Antibacterial compounds

Abstract

Bacterial protein synthesis-inhibiting compounds having formula (I) and salts, prodrugs, salts of prodrugs, and metabolites thereof, processes for making the compounds and intermediates in the processes, compositions containing the compounds, and methods of using the compounds are disclosed.

1 . A compound having formula (I)  
       
         
           
           
               
               
           
         
       
       or a salt thereof, in which 
 R 1  is —OH, —OR 7 , —NH 2 , —NHR 7 , or —N(R 7 ) 2 ;  
 R 2  is hydrogen or R P , in which R P  is —C(CH 3 ) 3 , —O(CH 2 CH═CH 2 ), or (2,4-dimethoxyphenyl)methyl;  
 R 3  is hydrogen, R 8 , —C(O)R 8 , —C(O)OR 8 , R 9 , —C(O)OCH 2 R 9 , or R 10 ;  
 R 4  is hydrogen, R 11 , —C(O)R 11 , R 12 , R 13 , R 14 , or R 15 ;  
 R 5  is hydrogen, R 16 , —C(O)R 16 , R 17 , R 18 , R 19 , or R 20 ;  
 R 6  is hydrogen, R 21 , —OH, —OR 21 , —NH 2 , —NHR 21 , —N(R 21 ) 2 ;  
 R 7  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 8  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 8a ; R 8a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 8b , —NH 2 , —NHR 8b , —N(R 8b ) 2 , or R 8c  substituents; R 8b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 8c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 8d , —F, —Cl, —Br, —I, —OH, —OR 8d , —NO 2 , —NH 2 , —NHR 8d , or —N(R 8d ) 2  substituents; R 8d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 9  is phenyl which is unfused or fused with benzene, cyclopentane, cyclopentene, cyclohexane, or cyclohexene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 9a , —F, —Cl, —Br, —I, —OH, —OR 9a , —NO 2 , —NH 2 , —NHR 9a , or —N(R 9a ) 2  substituents; R 9a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 10  is furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 10a , —F, —Cl, —Br, —I, —OH, —OR 10a , —NO 2 , —NH 2 , —NHR 10a , or —N(R 10a ) 2  substituents; R 10a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 11  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 11a ; R 11a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 11b , —NH 2 , —NHR 11b , —N(R 11b ) 2 , or R 11c  substituents; R 11b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 11c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 11d , —F, —Cl, —Br, —I, —OH, —OR 11d , —NO 2 , —NH 2 , —NHR 11d , or —N(R 11d ) 2  substituents; R 11d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 12  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, or R 12a ; R 12a  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 12b , —NH 2 , —NHR 12b , —N(R 12b ) 2 , or R 12c  substituents; R 12b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 12c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 12d , —F, —Cl, —Br, —I, —OH, —OR 12d , —NO 2 , —NH 2 , —NHR 12d , or —N(R 12d ) 2  substituents; R 12d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 13  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 13a ; R 13a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 13b , —NH 2 , —NHR 13b —N(R 13b ) 2 , or R 13c  substituents; R 13b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 13c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 13d , —F, —Cl, —Br, —I, —OH, —OR 13d , —NO 2 , —NH 2 , —NHR 13d , or —N(R 13d ) 2  substituents; R 13d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 14  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 14a ; R 14a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 14b , —NH 2 , —NHR 14b , —N(R 14b ) 2 , or R 14c  substituents; R 14b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 14c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 14d , —F, —Cl, —Br, —I, —OH, —OR 14d , —NO 2 , —NH 2 , —NHR 14d , or —N(R 14d ) 2  substituents; R 14d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 15  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 15a , —F, —Cl, —Br, —I, —OH, —OR 15a , —NO 2 , —NH 2 , —NHR 15a , or —N(R 15a ) 2  substituents; R 15a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 16  