Resolution of racemic cis-ketoacid (I) by precipitation with (R)-(+)-alpha-methylbenzylamine (II) in EtOH, followed by recrystallization of the corresponding salt from EtOH and final acidic treatment, allows isolation of (+)-ketoacid (IV). Finally, cyclization of (IV) with 2-adamantylhydrazine (V) gives rise to the title phthalazinone.