Compounds of formula (I):
wherein:
X represents an oxygen atom or a sulphur atom,
R
1
, R
2
, R
3
and R
4
are as defined in the description,
A represents an alkylene chain as defined in the description, and
B represents an alkyl or alkenyl group substituted by a group
or R
7
, or B represents a group
or R
7
.
化合物式(I):其中:X代表氧原子或
硫原子,R1、R2、R3和R4如描述中所定义,A代表如描述中所定义的烷基链,B代表被基团或R7取代的烷基或烯基,或B代表基团或R7。