A BORONIC ACID-DIOL INTERACTION IS USEFUL FOR CHIROSELECTIVE TRANSCRIPTION OF THE SUGAR STRUCTURE TO THE DELTA-[CO-III(BPY)(3)](3-[CO-III(BPY)(3)](3+) RATIO() VERSUS LAMBDA)

In order to apply boronic acid-saccharide interactions to the chirosel ective synthesis of Delta- and Lambda-[Co-III(bpy)(3)](3+) saccharide- binding ligands, 2,2'-bipyridine-4-boronic acid (bpymb) and 2,2'-bipyr idine-4,4'-diboronic acid (bpydb) were newly synthesized. It was shown that most D-saccharides form cyclic 1 : 1 complexes with bpydb to aff ord the CD-active species. The positive exciton coupling band implies that two pyridine rings are twisted in a clockwise direction ((R)-chir ality). In contrast, such a CD-active species was not yielded from bpy mb. The treatment of the bpydb-D-saccharide complexes with Co(OAc)(2) gave the substitution-active [Co-II(bpyba)(3)](4-)-saccharide complexe s, which were oxidized to the substitution-inactive [Co-III(bpyba)(3)] (3-)-saccharide complexes. In this stage, the Delta vs, Lambda ratio w as fixed. The complexes were converted to [Co-III(bpy)](3+) by treatme nt with AgNO3, and the e.e. was determined by comparison with authenti c Delta- or Lambda-[Co-III(bpy)](3+). The Delta-isomer was obtained in excess from most D-saccharides but the h-isomer was also obtained fro m D-fructose and D-fucose. At 4 degrees C, the largest e.e. for bpydb was attained with D-glucose (47% e.e.; Delta excess). Under the same r eaction conditions the bpymb + D-glucose system gave 16% e.e. (Delta e xcess). The e.e. of the bpydb + D-glucose system increased with loweri ng the reaction temperature and at -25 degrees C it reached 79% e.e. T he foregoing results clearly establish that the saccharide-templated s ynthesis is useful as a new concept for the preparation of chiral tris (2,2'-bipyridine)-metal complexes. Furthermore, the Delta vs. A equili brium can be shifted in either direction by the selection of saccharid e enantiomers.