To a mixture of the above 2-amino-5-hydroxymethyl-4,5,6,7-tetrahydro-thieno[2,3-c]pyridine-3,6-dicarboxylic acid di-tert-butyl ester (2.00 g, 5.20 mmol), phthalimide (0.92 g, 6.24 mmol) and triphenylphosphine (1.64 g, 6.24 mmol) in dry tetrahydrofuran (30 ml) cooled to 0° C. under a nitrogen atmosphere was added diethyl azodicarboxylate (DEAD) (0.98 ml, 6.24 mmol). The reaction mixture was allowed to stir overnight, slowly warming to room temperature. Next day the reaction mixture was again cooled to 0° C. and phthalimide (0.46 g, 3.12 mmol), triphenylphosphine (0.82 g, 3.12 mmol) and diethyl azodicarboxylate (DEAD) (0.49 ml, 3.12 mmol) was added in sequence and the reaction mixture was allowed to stir overnight, slowly warming to room temperature. The volatiles were evaporated in vacuo and the resultant solid dissolved in dichlorormethane (20 ml). The residue was subjected to flash column chromatography using a mixture of ethyl acetate/hexane (1:2) as eluant. Fractions were collected affording after evaporation in vacuo 1.0 g of the desired compound contaminated with phthalimide. Recrystallization from ethanol gave 0.23 g (9%) of pure 2-amino-5-(1,3-dioxo-1,3-dihydro-isoindol-2-ylmethyl)-4,5,6,7-tetrahydro-thieno[2,3-c]pyridine-3,6-dicarboxylic acid di-tert-butyl ester as a solid.