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 16a ; R 16a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 11b , —NH 2 , —NHR 11b , —N(R 11b ) 2 , or R 16c  substituents; R 16b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 16c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 16d , —F, —Cl, —Br, —I, —OH, —OR 16d , —NO 2 , —NH 2 , —NHR 16d , or —N(R 16d ) 2  substituents; R 16d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 17  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, or R 17a ; R 17a  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 17b , —NH 2 , —NHR 17b , —N(R 17b ) 2 , or R 17c  substituents; R 17b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 17c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 17d , —F, —Cl, —Br, —I, —OH, —OR 17d , —NO 2 , —NH 2 , —NHR 17d , or —N(R 17d  d) 2  substituents; R 17d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 18  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 18a ; R 18a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 18b , —NH 2 , —NHR 18b , —N(R 18b ) 2 , or R 18c  substituents; R 18b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 18c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 18d , —F, —Cl, —Br, —I, —OH, —OR 18d , —NO 2 , —NH 2 , —NHR 18d , or —N(R 18d ) 2  substituents; R 18d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 19  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 19a ; R 19a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 19b , —NH 2 , —NHR 19b , —N(R 19b ) 2 , or R 19c  substituents; R 19b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 19c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 19d , —F, —Cl, —Br, —I, —OH, —OR 19d , —NO 2 , —NH 2 , —NHR 19d , or —N(R 19d ) 2  substituents; R 19d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 20  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 20a , —F, —Cl, —Br, —I, —OH, —OR 20a , —NO 2 , —NH 2 , —NHR 20a , or —N(R 20a ) 2  substituents; R 20a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 21  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 21a ; R 21a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 21b , —NH 2 , —NHR 21b , —N(R 21b ) 2 , or R 21c  substituents; R 21b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 21c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 21d , —F, —Cl, —Br, —I, —OH, —OR 21d , —NO 2 , —NH 2 , —NHR 21d , or —N(R 21d ) 2  substituents; R 21d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 X 1  is —O—, —S—, —S(O)—, —SO 2 —, —NH—, or —NR 22 —;  
 R 22  is R 23 , —OH, —OR 23 , —C(O)R 23 , —C(O)OR 23 , —C(O)OCH 2 R 24 , —R 25 , or —CH 2 R 25 ;  
 R 23  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 23a ; R 23a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 23b , —NH 2 , —NHR 23b , —N(R 23b ) 2 , or R 23c  substituents; R 23b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 23c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 23d , —F, —Cl, —Br, —I, —OH, —OR 23d , —NO 2 , —NH 2 , —NHR 23d , or —N(R 23d ) 2  substituents; R 23d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;  
 R 24  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 24a ; R 24a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 24b , —NH 2 , —NHR 24b , —N(R 24b ) 2 , or R 24c  substituents; R 24b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 24c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 24d , —F, —Cl, —Br, —I, —OH, —OR 24d , —NO 2 , —NH 2 , —NHR 24d , or —N(R 24d ) 2  substituents; R 24d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; and  
 R 25  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unfused or fused with benzene, furan, imidazole, isothiazole, isoxazole, 1,2,3-oxadiazole, oxazole, pyrazine, pyrazole, pyridazine, pyridine, pyrrole, thiazole, or thiophene, in which each ring is independently unsubstituted or substituted with one or two or three independently selected R 25a , —F, —Cl, —Br, —I, —OH, —OR 25a —NO 2 , —NH 2 , —NHR 25a , or —N(R 25a ) 2  substituents; R 25a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl.  
 
