Thermal and photochemical epimerization/equilibration of carbohydrate cobaloximes

Epimeric carbohydrate alkyl cobaloximes 4:5, 9:10, and 12:13 can be equilib rated thermally or photochemically. In each case, one isomer is strongly fa vored: exo-3-deoxy-3-pyridyldimethylgly-oximatocobalt-1,2:5,6-di-O-isopropy lidene-alpha -D-glucofuranose 4 for the 4:5 epimer pair, exo-3-deoxy-3-pyri dyldimethylglyoximatocobalt-5-O-carboxymethyl-1,2-O-isopropylidene-alpha -D -xylofuranose 9 for the 9:10 epimer pair, and equatorial 1-deoxy-1-pyridyld imethylglyoximatocobalt-2,3,4,6-tetra-O-benzyl-beta -D-glucopyranose 12 for the 12:13 epimer pair. These data indicate that there is a strong facial p reference for the coupling of py(dmgH)(2)Co-. radicals with alkyl R-. free radicals, with the preferred kinetic path leading to the more stable produc t.