抄録
DDQ is the only reagent for the selective oxidation of side chains at C-3 of indoles, while oxidation of 3-substituted and 2,3-disubstituted indoles with the other oxidants usually occurs on the pyrrole ring and at the 2-substituent, not at the 3-substituent. This selective oxidation has now been applied to a synthetic work of oxidized diketopiperazine alkaloids, neoechinulin A (NEA) and fumitremorgine B (FTB). Model compounds, diketopiperazines (7 and 8) of tryptophan with glycine and L-alanine, was heated with DDQ in THF to give the corresponding dehydro compounds, 9 (21%) and 10 (60%). The DDQ oxidation of dihydro NEA (12c and 12t) synthesized from indole via several steps was next examined. In t-BuOH, 12 was cleaved to an indole-3-aldehyde (14) through three consecutive reactions, dehydration, addition of t-BuOH, and a retro-aldol reaction. When trichloroethanol was used as a solvent to avoid this side reaction, NEA (1) was isolated in 25% yield. Among other protic and aprotic solvents, dioxane was an only useful solvent. In order to synthesize FTB (15), model experiments via two different pathways involving DDQ oxidation were successfully examined.