     
     
         2 . A compound of  claim 1  having formula (I), or a salt thereof, in which R 1  is —OH, —OR 7 , —NH 2 , —NHR 7 , or —N(R 7 ) 2 ; 
 R 2  is hydrogen or R P , in which R P  is —C(CH 3 ) 3 , —O(CH 2 CH═CH 2 ), or (2,4-dimethoxyphenyl)methyl;    R 3  is hydrogen, R 8 , —C(O)R 8 , —C(O)OR 8 , R 9 , —C(O)OCH 2 R 9 , or R 10 ;    R 4  is hydrogen, R 11 , —C(O)R 11 , R 12 , R 13 , R 14 , R 15 ;    R 5  is hydrogen, R 16 , —C(O)R 16 , R 17 , R 18 , R 19 , or R 20 ;    R 6  is hydrogen, R 21 , —OH, —OR 21 , —NH 2 , —NHR 21 , —N(R 21 ) 2 ;    R 7  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 8  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 8a ; R 8a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 8b , —NH 2 , —NHR 8b , —N(R 8b ) 2 , or R 8c  substituents; R 8b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 8c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 8d , —F, —Cl, —Br, —I, —OH, —OR 8d , —NO 2 , —NH 2 , —NHR 8d , or —N(R 8d ) 2  substituents; R 8d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 9  is phenyl which is unsubstituted or substituted with one or two or three independently selected R 9a , —F, —Cl, —Br, —I, —OH, —OR 9a , —NO 2 , —NH 2 , —NHR 9a , or —N(R 9a ) 2  substituents; R 9a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 10  is furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 10a , —F, —Cl, —Br, —I, —OH, —OR 10a , —NO 2 , —NH 2 , —NHR 10a , or —N(R 10a ) 2  substituents; R 10a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 11  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 11a ; R 11a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 11b , —NH 2 , —NHR 11b , —N(R 11b ) 2 , or R 11c  substituents; R 11b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 11c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 11d , —F, —Cl, —Br, —I, —OH, —OR 11d , —NO 2 , —NH 2 , —NHR 11d , or —N(R 11d ) 2  substituents; R 11d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 12  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, or R 12a ; R 12a  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 12b , —NH 2 , —NHR 12b , —N(R 12b ) 2 , or R 12c  substituents; R 12b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 12c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 12d , —F, —Cl, —Br, —I, —OH, —OR 12d  —NO 2 , —NH 2 , —NHR 12d , or —N(R 12d ) 2  substituents; R 12d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 13  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 13a ; R 13a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 13b , —NH 2 , NHR 13b , —N(R 13b ) 2 , or R 13c  substituents; R 13b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 13c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 13d , —F, —Cl, —Br, —I, —OH, —OR 13d , —NO 2 , —NH 2 , —NHR 13d , or —N(R 13d ) 2  substituents; R 13d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 14  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 14a ; R 14a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 14b , —NH 2 , —NHR 14b , —N(R 14b ) 2 , or R 14c  substituents; R 14b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 14c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 14d , —F, —Cl, —Br, —I, —OH, —OR 14d , —NO 2 , —NH 2 , —NHR 14d , or —N(R 14d ) 2  substituents; R 14d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 15  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 15a , —F, —Cl, —Br, —I, —OH, —OR 15a —NO 2 , —NH 2 , —NHR 15a , or —N(R 15a ) 2  substituents; R 15a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 16  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 16a ; R 16a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 16b , —NH 2 , —NHR 16b , —N(R 16b ) 2 , or R 16c  substituents; R 16b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 16c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 16d , —F, —Cl, —Br, —I, —OH, —OR 16d , —NO 2 , —NH 2 , —NHR 16d , or —N(R 16d ) 2  substituents; R 16d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 17  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, or R 17a ; R 17a  is C 2 -alkenyl, C 3 -alkenyl, C 4 -alkenyl, C 5 -alkenyl, C 6 -alkenyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 17b , —NH 2 , —NHR 17b , —N(R 17b ) 2 , or R 17c  substituents; R 17c  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 17c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 17d , —F, —Cl, —Br, —I, —OH, —OR 17d , —NO 2 , —NH 2 , —NHR 17d , or —N(R 17d ) 2  substituents; R 17d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 18  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 18a ; R 18a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 18b , —NH 2 , —NHR 18b , —N(R 18b ) 2 , or R 18c  substituents; R 18b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 18c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 18d , —F, —Cl, —Br, —I, —OH, —OR 18d , —NO 2 , —NH 2 , —NHR 18d , or —N(R 18d ) 2  substituents; R 18d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 19  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkenyl, or R 19a ; R 19a  is C 2 -alkynyl, C 3 -alkynyl, C 4 -alkynyl, C 5 -alkynyl, C 6 -alkynyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 19b , —NH 2 , —NHR 19b —N(R 19b ) 2 , or R 19c  substituents; R 19b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 19c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 19d , —F, —Cl, —Br, —I, —OH, —OR 19d , —NO 2 , —NH 2 , —NHR 19d , or —N(R 19d ) 2  substituents; R 19d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 20  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 20a , —F, —Cl, —Br, —I, —OH, —OR 20a , —NO 2 , —NH 2 , —NHR 20a , or —N(R 20a ) 2  substituents; R 20a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 21  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 21a ; R 21a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 21b , —NH 2 , —NHR 21b , —N(R 21b ) 2 , or R 21c  substituents; R 21b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 21c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 21d , —F, —Cl, —Br, —I, —OH, —OR 21d , —NO 2 , —NH 2 , —NHR 21d , or —N(R 21d ) 2  substituents; R 21d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    X 1  is —O—, —S—, —S(O)—, —SO 2 —, —NH—, or —NR 22 —;    R 22  is R 23 , —OH, —OR 23 , —C(O)R 23 , —C(O)OR 23 , —C(O)OCH 2 R 24 , —R 25 , or —CH 2 R 25 ;    R 23  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 23a ; R 23a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 23b , —NH 2 , —NHR 23b , —N(R 23b ) 2 , or R 23c  substituents; R 23b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 23c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 23d , —F, —Cl, —Br, —I, —OH, —OR 23d , —NO 2 , —NH 2 , —NHR 23d , or —N(R 23d ) 2  substituents; R 23d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl;    R 24  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 24a ; R 24a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 24b , —NH 2 , —NHR 24b , —N(R 24b ) 2 , or R 24c  substituents; R 24b  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 24c  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 24d , —F, —Cl, —Br, —I, —OH, —OR 24d , —NO 2 , —NH 2 , —NHR 24d , or —N(R 24d ) 2  substituents; R 24d  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; and    R 25  is phenyl, furanyl, imidazolyl, isothiazolyl, isoxazolyl, 1,2,3-oxadiazoyl, oxazolyl, pyrazinyl, pyrazolyl, pyridazinyl, pyridinyl, pyrrolyl, thiazolyl, thiophenyl, triazinyl, or 1,2,3-triazolyl, each of which is unsubstituted or substituted with one or two or three independently selected R 25a , —F, —Cl, —Br, —I, —OH, —OR 25a  —NO 2 , —NH 2 , —NHR 25a , or —N(R 25a ) 2  substituents; R 25a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl.    
     
