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44.2.2.5.1 Method 1: Nitrogen Elimination by Oxidation of Pyrazol-3-ones Using Lead(IV) Acetate

DOI: 10.1055/sos-SD-044-00163

Gandon, V.; Malacria, M.Science of Synthesis, (200844221.

The oxidation of pyrazol-3-ones with lead(IV) acetate in methanol provides alkynyl esters or methyl alka-2,3-dienoates in moderate to high yields (Scheme 70).[‌159‌] With 4-unsubstituted pyrazol-3-ones 122, alkynes 123 are formed in 3550% yields via elimination of nitrogen. Addition of boron trifluoridediethyl ether complex to the reaction mixture does not improve the yields. On the other hand, 4,5-disubstituted 2,4-dihydro-3H-pyrazol-3-ones 124 give allenes 125 in 6076% yields exclusively with addition of boron trifluoridediethyl ether complex. The role of this Lewis acid is to facilitate the deprotonationisomerization of an intermediate dihydropyrazolium salt.

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