resulting white precipitate was collected by filtration washed with ether and dried at 0 1 torr to yield 38 1 mg 12 1 of the pure title compound partial 360 mhz 1 h nmr cdcl 3 u03b4 6 80 s 1h 6 53 s 1h 6 24 brs 2h 5 98 d 2h 4 89 d 1h 4 73 q 1h 4 65 d 1h 4 59 d 1h 4 40 m 1h 4 22 4 11 m 2h 3 74 s 6h 3 73 m 1h 3 65 s 2h 3 56 m 1h 3 47 3 21 m 6h 2 92 2 82 m h 2 31 t 2h 2 08 s 3h 1 38 d 3h etoposide 3 10 g 5 27 mmol dicyclohexylcarbodiimide dcc 1 76 g 8 53 mmol and methylthioacetic acid 792 mg 7 46 mmol was treated under nitrogen atmosphere with dry tetrahydrofuran 125 ml and the resulting colorless solution was stirred at room temperature for 100 h and additional methylthioacetic acid 180 mg was added after stirring at room temperature for 17 h additional dcc 250 mg was added and the reaction mixture was kept at room temperature for 48 h and then concentrated in vacuo the resulting white solid was treated with ethyl acetate 175 ml stirred for 5 min and filtered through a pad of celite the solids were washed with ethyl acetate 4 u00d7 75 ml and the combined filtrate was partitioned.