5-(methoxycarbonyl)-1H-indole-2-carboxylic acid → 2-(tert-Butyl)5-methyl1H-indole-2,5-dicarboxylate

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Reaction Steps

To 13 g 1H-Indole-2,5-dicarboxylic acid 5-methyl ester in 300 ml toluene, 59 ml Di-tert-butoxymethyl-dimethyl-amine were added dropwise at 80° C. Then, the reaction mixture was heated under reflux for additional 6 h. After removal of the solvents under reduced pressure the residue was dissolved in 300 ml DCM and washed with sat. aqueous NaHCO3 solution (2×100 ml). The organic layer was dried over MgSO4 and concentrated under reduced pressure. The residue was purified by chromatography on silica gel eluting with a n-heptane/ethyl acetate gradient. The fractions containing the product were collected and concentrated under reduced pressure. Yield: 8.3 g. MS (ES+): m/e=276.

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