Diastereoselective Synthesis of Spirocyclopropane-Linked Pyrazolones from Azomethine Ylides via C(sp3)-H Activation

A novel synthesis of spirocyclopropane-1,4-pyrazolin-5-one derivatives via iodine-mediated C(sp(3))-H activation is described. Thus, a diastereoselective synthesis of spirocyclopropane-linked pyrazolones from 4-arylidene-3-methyl-1-phenylpyrazolin-5-ones and azomethine ylides, generated insitu from iodine-catalyzed reaction of 2-methylquinoline or acetophenones with pyridine in the presence of base, has been developed. These transformations proceeded via a cyclopropanation reaction followed by anti-elimination of pyridine. Unequivocal evidence for the structures of spirocycles rac-anti-5a and rac-anti-12a were obtained from single-crystal X-ray analyses. These methods offer several advantages, such as being inexpensive, moderate to high yield, high atom economy, and ease of product isolation, which make them attractive processes for the synthesis of spirocyclopropane-linked pyrazolones. The spirans reported in this work may deserve further attention not only from a synthetic but also from a pharmacological point of view.