REACTIONS OF N-ACYLATED INDOLES WITH SINGLET OXYGEN

Reactions of seven N-acyl indole derivatives with singlet oxygen have been investigated. 1-Acetyl-(1a) and 1-(chloroacetyl)-2,3-dimethylindole (1b) gave exclusively 1-acetyl-(2a) and 1-(chloroacetyl)-2-hydroperoxy-3-methyleneindoline (2b), respectively, in high yields via ene reaction on irradiation with oxygen in the presence of TPP sensitizer. 1-Acetyl-3-methylindole (1c) gave only 1-acetyl-2-hydroperoxy-3-methyleneindoline (2c) in low yield under the same conditions. In contrast, 1-acetyl-2-methyl-3-ethylindole (3a) and 1-acetyl-2-methyl-3-isopropylindole (3b) gave a mixture of ene products, 2-hydroperoxy- and 3-hydroperoxyindolines (4a and b and 5a and b), in addition to 2,3-bond cleavage products 6a and 6b, respectively. In the case of 1-acetyl-2-methyl-3-tert-butylindole (7), only the product of 2,3-bond cleavage (8) was obtained. A 1,2-dioxetane is intermediate in the cleavage and could be observed after photooxygenation of 7 at -5-degrees-C by NMR and was reduced by trimethylphosphite and dimethyl sulfide. The decomposition rate constant of 1,2-dioxetane 10 was measured by NMR; E(a) is 24.6 kcal/mol.