the radical oxy-Cope rearrangement. [17] For this purpose, the sulfoxide 10 was prepared from norbornenone 8 via the allylic alcohol 9 (the direct synthesis with the procedure of Evans et al. does not work in this particular case). By treating the sulfoxide 10 with Bu 3 SnH and azobisisobutyronitrile (AIBN) in refluxing toluene, the desired rearranged compound 11 is obtained in an unoptimized 46 % yield (Scheme 3). This reaction consists of a sequence of a [2,3]-sigmatropic rearrangement followed by generation of an allyloxyl radical, regioselective fragmentation to an allyl radical, and finally a 6endo cyclization. [17] In conclusion, we have demonstrated that allylsulfoxides, which are easily available from ketones, are suitable precursors for allyloxyl radicals. An unique method for the twocarbon ring expansion of cyclobutanones has been developed based on a sequential radical-chain reaction. To the best of our knowledge, this is the first cascade process in which the radical precursor is continuously generated in an equilibrium reaction as a minor component of the reaction mixture. The stability of the sulfoxide radical precursors and the mild reaction conditions of the ring expansion renders this reaction attractive for preparative purposes. The control of the regioselectivity of the process requires a proper design of the system. Extension of this reaction to other ring sizes and more complexed systems is currently under investigation.
Experimental Section
General procedure for the radical ring expansion: A solution of Bu 3 SnH (0.49 mL, 1.84 mmol) and AIBN (10 mg, 0.06 mmol) in C 6 H 6 (3 mL) was added over 12 h with a syringe pump to a refluxing solution of the allylic sulfoxide (1.23 mmol) in C 6 H 6 (123 mL, 0.01m). (For sulfoxide 1 e, better results were obtained when irradiation with a 300-W sunlamp was performed during the reaction.) The solvent was removed under reduced pressure and the crude product was purified by flash chromatography.