Among the compounds of formula (1), those in which at least one of R1 and R2 is an alkyl group, especially having 1 to 10 carbon atoms, above all 1 to 5 carbon atoms, or R1 and R2 are bonded together to form a trimethylene group or a group of the formula ##STR13## wherein R3 and R4 are as defined above, and preferably represent an alkyl group having an alkyl group with 1 to 10 carbon atoms, especi