A method of preparing a compound of formula (III): (M)x-(S)y-(M′)z  (III) wherein x is from 1 to 6, y is from 1 to 12, z is from 0 to 10, each M and each M′ is independently a mycolic acid residue including a β-hydroxy acid moiety and each S is a monosaccharide unit; the method comprising reacting one or more mycolic acids with one or more saccharide units wherein the hydroxyl group of each β-hydroxy acid moiety is protected prior to reaction with the one or more saccharide units.