The van AIphen-HUttel Rearrangement

Baumes, J. Elguero, R. Jacquier, and G. Tarrago, Tetrahedron Lett., 3781 (1973). 

Subsequently, the rearrangement product of 118, formulated earlier as 117, was also shown by two independent groups to be 128, proving 4//-pyrazoIes to be major products from the van Alphen-Hiittel rearrangement. 

It has been suggested that the thermal rearrangement proceeds via a radical pair, whereas the acid-catalyzed process is via a radical-radical cation 

Rearrangement was believed to proceed via 137 and 138, the ratio 135 136 depending upon K, and also on kN and Arc.M The same authors in a related study concluded that there was substantial charge transfer in the transition state of acid catalyzed rearrangements, as compared with those in neutral media.110 

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This is known as the van AIphen-Huttel rearrangement its mechanism, scope, and limitations were discussed in the preceding chapter.1 In cases where the 4//-pyrazole has been isolated, the migrating group R2 has most frequently been phenyl.40-46 In one example it has been methyl,47 and in an... 

Zinc in acetic acid reduced 31 to 98.16 The same reagent converted 99 to a mixture of 100 and 101 (Scheme 24), thus establishing 99 as a product from the van AIphen-Huttel rearrangement.41... 

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