Regioselectivity epoxide hydrogenolysis

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

In some cases, a simple tosylation can be equally regioselective, especially when one of the hydroxyl substituents is more sterically hindered then the other. This approach served as a key step in an expeditious approach towards naproxen 217 (Scheme 55). The primary alcohol function of the optically active diol 214, of 98% ee, was selectively activated with tosyl chloride [135]. The resulting to-sylate, upon treatment with NaH, underwent smooth cycHzation to the epoxide 215. Hydrogenolysis proved to be highly facial selective, delivering the primary alcohol 216 in high enantiopuxity. A final Jones oxidation then furnished naproxen of 96% ee. 

Epoxides react with hydrogen and a catalyst by a hydrogenolysis reaction, but it is relatively slow and many functional groups can be reduced preferentially, which means that epoxides can be reduced if there is little or no other functionality. If reduction of the epoxide is to be avoided, rhodium on carbon (Rh/C) or Wilkinson s catalyst is preferred. Predicting the direction of ring opening (regioselectivity in the reduction)... 

---