To 2-amino-5-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-4,7-dihydro-5H-thieno[2,3-c]pyran-3-carboxylic acid tert-butyl ester (4.5 g, 0.011 mole) dissolved in dichloromethane (30 ml), was added sodium bicarbonate (1.0 g, 0.011 mole) dissolved in water (16 ml). The reaction mixture was cooled to 0° C. and 9-fluorenylmethyl chloroformate (3.0 g, 0.012 mole) was added. After stirring for 5 minutes the reaction mixture was warmed to room temperature and stirred vigorously for 16 h. The organic layer was separated and washed with brine (10 ml). The aqueous phase was extracted with dichloromethane (2×20 ml) and the combined organic phases were dried (MgSO4), filtered and evaporated in vacuo to give an orange solid which was purified by flash chromatography using dichloromethane as eluent. Pure fractions were collected and evaporated in vacuo affording 5.6 g (81%) of 5-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-2-(9H-fluoren-9-ylmethoxycarbonylamino)-4,7-dihydro-5H-thieno[2,3-c]pyran-3-carboxylic acid tert-butyl ester as a solid.
