Reductive amination of 3-amino-thiophene-2-carboxylic acid methyl ester (3.0 g, 19.1 mmol) and 4-oxo-piperidine-1-carboxylic acid benzyl ester (4.46 g, 19.1 mmol) in THF (4.6 mL) employing Bu2SnCl2 (0.598 g, 1.92 mmol) and PhSiH3 (2.58 mL, 21.6 mmol) was carried out as described for example 19, step 1 gave 4-(2-methoxycarbonyl-thiophen-3-ylamino)-piperidine-1-carboxylic acid benzyl ester (7.25 g, quantitative). NMR 1H (CDCl3, 400 MHz): 7.4-7.3 (m, 6H), 6.9-6.78 (m, 1H), 6.7-6.6 (m, 1H), 5.14 (s, 2H), 4.15-4.0 (m, 2H), 3.81 (s, 3H), 3.6-3.45 (m), 3.1 (brt, 2H), 2.1-1.9 (m, 2H), 1.6-1.45 (m, 2H).
