Synthesis
The general procedure for the synthesis of tert-butyl 1-Boc-4-aminoindole and 4-amino-2,3-difluoroindole-1-carboxylate from 1-Boc-4-nitroindole is as follows: to a 2.5 liter Paar oscillator hydrogenation flask were added 40.0 grams (0.152 mol) of 1-Boc-4-nitroindole, 500 ml of toluene, and 1.0 gram of 5 wt% Pd/C Degussa Type E101NO/W (50% water wet). The yellow-green mixture is hydrogenated at room temperature for approximately 3 hours at approximately 50 psi hydrogen pressure until hydrogen uptake ceases and HPLC analysis shows that the reaction is complete. Note: Batch analysis should be performed immediately after completion of hydrogen uptake as excessive hydrogenation results in the formation of approximately 5-15% indole by-products (extended reaction times will result in decreased yields due to excessive hydrogenation). The reaction mixture is filtered through Celite, washed with toluene and concentrated. Multiple hydrogenation batches were combined and concentrated to 140 g of crude orange oil. Purification by column chromatography with 25% ethyl acetate: 75% hexane as eluent gives 121 g of pure product as an orange oil with 100% HPLC purity and 88% overall yield. The indole by-product must be removed in this step or it will interfere with subsequent steps.
