Pyranos-1-yl and furanos-1-yl radicals add to protonated hetero­aro­matics to produce C -nuc­leoside derivatives. 48–54 A proposed mechanism for this type of reaction is given in Scheme 12. 50 Protonation drama­tically increases the rate of addition of a carbon-centered radical to a heteroaromatic compound; in fact, the rate is so fast that it is not necessary to conduct the reaction in an inert atmosphere. Not only does reaction take place in the pres­ence of molecular oxygen but oxygen is a likely participant in the rearomat­ization stage of this process (Scheme 12). In reactions of this type the initially formed radical (R·) usually is generated by photolysis of an O‑ ac­yl- N -hy­droxy-2-thiopyridone (eq 15). 48 (Reactions of O‑ acyl- N -hydroxy-2-thiopyridones are dis­cussed in Chapter 13 .)