Transformation of the cyclohexane ring to the cyclopentane fragment of biologically active compounds.

The review is devoted to strategies for contraction of six-membered cycles in the synthesis of functionalized cyclopentane/enones, which are biologically active compounds. The main synthetic methods of ring contraction (ozonolysis-aldol condensation, ozonolysis-Dieckmann reaction, Baeyer-Villiger cleavage-Dieckmann reaction) and rearrangements (benzil, semipinacol, with the participation of thallium- and iodine-based oxidants, photochemical, Wolff, Meinwald, Wagner-Meerwein and Favorskii) are presented. The review summarizes literature data covering the last 12 years, with some exceptions of earlier works due to the importance of the published information.