The Regiochemistry of Cyclization of α-Sulfenyl-, α-Sulfinyl-, and α-Sulfonyl-5-hexenyl Radicals:  Procedures Leading to Regioselective Syntheses of Cyclic Sulfones and Sulfoxides

A study of the cyclization of alpha-sulfenyl-, alpha-sulfinyl-, and alpha-sulfonyl-5-hexenyl and 5-methyl-5-hexenyl radicals reveals a unique contrast in the mode of ring closure of the radicals. In the case of the 5-hexenyl radicals, the sulfinyl-substituted species displays unexpected regioselectivity relative to its analogues. Thus, while the alpha-S- and alpha-SO(2)-5-hexenyl radicals give measurable and increasing quantities of 6-endo product, the alpha-sulfinyl species cyclizes with high selectivity (95.5:4.5) via a 5-exo mode. By contrast, ring closure of the 5-methyl-5-hexenyl radicals is found to give substantially the 6-endo product in all cases. It is the alpha-sulfonyl-5-methyl-5-hexenyl radical that now exhibits high regioselectivity (97.5:2.5) for 6-endo closure: an illustration of the synthetic value of this observation is the independent synthesis of the model cyclohexyl sulfone 61 in high yield. It is found that ring closure under the conditions employed occurs irreversibly in all cases.