Asymmetrische Katalyse, 67. Diastereoselektive Hydrierung der Folsäure mit optisch aktiven Rhodium(I)‐Diphosphan‐Komplexen

Asymmetric Catalysis, 67 [1] . – Diastereoselective Hydrogenation of Folic Acid with Optically Active Rhodium(I)‐Diphosphane Complexes With immobilized rhodium(I)‐diphosphane catalysts supported on silica gel, the C = N bonds of the pyrazine ring of folic acid ( 1 ) are reduced with hydrogen in aqueous solution to give 5,6,7,8‐tetrahydrofolic acid. A mixture of the two diastereomers 2a and 3a is obtained with (6 S )‐ and (6 R )‐configuration, respectively, at the newly formed asymmetric center in the pterine system and ( S )‐configuration in the L‐glutamic acid moiety. The unstable hydrogenation products are derivatized with (‐)‐menthyl chloroformate. An improved HPLC procedure for the analysis of the products has been developed. By using optically active chelate phosphanes as cocatalysts together with [Rh(cod)Cl] 2 , a diastereomeric excess of up to 24% of the natural isomer 2a with (6 S,S )‐configuration is attained in the heterogeneous hydrogenation of folic acid.