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Development of new methodology for synthesis of small ring compounds is an intriguing subject because of important biological activities of derivatives and their inherent chemical properties. Recently, we have elaborated several tools based on cascade reactions, which are useful for the synthesis of natural products. (A) Tandem Intramolecular Michael-Aldol Reaction Treatments of symmetrical (8) and α'-protected ketones (11 and 14) with TBSOTf in the presence of Et_3N provided four membered polycyclic compounds (9), (12) and (15), respectively, whereas (16) and (20) bearing two kinds of hydrogens adjacent to the ketocarbonyl group were transformed to (19) and (21) by the tandem reaction conducted under TMSI in the presence of (TMS)_2NH. It is noteworthy that the two reaction conditions were complementary. (B) Rearrangement of Cyclobutanes to Cyclopropanes A novel rearrangement of cyclobutanes (31) and (35) leading to cyclopropanes (32), (33) and (36) was achieved under three different conditions, using BF_3・OEt_2 or POCl_3-pyridine or Raney Ni (W-2). (C) Cleavage of Cyclopropyl Ketone Accompanied by Tandem Intramolecular Michael-Aldol Reaction A novel cascade reaction of α, β-unsaturated esters (44) and (47) possessing a cyclopropyl ketone moiety affording polycyclic cyclobutane (45) and (48) has been performed by the treatment with TMSI in the presence of (TMS)_2NH. (D) Total Synthesis of (±)-Anthoplalone and (±)-Lepidozene A synthesis of (±)-anthoplalone (49) and a formal synthesis of (±)-lepidozene (50) were achieved via the bicyclic compound (36), obtained by the tandem intramolecular Michael-aldol reaction and the rearrangement. (E) Synthetic Study of (±)-Filifolone The cascade reaction of the cyclopropyl aldehyde (56) produced the bicyclic iodide (57), which was converted into the potential intermediate (60) of filifolone (7).