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.
