the major impurity was the exomethylene carbazolone which amounted to 2 6 of the crude ondansetron mixture the crude ondansetron mixture prepared as described in example 2 11 7 g was dissolved in methanol 468 ml at 62 u00b0 c the resulting solution was stirred over charcoal 0 6 g at 62 u00b0 c for 15 min and then the charcoal was filtered off an approximately 100 ml portion of the methanol was distilled off under reduced pressure the solution was then cooled to 0 u2013 5 u00b0 c and stirred for an hour precipitated crystals were filtered off and washed twice with cold 0 u2013 5 u00b0 c methanol 7 3 ml crystals were dried under reduced pressure at 65 u00b0 c to give ondansetron u02dc 81 yield in an hplc purity of 99 6 and with 0 25 contamination by the exomethylene carbazolone crystallized ondansetron prepared as described in example 3 9 5 g was dissolved in methanol 380 ml at 62 u00b0 c the solution was stirred over charcoal at 62 u00b0 c for 15 min and then the charcoal was filtered off an approximately 95 ml portion of methanol was distilled off the solution was then cooled to 0 u2013 5 u00b0 c and stirred for an hour precipitated crystals were filtered off and washed twice with cold 0 u2013 5 u00b0 c methanol 5 9 ml the crystals were then dried under reduced pressure at 65 u00b0 c to give ondansetron u02dc 81 yield in 99 9 hplc purity and with.