Rearrangement chromate, intermediates

The formation of an intermediate with electron-deficient oxygen is also one of the possible paths for the oxidation of alcohols. An intermediate such as LVI, or the chromate ester LVII which might behave in similar fashion, could rearrange to give "abnormal products or lose a proton to give the usual, expected product.889-882... 

The first step in Cr(VI) oxidations can take place to give a chromate ester (Chapter 24) but this intermediate has no proton to lose so it transfers the chromate to the other end of the allylic system where there is a proton. The chromate transfer can be drawn as a [3,3]-sigmatropic rearrangement. 

The olefin 157 was epoxidized to an equimolar mixture (158a,b). Both epoxides opened with lithium di-/t-propy]amide to the desired allylic alcohols (159). Expoxide 158a also yielded unwanted tertiary alcohol 160 (40%). Reaction of the crude epoxide mixture with lithium di-/i-propylamide followed by oxidation (Ct03-py2) and purification resulted in a 71 29 mixture of enones 152 and 161, with an overall yield of 152 from 155 of 16%. The oxidation of 160 to 161 occurs by allylic rearrangement of the intermediate chromate ester. The reduction product from 154 was resolved into its enantiomers via the brucine salt of the hydrogen phthalate 162. The levorotatory enantiomer was found to have the absolute configuration corresponding to that of the pentacyclic triteipenes. 

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