Pharmaceutical compounds

A compound of the formula (1):

• or a salt thereof, wherein:
• X1 and X2 are saturated hydrocarbon groups which together contain a total of five to nine carbon atoms and zero or one oxygen atoms and which link together such that the moiety:
• forms a monocyclic or bicyclic ring system optionally substituted with one or more fluorine atoms;
• R1 is selected from OR5;
• NR5R6;
• COR5;
• COOR5;
• CONR5R6;
• CONR5OR6;
• C(═NR5)R6;
• C(═NOR5)R6;
• OCOR5;
• NR7COR5;
• NR7CONR5R6;
• NR7COOR5;
• OCONR5R6;
• CH2OR5;
• CH2NR5R6;
• CH2COR5;
• CH2COOR5;
• CH2CONR5R6;
• CH2CONR5OR6;
• CH2C(═NR5)R6;
• CH2C(═NOR5)R6;
• CH2OCOR5;
• CH2NR7COR5;
• CH2NR7CONR5R6;
• CH2NR7COOR5;
• CH2OCONR5R6;
• a C1-6 non-aromatic hydrocarbon group which is optionally substituted with one to six fluorine atoms and wherein one or two, but not all, carbon atoms of the hydrocarbon group may optionally be replaced by a heteroatom selected from O, N and S and oxidized forms thereof;
• and an optionally substituted 4, 5- or 6-membered ring containing 0, 1, 2 or 3 heteroatoms selected from O, N and S and oxidized forms thereof;
• R4 is H or a C1-6 non-aromatic hydrocarbon group which is optionally substituted with one to six fluorine atoms and wherein one or two, but not all, carbon atoms of the hydrocarbon group may optionally be replaced by a heteroatom selected from O, N and S and oxidised forms thereof;
• and
• R5, R6 and R7 are the same or different and each is independently selected from hydrogen, a non-aromatic C1-6 hydrocarbon group optionally substituted with one or more fluorine atoms or optionally substituted with a 4, 5- or 6-membered ring containing 0, 1, 2 or 3 heteroatoms selected from O, N and S and oxidized forms thereof, or R5 and R6 can be joined together to form an optionally substituted monocyclic or bicyclic ring containing 0, 1, 2 or 3 heteroatoms selected from O, N and S and oxidized forms thereof.