Skeletal Photochemical Rearrangement of 7,9-Di-tert-butyl-1-oxaspiro[4.5]deca-6,9-dien-2,8-dione

Abstract

Abstract: An unusual skeletal photochemical transformation of 7,9-di-tert-butyl-1-oxaspiro[4.5]deca-6,9-dien-2,8-dione (2) into 5,7-di-tert-butylspiro[2.5]octa-4,7-dien-6-one (3) and 7,10-di-tert-butyl-1-oxaspiro[4.5]deca-7,9-dien-2,6-dione (4) was observed upon irradiation of a dilute hexane solution of 2 with high-pressure UV light for 1 h. The products were isolated by column chromatography on silica gel in yields of 74% for compound 4 and 14% for compound 3. Both products were reliably characterized by NMR and HRMS. The structure of 4 was further confirmed by X-ray diffraction analysis. According to the proposed mechanism, formation of compound 3 is presumably homolytic and involves homolysis of the lactone C–O bond in the starting compound 2, release of CO2, and intramolecular recombination of the resulting biradical to form a cyclopropyl ring. The proposed pathway for the formation of oxaspiro dienone 4 from 2 may be either radical or ionic in nature. It is assumed that the initial stage of the reaction partially resembles an ionic Nazarov cyclization and proceeds through electrocyclization with the formation of cyclopentenyl-type oxyallyl cations, which, through a sequence of transformations accompanied by charge or radical transfer and extensive bond rearrangement, lead to the final product 4. The initial oxaspiro compound 2 is a known oxidative transformation product of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid (1) and its esters, which are widely used industrial antioxidants for plastics and rubbers. Compound 1, also known as phenosanoic acid or dibufelone, is used as a neuroprotective and antiepileptic drug, making this study relevant from both environmental and pharmaceutical quality control perspectives.