     
         3 . A compound of  claim 2  having formula (I), or a salt thereof, in which R 1  is —OH, —OR 7 , —NH 2 , —NHR 7 , or —N(R 7 ) 2 ; R 2  is hydrogen or R P , in which R P  is —C(CH 3 ) 3 , —O(CH 2 CH═CH 2 ), or (2,4-dimethoxyphenyl)methyl; R 3  is hydrogen, R 8 , —C(O)R 8 , or —C(O)OR 8 ; R 4  is hydrogen; R 5  is hydrogen or R 20 ; R 6  is hydrogen; R 7  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 8  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 8a ; R 8a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 8b , —NH 2 , —NHR 8b , or —N(R 8b ) 2  substituents; R 20  is phenyl, furanyl, pyrrolyl, or thiophenyl, each of which is unsubstituted or substituted with one or two or three independently selected R 20a , —F, —Cl, —Br, —I, —OH, —OR 20a , —NO 2 , —NH 2 , —NHR 20a , or —N(R 20a ) 2  substituents; R 20a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; X 1  is —O—, —S—, —S(O)—, —SO 2 —, —NH—, or —NR 22 —; and R 22  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, C 6 -alkyl, or R 23a ; R 23a  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl, each of which is substituted with one or two or three independently selected —F, —Cl, —Br, —I, —CF 3 , —CF 2 CF 3 , —OH, —OR 23b , —NH 2 , —NHR 23b , or —N(R 23b ) 2  substituents.  
     
     
         4 . A compound of  claim 3  having formula (I), or a salt thereof, in which R 1  is —OH, —OR 7 , —NH 2 , —NHR 7 , or —N(R 7 ) 2 ; R 2  is hydrogen or R P , in which R P  is —C(CH 3 ) 3 , —O(CH 2 CH═CH 2 ), or (2,4-dimethoxyphenyl)methyl; R 3  is hydrogen, C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; R 4  is hydrogen; R 5  is hydrogen, phenyl, furanyl, pyrrolyl, or thiophenyl; R 6  is hydrogen; R 7  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl; X 1  is —O—, —S—, —S(O)—, —SO 2 —, —NH—, or —NR 22 —; and R 22  is C 1 -alkyl, C 2 -alkyl, C 3 -alkyl, C 4 -alkyl, C 5 -alkyl, or C 6 -alkyl.  
     
     
         5 . A compound of  claim 4  having formula (I), or a salt thereof, in which R 1  is —OH or —O(ethyl); R 2  is hydrogen or (2,4-dimethoxyphenyl)methyl; R 3  is methyl or ethyl; R 4  is hydrogen; R 5  is hydrogen or phenyl; and R 6  is hydrogen.  
     
     
         6 . A compound of  claim 5  having formula (I), or a salt thereof, which is 
 6-(2,4-dimethoxybenzyl)-4-methyl-9-oxo-2-phenyl-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid,    4-methyl-9-oxo-2-phenyl-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid 6-(2,4-dimethoxybenzyl)-4-methyl-9-oxo-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid,    4-methyl-9-oxo-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid,    6-(2,4-dimethoxybenzyl)-4-ethyl-9-oxo-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid, or    4-ethyl-9-oxo-3,4,6,9-tetrahydro-2H-[1,4]oxazino[3,2-b][1,8]naphthyridine-8-carboxylic acid.    
     
     
         7 . A composition for treating bacterial infections in a fish or a mammal, the compositions comprising a therapeutically effective amount of a compound having formula (I), or a salt thereof, and an excipient.  
     
     
         8 . A method for treating bacterial infections in a fish or a mammal, the method comprising administering thereto a therapeutically effective amount of compound having formula (I), or a salt thereof.