作者:Alan C. Weedon、Boke Zhang
DOI:10.1055/s-1992-34165
日期:——
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.
我们研究了一系列氮取代基激活吲哚环化光化学反应的能力;选择这些氮取代基是为了使它们能够在温和的中性、酸性或碱性条件下进行环化反应后被去除。 我们制备了 N 取代吲哚 1a-g,其中 N 取代基分别为 COPh、CO2Et、CO2CH2CH2SiMe3、CO2CH2CH2CN、CO2Bu-t、CO2Ph 和 CO2CH2Ph。在紫外线的直接照射下,化合物 1a-g 与环戊烯发生光化学环化加成反应,生成环丁烷融合吲哚,这是由于烯的末端与吲哚的 2-位和 3-位键合。该反应的化学收率在 32%-80% 之间,光化学效率在 0.00028-0.054 之间。当反应用苯乙酮敏化时,化学收率提高到 64-95%。这种改进是由于避免了竞争单激发态衍生的光-弗里斯重排。用氟离子在二氯甲烷中处理由 1c 生成的光加合物,可以去除其中的硅乙氧基羰基;用三氟乙酸在室温下处理由 1e 生成的加合物,可以去除其中的叔丁氧基羰基;用氢解法去除由 1g 生成的加合物中的苄氧基羰基;用碳酸醇在室温下处理由 1d 生成的加合物中的氰乙氧基羰基。所有这些脱保护反应的产率都非常高。去除 1f 衍生的加合物中的苯氧羰基和 1b 衍生的加合物中的乙氧羰基需要用热浓碱处理,收率较低,而 1a 衍生的加合物中的苯甲酰基对热浓酸或碱都很稳定,只有用氢化铝锂处理才能去除。