Polymyxin derivatives as antimicrobial compounds

A compound of formula (IIb):

• wherein:
• R1 is selected from hexanoyl, hepatanoyl, octanoyl, nonanoyl, decanoyl, dodecanoyl, 7 methyloctanoyl, S-5-methylheptanoyl, R-5-methylheptanoyl, S,R-5-methylheptanoyl (racemic mixture), 4-biphenylcarboxyl, 4-trifluoromethylbenzoyl, 4-ethylbenzoyl, 3,4-dichlorobenzoyl, 4-chlorobenzoyl, 3-chlorobenzoyl, pentafluorobenzoyl, 4-methylbenzoyl, 4-ethylphenylacetyl, phenylacetyl, 4-methylphenylacetyl, 4-trifluoromethylphenylacetyl, pentafluorophenylacetyl, 3,4-dichlorophenylacetyl, 4-chlorophenylacetyl, 3-chlorophenylacetyl, 2-chlorobenzoyl, 2-fluorobenzoyl, 2-methylbenzoyl, 2-chlorophenylacetyl, 2-fluorophenylacetyl, 2-methylphenylacetyl, 2,3-dichlorobenzoyl, 2,3-dimethylbenzoyl, 2,4-dichlorophenylacetyl, 2,4-dichlorobenzoyl, 2,4-dimethylbenzoyl, 2-chloro-4-methylbenzoyl, 2-chloro-4-trifluoromethylbenzoyl, 3-fluorobenzoyl, 3-methylbenzoyl, 3-trifuoromethylbenzoyl, 3,4-dimethylbenzoyl, 3-fluoro-4-methylbenzoyl, 4-chloro-3-methylbenzoyl, 3,4-dimethylphenylacetyl, 3-chloro-4-methylbenzoyl, 4-chloro-3-fluorobenzoyl, 3-fluoro-4-trifluoromethylbenzoyl, 3-chloro-4-fluorobenzoyl, 4-methyl-3-trifluoromethylbenzoyl, 3-methyl-4-trifluoromethylbenzoyl, 3-methyl-5-trifluoromethylbenzoyl, 3,5-dimethylbenzoyl, 3,5-dichlorobenzoyl, 3,5-bis(trifluoromethyl) benzoyl, 3-fluoro-5-trifluoromethylbenzoyl, 3-chloro-5-methylbenzoyl, 3-chloro-5-fluorobenzoyl, 2,4,6-trimethylbenzoyl, 2,4,6-trichlorobenzoyl, 2-chloro-4-fluorobenzoyl, 4-chloro-2-fluorobenzoyl, 3,4,5-trifluoromethylbenzoyl, 4-chloro-2-trifluoromethylbenzoyl, 2-fluoro-4-trifluoromethylbenzoyl, 3-biphenylcarboxyl, 4-chlorobiphenyl-4-carboxyl, 3-phenylproponyl, 4-phenylbutanoyl, 2,4-dichlorophenylsulfonyl, 4-chloro-3-trifluoromethylbenzoyl, 4-isopropylbenzoyl, 4-chloro-3-fluorobenzoyl, and 3-chloro-4-trifluoromethylbenzoyl;
• R3 represents a side chain of an amino acid selected from leucine, phenylalanine, norleucine, norvaline or t-butylglycine;
• R4 represents a side chain of an amino acid selected from alanine, threonine, serine, valine, t-butylglycine, 2-aminobutyric acid or 2-aminoisobutyric acid;
• and
• k and q are individually selected from 1, 2, or 3;
• or
• pharmaceutically acceptable salts thereof.