Cleavage  of  five  different  nucleoside  diphos-

phosugars  has  been  studied  in  the  presence  of  Cu 2 +  and

Zn 2 + complexes. The results show that metal ion catalysts

promote  the  cleavage  via  intramolecular  transesterifica -

tion whenever a neighbouring HO group can adopt a cis-

orientation  with  respect  to  the  phosphate. The  HO  group

attacks the phosphate and two monophosphate products are

formed. If such a nucleophile is not available, Cu 2 + com-

plexes are able to promote a nucleophilic attack of an exter-

nal nucleophile, e.g. a water molecule or metal ion coor-

dinated HO ligand, on phosphate. With the Zn 2 + complex,

this was not observed.