A compound of formula (I)R-L-CO—X (I)wherein R is a C10-24 unsaturated hydrocarbon group optionally interrupted by one or more heteroatoms or groups of heteroatoms selected from S, O, N, SO, SO2, said hydrocarbon group comprising at least 4 non-conjugated double bonds；L is a linking group forming a bridge of 1 to 5 atoms between the R group and the carbonyl CO wherein L comprises at least one heteroatom in the backbone of the linking group； andX is an electron withdrawing group or a salt thereoffor use in the treatment of skin cancer such as basal cell carcinoma.
