Indole-3-carboxylic acid (6.43 gr, 39.9 mmol) is added in portions to a solution of 5.33 mL (37.7 mmol) of trifluoroacetic anhydride in dichloromethane (172 mL) and under nitrogen atmosphere. The resulting suspension is left under stirring for 30 minutes. 7-hydroxy-9-methoxycarbonylmethyl-9-aza-bicyclo[3.3.1]nonan-3-carboxylic acid methyl ester (6 gr, 22.1 mmol) dissolved in dichloromethane (15 mL) is then added dropwise. Next, a catalytic amount of DMAP is added, and the suspension is then left under stirring for 4 h. Following that time, and once the limiting starting product has been used up as shown by thin-layer chromatography (dichloromethane/methanol/ammonia 90:10:1, which reveals potassium permanganate), water (50 mL) is added to the reaction mixture and the mixture is basified with a saturated aqueous solution of sodium bicarbonate. Following filtration to remove the remains of indole-3-carboxylic acid in excess, the resulting phases are separated. The organic phase is dried over magnesium sulphate and evaporated to dryness, to give 7-(1H-Indole-3-carbonyloxy)-9-methoxycarbonylmethyl-9-aza-bicyclo[3.3.1]nonane-3-carboxylic acid methyl ester as a white solid (8.20 gr, 89%) that can be used directly in the following synthesis step.
