Disclosed are compounds of Formula (I), crystalline forms thereof, and methods for their preparation. Specific compounds of formula (I) include: [(2R)-3-{ 2-[6-(1-Azetidinyl)-5-fluoro-2-methyl-4-pyrimidinyl]hydrazino} -2-(cyclopentylmethyl)-3-oxopropyl]hydroxyformamide, 6-[(4S)-4-hydroxy-3,3-dimethyl-1-pyrrolidinyl]-2-methyl-4-pyrimidinyl} hydrazino)-3-oxopropyl]hydroxyformamide, and [(2R)-2-(Cyclopentylmethyl)-3-(2-{ 5-fluoro-6-[(9aS)-hexahydropyrazino[2,1-c][1,4]oxazin-8(1H)-yl]-2-methyl-4-pyrimidinyl} hydrazino)-3-oxopropyl]hydroxyformamide. Compounds of formula (I) are useful for treating bacterial infections, particularly where the bacterial infection is caused by Streptococcus, Staphylococcus, Moraxella, Haemophilus, Neisseria, Mycoplasma, Legionella, Chlamydia, Bacteroides, Clostridium, Fusobacterium, Propionibacterium, or Peptostreptococcus. Also disclosed are three different crystalline forms of [(2R)-2-(cyciopentylmethyl)-3-(2-{ 5-fluoro-6-[(9aS)-hexahydropyrazino[2,1-c][1,4]oxazin-8(1H)yl]-2-methyl-4-pyrimidinyl} hydrazino)-3-oxopropyl]hydroxyformamide. The crystalline polymorphs are characterised with infra-red spectroscopy, X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry
