Oxygen oxidative rearrangements

We had two possible routes in which alcohol 72 could be used (Scheme 8.19). Route A would involve rearrangement of tertiary alcohol 72 to enone 76. Deprotonation at C5 and generation of the enolate followed by exposure to an oxaziridine or other oxygen electrophile equivalents might directly afford the hydrated furan C-ring of phomactin A (see 82) via hydroxy enone 81. We had also hoped to make use of a chromium-mediated oxidative rearrangement of tertiary allylic alcohols. Unfortunately, treatment of 72 to PCC produced only unidentified baseline materials, thereby quickly eliminating this route. 

A novel and unusual oxidative rearrangement of 6-methoxypyran-2-ones to form highly functionalized cr,/3-buteno-lides has been reported <20050L3705>. The oxidation occurs at 20 °C in the presence of molecular oxygen. A mechanism was proposed for this transformation and is shown in Scheme 21. 

The biopterin product is recycled by elimination of water, reduction using NADPH as the reagent, and reaction with molecular oxygen. The other product, the phenylalanine oxide, rearranges with a hydride shift followed by the loss of a proton to give tyrosine. 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized stereospecifically with rearrangement to the allylic alcohol in good yield by simple exposure to a palladium/caibon catalyst, as depicted in equation (12). Adventitious oxygen preadsotbed on the catalyst seems the likely oxida The reaction fails on the parent ccphem or its monoxide, or on the free acid of the dioxide. This reaction would seem to hold some promise for furdier utility in the cephem field and odier related systems. 

The same group of workers have also investigated the dye-sensitized photo-oxygenation of vincadifformine. After reduction of the reaction mixture with sodium thiosulfate, the related 16-hydroxyindolenine derivative 285 was obtained, which (without isolation) was rearranged in acetic acid to vincamine in 46% yield. Tabersonine behaved similarly (277). These results are broadly in agreement with those obtained by L6vy and his collaborators in an independent study of the photochemical oxidative rearrangement of vincadifformine (218). 

The oxidative rearrangement of 28 and 2 has also been affected with cupric ion in aqueous dimethylformamide in an oxygen atmosphere at... 

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