We have found that ninhydrin reacted with N-hydroxyurea and N-alkoxyureas in acetic acid with the predominant formation of diastereomers of 1,3a,8a-trihydroxy-1,3,3a,8a-tetrahydroindeno[1,2-d]imidazole-2,8-dione and 1-alkoxy-3a,8a-dihydroxy-1,3,3a,8a-tetrahydroindeno[1,2-d]imidazole-2,8-diones, respectively. with sic-orientation of 3a,8a-HO-groups. The X-ray structural analysis of 1,3aS,8aR-trihydroxy-1,3,3a,8a-tetrahydroindeno[1,2-d]imidazole-2,8-dione and 1-n-butyloxy-3aS,8aR-dihydroxy-1,3,3a,8a-tetrahydro-indeno[1,2-d]imidazole-2,8-dione has demonstrated their specific structural features.