1-inyl; a C1-6 -3-alkanoyloxyprop-1-inyl, C1-6 -3-alkanoyloxyprop-1-enyl or C1-6 -3-alkanoyloxypropyl group; or a 3-hydroxyprop-1-enyl or 3-hydroxypropyl group; and R5 is H; or B) R1 is a methyl or ethyl group; R2 is a methoxy, thiomethyl, cyano, formyl, acetyl group or a 1-alkoxyethyl group having 1 to 6 carbon atoms in the alkoxy radical; R3 is an alkoxymethyl, alkoxy, 1-alkoxyethyl, alkoxymethy