A compound of the formula 1 ##STR11## having the E configuraiton wherein R1 and R2 may be the same or different, provided that, when R1 and R2 are the same, each of them is a methyl or ethyl radical, and when R1 and R2 are different, one of them is hydrogen and the other is a methyl or ethyl radical; or a non-toxic, pharmaceutically acceptable salt thereof.