スクアリン酸を合成素子とする多官能型炭素および複素環化合物の高選択的合成

抄録

New method for derivatization of squaric acid was developed by means of Lewis acid-catalyzed reaction of the acid family with unsaturated organosilanes to give 4-substituted 4-hydroxy (or, 4- ethoxy) -2-cyclobutenones. Thermal electrocyclic ring opening of 4-allylated and 4-acylmethylated products was followed by intramolecular [2 + 2] cycloaddition and cyclization of minor stereoisomer of enol ketene to give bicyclo [3.2.0] heptenones and tetronates, respectively. Further, new synthetic potential as a C₄-synthon was found in ring transformation of 4-hydroxycyclobutenones involving radical- and cation-mediated ring opening and reclosure processes, which afforded highly substituted 2- (5H) furanones and 4-cyclopentene-1, 3-diones. These reactions included interesting mechanistic features such as 5-endo cyclization of acyl radical, ionic rearrangement of hypoiodite and 1, 2-silylmigration of alkynylsilane. In particular, ab initio calculation for 5-endo cyclization of 5-oxa-2, 4-pentadienoyl radical suggested unusual non-radical cyclization mechanism. The synthetic utility of the above ring transformation strategies was demonstrated in natural product synthesis of (E) -basidalin and (Z) -multicolanate and construction of a triquinane skeleton.