Removable Groups for Activation of Indole Photochemistry

The ability of a series of nitrogen-substituents to activate the cycloaddition photochemistry of indoles has been examined; the N-substituents were chosen in order to enable their removal following cycloaddition under mild neutral, acidic or basic conditions. N-Substituted indoles, 1a-g, were prepared in which the N-substituent is COPh, CO₂Et, CO₂CH₂CH₂SiMe₃, CO₂CH₂CH₂CN, CO₂Bu-t, CO₂Ph, and CO₂CH₂Ph, respectively. Under direct irradiation with ultraviolet light the photochemical cycloaddition reaction of compounds 1a-g with cyclopentene yields cyclobutane fused indolines resulting from bonding of the alkene termini to the 2- and 3-positions of the indole. The reaction proceeds with chemical yields in the range 32%-80% and photochemical efficiencies in the range 0.00028-0.054. When the reaction is sensitized with acetophenone the chemical yields are raised to 64-95%. This improvement arises from avoidance of competing singlet excited state derived photo-Fries rearrangement. The silyl ethoxycarbonyl group of photoadducts derived from 1c can be removed by treatment of the adduct with fluoride ion in dichloromethane, the tert-butyloxycarbonyl group of adducts derived from 1e by treatment with trifluoroacetic acid at room temperature, the benzyloxycarbonyl group of adducts derived from 1g by hydrogenolysis, and the cyanoethoxycarbonyl group of adducts derived from 1d with alcoholic carbonate at room temperature. All of these deprotection reactions proceed in very high yield. Removal of the phenoxycarbonyl group of adducts derived from 1f and the ethoxycarbonyl group of adducts derived from 1b required treament with hot concentrated base and the yields were lower, while the benzoyl group of adducts derived from 1a was stable to hot concentrated acid or base and could be removed only by treatment with lithium aluminum hydride.