A compound of formula (I) wherein n is 0 to 4； m is 0 or 1 with the proviso that the sulphur atom and R3 are in vicinal position (if m=0 then R3 is in position 2′；, and if m=1 then R3 is on position 1′；)； R is ethyl or vinyl； R1 is hydrogen or (C1-6)alkyl； R2 is hydrogen or —；(C3-6)cycloalkyl, or —；unsubstituted (C1-6)alkyl, or —；(C1-6)alkyl substituted by one or more of —；hydroxy； preferably one or two, —；methoxy, —；halogen, —；(C3-6)cycloalkyl, or R1 and R2 together with the nitrogen atom to which they are attached form a 5 to 7 membered heterocyclic ring containing at least 1 nitrogen atom or 1 nitrogen and 1 additional heteroatome e.g. selected from N or O, or R1 is hydroxy and R2 is formyl； R3 is OH, OR4, a halogen atom, or —；with the proviso that R3 is bound to 2′； R3 represents —；O—；(CH2)P—；O—； with p is 2 or 3； R4 is unsubstituted (C1-6)alkyl or (C3-6)cycloalkyl.
