Alpha- and beta-thujone radical rearrangements and isomerizations. A new radical clock.
Radical clocks have been extensively used in chemical and biochemical mechanistic studies. The C4 radicals of alpha- and beta-thujone can undergo two distinct rearrangement reactions that could, in principle, serve as simultaneous but independent radical clocks. We have therefore generated these C4 radicals by photolysis of the corresponding N-hydroxypyridine-2-thione ester precursors and have investigated their fates and lifetimes. Photolysis of either alpha- or beta-thujone generates the same 6:100 mixture of alpha- and beta-thujone when the radicals are quenched by thiophenol. Hydrogen atom transfer from thiophenol to the radical thus occurs preferentially from the less sterically hindered alpha-face to give beta-thujone. The third product formed in the photolysis via opening of the cyclopropyl ring is 2-methyl-5-isopropylcyclopent-2-enone. The ratio of ring opened to unopened products gives very similar values of kralpha = 4.4 x 10(7) s(-1) and krbeta = 1.0 x 10(8) s(-1) for ring opening of the radicals generated from alpha- and beta-thujone, respectively. If the C4 cation rather than radical is generated, it is converted to carvacrol, a phenol that is not obtained in the radical reactions. Thujone therefore differentiates between radical and cation pathways and provides a measure of the radical lifetime.
He
Xiang
X
Department of Pharmaceutical Chemistry, University of California, 600 16th Street, San Francisco, California 94143-2280, USA.
Ortiz de Montellano
Paul R
PR
D000081008:Bicyclic Monoterpenes; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D015394:Molecular Structure; D039821:Monoterpenes / Q000138:chemical synthesis* / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D013237:Stereoisomerism
D000081008:Bicyclic Monoterpenes; D039821:Monoterpenes; C005790:beta-thujone