THF (catalyst) in tetrahydrofuran; anhydrous butyl ether under N2, educts heated under reflux in n-Bu2O-THF for 21 h in the dark; cooled soln. filtered through Kieselguhr, evapd. in vac., solid dissolved in acetone, filtered, pptd. by addn. of light petroleum, recrystd. from acetone/light petroleum; elem. anal.;