title: A Pyrazole to Furan Rearrangement. Thermolysis of 5-Azido-4-formylpyrazoles.
abstract: 5-Azido-3-benzyl-4-formyl-1-phenylpyrazoles 1a-c extrude dinitrogen upon heating in toluene to give the corresponding nitrenes, which immediately rearrange via a new ring-opening ring-closure reaction to produce an equimolar mixture of 4-cyano-2-phenyl-3-phenylazofurans 2a-c and 3-benzyl-4-cyano-1-phenylpyrazoles 3a-c. The formation of the 4-cyano-2-phenyl-3-phenylazofurans 2a-c is the first example in the pyrazole series of a nitrene rearrangement, in which the parent heterocyclic system of the product differs from that of the starting material. The isolation of equimolar amounts of the two products points to the fact that their formation occurs by two mechanistically interconnected pathways, between which the exchange of a redox equivalent takes place. Evidence for the existence of two mechanistically interlinked pathways is presented, and the insight into the stoechiometry of the reaction is taken advantage of to optimize the reaction with respect to either of the two products 2 or 3. Thus, it is demonstrated how one can bias the two pathways using external reagents, thereby changing the product distribution ratio 2:3 from 1:1 in the unbiased case, to 1:4 in one direction, and to better than 20:1 in the other direction.
authors.affiliation: Department of Chemistry, Odense University, Campusvej 55, DK-5230 Odense, Denmark, Department of Chemistry, The Technical University of Denmark, DK-2800 Lyngby, Denmark, and Department of Chemistry, University of Leuven, B-3001 Heverlee, Belgium.
authors.forename: Niels
authors.initials: N
authors.lastname: Svenstrup
authors.forename: Klaus B.
authors.initials: KB
authors.lastname: Simonsen
authors.forename: Niels
authors.initials: N
authors.lastname: Thorup
authors.forename: Jørgen
authors.initials: J
authors.lastname: Brodersen
authors.forename: Wim
authors.initials: W
authors.lastname: Dehaen
authors.forename: Jan
authors.initials: J
authors.lastname: Becher