in toluene to a soln. of O(B(OAc)2)2 benzothiazoline (molar ratio 1:2) was aded; amixture was refluxed for 27 h, acetic acid liberated being removed azeotripically; concn., residue was dried at 60-65°C/0.05 torr, repeatedly washed with dry hexane; elem. anal.;