Abstract
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.
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