Radical reactions protecting groups

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

In a recent application of this strategy, nitrocyclohexane 143 (prepared from nitrosugar 142 by intramolecular Henry reaction) was subjected to a radical denitration by HSnBu3, after protection of the hydroxyl groups to avoid side reactions. Inositol 146 was selectively obtained in good yield, once the hydroxyl protecting groups were removed (Scheme 45).101... 

This radical cyclization strategy was utilized for the synthesis of the smaller fragment silyl ether 54 as well (Scheme 8). Evans aldol reaction of the boron eno-late derived from ent-32 with aldehyde 33, samarium(III)-mediated imide methyl ester conversion, and protecting group exchange led to tosylate 51. Elaboration of 51 to ketone 53 was achieved under the conditions used for construction of the second tetrahydrofuran moiety of 49 from 46. A highly diastereoselective reduc-... 

The removal of the tritylthio (Ph3CS) protecting group used in guanine derivatives has been accomplished in a good yield by the action of (TMS)3SiH under free-radical conditions (Reaction 4.22) [49]. 

---