抄録
The Pictet-Spengler reaction of N-hydroxytryptamine (6) with various aldehydes was examined. Reduction of 3-(2-nitroethyl) indole (5) with aluminum amalgam gave 6, which was not so stable and was readily oxidized to the azoxy compound (10) in solution under an oxygen atmosphere. Reaction of 6 with saturated aldehydes gave the corresponding nitrones (11b, c) without an acid catalyst at room temperature, while the reaction with α, β-unsaturated aldehydes provided nitrones (11d, e, f) in the presence of trifluoroacetic acid. Cyclization of the nitrones with trifluoroacetic acid in methylene chloride gave the 2-hydroxy-1, 2, 3, 4-tetrahydro-β-carbolines (2) in good yields. The cyclization of the saturated nitrones proceeded rapidly, whereas the similar reaction of the unsaturated nitrones was slow. Direct Pictet-Spengler reaction of 6 with aldehydes in methylene chloride in the presence of trifluoroacetic acid gave 2a, b, c in excellent yields. Dehydration of the 2-hydroxy-β-carboline (2b) with trifluoroacetic anhydride in benzene gave the 3, 4-dihydro-β-carboline (15). 6-Bromo-5-methoxy-N-hydroxytryptamine (25) was prepared from 3-bromo-4-methyoxyaniline (17) via the indole (22). The Pictet-Spengler reaction of 25 with isovaleraldehyde gave the 2-hydroxy-tetrahydro-β-carboline (26).
