The compounds illustrated above have the general formula [I] wherein R3 and R4 are hydrogen atoms, R5 is methyl, Y is --CO-- and n is 2, but the compounds having the general formula [I] wherein R3 is alkoxy having 1 to 3 carbon atoms, R4 is alkyl having 1 to 3 carbon atoms, R5 is hydrogen or alkyl other than methyl, Y is --CO-- and n is 3 or 4 and the others are as herein defined, for example,