable from reaction of pyrazolones with conventional alkylating agents.

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