abstract |
Compounds of formula (I): n n n n n n n n n n wherein:n 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 nn n n n n n n n n n or R 7 , or B represents a group n n n n n n n n n n or R 7 . |