VI. Summary

An effective procedure for deoxygen­ation begins with the conversion of a hydroxyl group in a carbohydrate into an O‑thio­carbonyl group and ends with a radical reaction (the Barton- McCombie reaction) that replaces the O‑thio­carbonyl group with a hydrogen atom. Xanthates, (thio­car­bon­yl)­imidazolides, and thiono­car­bonates are the most common substrates for the Barton- McCombie reaction. Although a number of hydrogen-atom donors can be used in this reac­tion, the usual choice is tri-n-butyl­tin hydride. Safety con­cerns about tin hydrides and prob­lems with product pur­i­fi­cation have caused chem­ists to turn increas­ingly to other hydro­gen atom sources, in par­tic­ular, tris(trimethylsilyl)­silane. Barton-McCombie reaction is sometimes compli­cated by com­peting reac­­tions, the most common of which regenerates the partially protected carbo­hydrate from which the O‑thio­car­bonyl-containing substrate was synthesized.

Differences in reactivity sometimes favor selecting a particular type of O-thiocarbonyl com­pound. Phenyl thionocarbonates are particularly valu­able for hydroxyl group replacement by a hydrogen atom during nucleoside syn­thesis, and they are the least likely O-thiocarbonyl derivative to undergo alco­hol regeneration. If reaction of a xanthate is attempted with a hydro­gen-atom donor much less effective than tri-n-butyltin hydride, xanthate-dithiocarbonate rear­rangement can take place. Reactions of tertiary xanthates can be compli­cated by Chugaev elimination unless these reactions are conducted at low temper­ature.

Deoxygenation involving cyclic thionocarbonates differs from that of other O-thiocarbonyl compounds because reac­tion involves ring opening. The direction of ring opening determines which of the two carbon atoms in the ring system will become the radical center. A high yield of a single product, therefore, depends on high regioselectivity in the ring-opening process. Cyclic thi­ono­car­bonates gen­er­ally react to give deoxy compounds resulting from forma­tion of the more stable intermediate radical. In some instances release of ring strain during ring opening becomes a factor in deter­mining where the radical center will be located. Mixtures of pro­ducts are a common result when the two radicals produced by ring opening are compar­able in stability.

Radicals produced by reaction of O-thiocarbonyl compounds undergo addition reactions when a compound with a reactive multiple bond is present, and they undergo cyclization when the radical itself has a properly positioned multiple bond. In either situation the reactions that take place are of the addition-abstraction or addition-elimination type. The latter includes reac­tions with double bond migration and those without. Observed radical cycli­zations are all addition- abstraction reactions. Most of these involve addition of a framework radical to either a framework multiple bond or to a substit­uent multiple bond.