| Abstract
| - The adamantylidene-substituted allylic alcohol 1a reacts diastereoselectively with singlet oxygen inCDCl3 to yield the dioxetane threo-2a, accompanied by the dioxolane 4, which is formed as a subsequentproduct of an initial ene reaction. In the more polar CD3OD/CCl4 solvent mixture, the diastereoselectivitydrops to 89:11, but the ene mode (dioxolane 4) is completely suppressed. In contrast, the photooxygenation ofthe acetate 1b in CDCl3 and in CD3OD yielded only ene product 3b. The observed threo-selective hydroxy-group directivity is explained in terms of hydrogen bonding in the exciplex between the singlet oxygen andthe hydroxy group, and the latter is appropriately conformationally aligned in the substrate by 1,3-allylic strain.This synergism between conformational strain and hydrogen bonding controls not only the stereochemicalcourse but also the mode selectivity (ene, [4+2], and [2+2]) in all three reaction types of singlet oxygen.
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