Deoxygenation of Alcohols Barton-McCombie Reaction

A relevant reductive process, which has found wide application in organic synthesis, is the deoxygenation of alcohols introduced in 1975 by Barton and McCombie [58]. Reaction (4.28) shows that the thiocarbonyl derivatives, easily obtained from the corresponding alcohol, can be reduced in the presence of BusSnH under free radical conditions. The reactivity of xanthates and thiocarbonyl imidazolides [58] was successfully extended to 0-arylthiocarbonates [59] and (9-thioxocarbamates [60]. Several reviews have appeared on this subject, thus providing an exhaustive view of this methodology and its application in natural product synthesis [61-64]. 

86% yield (Reaction 4.32), whereas other radical-based reducing systems give much poorer yields [69]. 

Radical deoxygenation via thioimidazolyloxy derivatives is also found to be efficient process. Two examples are reported in Reactions (4.33) and (4.34) [70,71]. In particular, dideoxygenation was useful to prove the structure of the stemodane ring system, achieved by other routes. 

The application of (TMS)3SiH has also been extended to the dideoxygenation of v/c-diols that readily affords the corresponding olefins. Reaction (4.39) shows an example of radical-based dideoxygenation of a bis-O-thioxocarba-mate derivative by this silane under standard conditions [77]. In this way, a 

---

The products were obtained in excellent yields after simple hexane extraction. The removal of bromine and iodine proceeded smoothly. The removal of PhSe afforded methylcyclohexanone, indicating that the decarbonylation of acyl radicals takes place. The efficiency of deoxygenation of alcohols (Barton-McCombie reaction) is independent of the type of thiocarbonyl derivative (i.e. 0-arylthio-carbonate, O-thioxocarbamate, thiocarbonylimidazole or xanthate), as previously reported for (TMS)3SiH in organic solvents.25... 

---