Abstract: Compounds of the general structure ##STR1## are disclosed, wherein X is O, C.sub.n, NH, or S, wherein n is 1, 2, 3 or 4; R1 is OH, CH.sub.3, CH.sub.2 OH, N.sub.3 or CH.sub.2 N.sub.3 ; R3 is H or CH.sub.3 ; R5 is Y-R2, wherein Y is a six-carbon chain optionally containing up to three double or triple bonds or a mixture of double and triple bonds up to a maximum of three; R2 is C.sub.1 -C.sub.10 alkyl OH, C.sub.1 -C.sub.10 alkyl N.sub.3 or COOR4, wherein R4 is H, a branched or unbranched C.sub.1 -C.sub.10 alkyl (including substituted alkyl radicals), cycloalkyl, preferably C.sub.5 or C.sub.6 cycloalkyl, or a five- or six-membered aryl radical (including substituted aryl radicals), i.e. R2 is COOH or an ester of R4; R6 is a seven-carbon chain optionally containing up to three double or triple bonds or a mixture of double and triple bonds up to a maximum of three; and . . . . . . indicates a single, double or triple bond.