A highly diastereoselective dioxetane formation by the hydroxy-directed [2+2] cycloaddition of singlet oxygen to a chiral allylic alcohol

The adamantylidene-substituted allylic alcohol la reacts diastereoselectively with singlet oxygen in CDCl3 to yield the dioxetane threo-2a, accompanied by the dioxolane 4, which is formed as a subsequent product of an initial ene reaction. In the more polar CD3OD/CCl4 solvent mixture, the diastereoselectivity drops to 89:11, but the ene mode (dioxolane 4) is completely suppressed. In contrast, the photooxygenation of the acetate Ib in CDCl3 and in CD3OD yielded only ene product 3b. The observed three-selective hydroxy-group directivity is explained in terms of hydrogen bonding in the exciplex between the singlet oxygen and the 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 stereochemical course but also the mode selectivity (ene, [4+2], and [2+2]) in all three reaction types of singlet oxygen.