The present invention relates to a compound of formula (I) wherein (i) when a and c are double bonds, R2 is absent, b is a single bond, R1 is formula (II) or (ii) when a and c are single bonds, b is a double bond, and R1 is H, R2 is formula (III) wherein X is straight or branched chain C¿1-6? alkyl, -(CH2)nAr, C1-6 cycloalkyl, or (CH2)nR', where R' is a cyclic hydrocarbyl group; Y is a natural or unnatural amino acid side chain; W is OH or NHR?3¿, wherein R3 is CH(Y')CO¿2?X', where X' and Y' are defined as for X and Y respectively, and may be the same or different to X and Y respectively; and Z1 and Z2 are each independently H, straight or branched chain C1-6 alkyl, straight or branched chain C1-6 alkenyl, -(CH2)nAr, -(CH2)n-CO2R', -(CH2)p-CH=CH-(CH2)qAr where p and q are each independently O to 5, R' is C1-6 alkyl; and each n may be the same or different and is from 1 to 5.