Synthesis and Structural Elucidation of Novel Spiropyrazolines Precursors to Spiro-gem-dimethylcyclopropanes

Abstract

1,3-dipolar cycloaddition reaction of 2-diazopropane (DAP) 1 with 2-arylidene-1-tetralone 2a–b led exclusively to spiro-Δ1-pyrazolines 3a–b. When 4-methyl-2-arylidene-1-tetralones 2c–d was used as starting dipolarophile a mixture of diastereiosomeric cycloadducts 4a–b and 5a–b was obtained. The photochemical nitrogen-extrusion studied for spiropyrazolines 3a–b and 4a–b led stereospecifically to spiro-gem-dimethylcyclopropanes. In all cases, the reaction regiochemistry and stereochemistry was discussed on the basis of 2D-NMR experiments.