Contrast agents for enhanced magnetic resonance imaging

A compound of the formula (IIa), (IIb), (Ia) or (Ib):

• or a salt thereof,
• wherein:
• R1 and R2 are each independently alkyl, arylalkyl or aryl;
• R11-R13 are each independently H, alkyl, a monosaccharide, a polysaccharide (e.g., a disaccharide, trisaccharide, and the like) or a polypeptide;
• R14-R16 are each independently H, alkyl or R14 and R15 or R15 and R16, together with the oxygen atoms to which they are attached, form a heterocyclyl group;
• and
• X2 is OH or NHR10, wherein R10 is H or acyl;
• or
• or a salt thereof,
• wherein:
• R1 and R2 are each independently alkyl, arylalkyl or aryl;
• R3, R4, and R6 are each independently alkyl, a protecting group, a monosaccharide, a polysaccharide (e.g., a disaccharide, trisaccharide, and the like) or a polypeptide;
• R7, R8, and R9 are each independently alkyl or R7 and R8 or R8 and R9, together with the oxygen atoms to which they are attached, form a heterocyclyl group;
• and
• X1 is OR5, wherein R5 is H, a protecting group, a monosaccharide, a polysaccharide (e.g., a disaccharide, trisaccharide, and the like), a polypeptide or NHR10, wherein R10 is H or acyl;