Improved chromium-catalyzed allylic oxidation of Δ5-steroids with t-butyl hydroperoxide

Versatile Construction of 6-Substituted cis-2,8-Dioxabicyclo[3.3.0]octan-3-ones: Short Enantioselective Total Syntheses of Cheloviolenes A and B and Dendrillolide C

“…Incorporation of the phenyl group in the alkenyl chloride fragment was crucial to the success of this ring-closing metathesis as reported by Dorta. 67,68 After much experimentation, we found that the demanding allylic oxidation of 58 could be accomplished with CrO 3 and tert-butylhydroperoxide 69 to reliably give butenolide 59 in 32% yield. 70−72 The opposite enantiomer of butenolide 59 was readily accessible by the same sequence, starting from (L)-menthol and employing the opposite enantiomer of the Trost ligand.…”

Optimal TBHP Allylic Oxidation of Δ5-Steroids Catalyzed by Dirhodium Caprolactamate

“…Conversion is slow at room temperature, but complete within 20 h at 40 °C; at 60 °C oxidation is considerably faster, but the catalyst undergoes hydrolysis at a measurable rate. With 2.0 equiv of T-HYDRO® oxidation reached 83% conversion within 20 h and was at 95% conversion with 4.0 equiv of T-HYDRO® over the same time, the reason for which is competitive formation of both stereoisomers of 7-hydroperoxy-(5) and 7-hydroxy-cholesterol (6) 15 that undergo slow oxidation to 7-ketocholesterol (Scheme 1). Dichloroethane was preferred over dichloromethane because of its higher boiling point.…”

Allylic Oxidations of Olefins to Enones

“…Herein, Table 3 gives an overview of common reagent combinations for the allylic oxidation of cholesteryl acetate (50) using tert-butyl hydroperoxide as a source of oxygen together with catalytic amounts of different chromium reagents. 17,[74][75][76][77][78] These protocols usually provide a good level of chemoand regioselectivity and give enones like 51 in moderate to good yields along with small amounts of epoxides like 52.…”