The present invention relates to an improved process for preparing
compounds of formula 1 on an industrial scale, ##STR00001## wherein
R.sup.1, R.sup.2 and R.sup.3 are independently C.sub.1-6-alkyl,
C.sub.2-6-alkenyl, C.sub.2-6-alkynyl or aryl; and R.sup.4 is H,
OC.sub.1-6-alkyl, Oaryl, OH, C.sub.1-6-alkyl, halogen, CN or NO.sub.2.