on atoms; R' is R, SiR3 or --CO(NR)--R', with R' being R or SiR3, with the proviso that R' may together form with R a single alkylene radical having from 4 to 6 carbon atoms; and R is an alkyl or alkenyl radical having from 1 to 4 carbon atoms, a cycloalkyl or cycloalkenyl radical having from 4 to 6 nuclear carbon atoms, a phenyl, tolyl or xylyl radical, or hydrogen if R' is not an amide.