The present invention relates to compounds of formula I or II:—wherein R is an alkyl, alkenyl, alkynyl, aryl, aralkyl, aralkenyl, or aralkynyl group, that may be substituted or unsubstituted, and that optionally includes at least one heteroatom in its carbon skeleton; R1 is hydrogen, or an alkyl, substituted alkyl, alkenyl, alkynyl, aryl, substituted aryl, or aralkyl group; and R2 is an alkyl, alkenyl, alkynyl, aryl, aralkyl, aralkenyl, or aralkynyl group containing 4-12 carbon atoms, that may be substituted or unsubstituted, and that optionally includes at least one heteroatom in its carbon skeleton; provided that in a compound of formula I, when R1 is an iso-propyl or phenyl group, R2 is not an acetyl or tert-butyldimethylsilyl group; and their use in therapeutic, diagnostic and research methods.