Thermal rearrangements of 3,3-spiroalkylated pyrazoles: Ring expansion andnovel cases of sequential 1,5-shifts

A 3,3-spiro-(cyclopentyl)pyrazole containing electron withdrawing ester gro ups undergoes readily ring expansion in the form of the van Alphen-Huettel rearrangement. Subsequent post van Alphen-Huettel rearrangement involved a sequence of 1,5-shifts different from that suggested earlier. Reactive inte rmediates have been isolated and identified. The corresponding phenyl analo g does not exhibit post van Alphen-Huettel rearrangement. A rationale for t he different behavior is offered. X-Ray crystallography has been applied to differentiate between structurally similar product.