DIRECT EPOXY HYDROXYLATION OF HYDROPEROXY HOMOALLYLIC ALCOHOLS - MULTIDENTATE OXYGEN DONOR AND OXYGEN ACCEPTOR SUBSTRATES IN TI(IV)-CATALYZED EPOXIDATIONS

The hydroperoxy homoallylic alcohols (R*,S*)-2a and (S*,S*)-2a-d, readily available through the photooxygenation of chiral allylic alcohols 1, were converted into epoxy diols 4 under the catalytic action of Ti(Oi-Pr)4. In these epoxy hydroxylations, the hydroperoxides 2 play the double role as oxygen atom donor and, in form of the in situ generated corresponding unsaturated diols 3, as substrate for oxygen transfer. Compared to Ti(IV)-catalyzed epoxidations of unsaturated diols by t-BuOOH, the advantage of this approach is that a large rate enhancement is obtained. Moreover, with the exception of (S*,S*)-2d, all reactions proceeded in unusually high diastereoselectivity. These results are rationalized in terms of the ability of the hydroxy-functionalized hydroperoxides (oxygen atom donors) as well as the corresponding unsaturated diols (oxygen atom acceptors) to chelate to the titanium metal in the catalytically operating template. For (S*,S*)-3a-c, bidentate binding is feasible, while for (R*,S*)-3a and (S*,S*)-3d, this is difficult due to unfavorable steric interactions. Important for synthetic applications is the fact that allylic alcohols 1 can directly be converted into epoxy diols 4 in a one-pot, two-step procedure simply by adding catalytic amounts of Ti(Oi-Pr)4 to a photooxygenated solution of 1.