Van Alphen rearrangement

Spiro intermediates (48) have been invoked by Dobeneck and Uhl93 to explain the formation of the pyrazolotetrahydropyrimidines 49 from the diazopyrrolinones 47 and DEAD. A van Alphen rearrangement,94 which may be a [1,5] shift, is involved. 

Pyrazole from pyrazolenine ring van Alphen rearrangement... 

Migrations of C- linked substituents around the ring, on to carbon or nitrogen atoms, are common amongst these compounds. This is the van Alphen-Huttel rearrangement and by it 3H-pyrazoles are converted into 1//-pyrazoles, and 2H-imidazoles are thermally iso-merized into IH-imidazoles. 

Rearrangements of the van Alphen-Hiittel type, involving migrations of carbon-linked substituents around the ring, and characteristic of the 2H-and 3//-pyrroles, the 3H- and 4//-pyrazoles, and the 2i/-imidazoles, are rare among 4//-imidazoles because of lack of suitably substituted examples. A number of other thermal reactions are known, including the extensively studied rearrangements of structure types 2 and 21. 

From a study of his and van Alphen s results, Huttel identified three classes of rearrangement (Scheme 40).137 Type A involved migration of a group from C-3 to unsubstituted C-4 (e.g., 114 - 11564), Type B an acyl migration from C-4 or C-5 to N (e.g., 116 -> 11764or 118 - 117137), and Type C an aryl migration from C-3 to N (e.g., 118 - 119137) the latter two modes were observed when all carbon atoms were fully substituted. 

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. 

Section IV,A,2 Competition between van Alphen-Huttel rearrangement and ring opening to a vinyldiazo compound is found to be substituent dependent for transient spiro-3//-pyrazoles formed from methyl propynoate and 4-diazopyrazolones.177... 

J. van Alphen, Pyrazolines and their rearrangement to form pyrazoles, Rec. Trav. Chim. Pays-bas 62, 491-496 (1943). 

When the isoxazole (105.1) is treated with an excess of diazomethane at laboratory temperature, a [1, S]sigmatropic shift occurs and the benzoyl group migrates from the bridgehead position to an adjacent carbon (that derived from diazomethane)—a move which resembles the Van Alphen-Hauttel rearrangement [B-4]. 

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