A 3:1 mixture (0.44 g, 1.4 mmol) of 6-bromo-2-(trifluoroacetyl)-1,2,3,4-tetrahydroisoquinoline and 8-bromo-2-(trifluoroacetyl)-1,2,3,4-tetrahydroisoquinoline (prepared according to Tetrahedron Lett. 1996, 37(31), 5453-5456) was stirred in ethanol (10 mL) containing a few drops of 25% ammonium hydroxide at RT. After 2.5 h, the solution was concentrated, dissolved in dry THF (1.0 mL) under argon and cooled on an ice-bath. DIPEA (0.41 mL, 2.4 mmol) was added followed by a solution of [(4S)-4-methyl-2,5-dioxoimidazolidin-4-yl]methanesulfonyl-chloride (0.27 g, 1.2 mmol) and dry THF (1.0 mL). The mixture was stirred at RT for 1 h and then concentrated. The crude product was taken up in water and extracted twice with EtOAc. The combined organic phases were washed with brine, dried, filtered and concentrated to give 0.55 g of a mixture of (5S)-5-({[6-bromo-3,4-dihydroisoquinolin-2(1H)-yl]sulfonyl}methyl)-5-methylimidazolidine-2,4-dione and (5S)-5-({[8-bromo-3,4-dihydroisoquinolin-2(1H)-yl]sulfonyl}methyl)-5-methylimidazolidine-2,4-dione. The regioisomers were separated by preparative HPLC.
