A mixture of 4.09 g of 1,1,1,3,3,3-hexamethyldisilazane and 0.41 g of methanol was added to a solution of 1.40 g of 3-[1'-(tert.butoxycarbonyl)-7,9-dihydrospiro[pyrido[1,2-a]indole-8(6H),3'-pyrrolidin]-10-yl]-4-(1-methyl-3-indolyl)furan-2,5-dione in 25 ml of dry DMF and stirred for 16 hours. The mixture was poured into 100 ml of water and extracted with ethyl acetate. The ethyl acetate extracts were washed with sodium chloride solution, dried and concentrated. There were obtained 820 mg of 3-[1'-(tert.butoxycarbonyl)-7,9-dihydrospiro[pyrido-[1,2-a]indole-8(6H).3'-pyrrolidin]-10-yl]-4-(1-methyl-3-indolyl)-1H-pyrrole-2,5-dione after chromatography on silica gel using dichloromethane/methanol (9:1) for the elution.
