Über Pterinchemie. 69 Mitteilung [1]. Konformationsanalyse von 5,6,7,8‐Tetrahydropteroinsäure und 5,6,7,8‐Tetrahydro‐ L ‐folsäure

Conformational analysis of 5,6,7,8‐tetrahydropteroic acid and 5,6,7,8‐tetrahydro‐ L ‐folic acid In the 360‐MHz‐ 1 H‐NMR.‐spectrum of (6 R , S )‐9,9‐dideuterio‐5, 6, 7, 8‐tetrahydropteroic acid (racemic) (XIII) ( AMX ‐System, Fig. 4 ) and (6 R , S )‐9,9‐dideuterio‐5, 6, 7, 8‐tetrahydro‐ L ‐folic acid (diastereomeric) (XVI) the H a –C(6) and H a –C(7) show a vicinal coupling constant of 6,7 Hz and the H a –C(6) and H e –C(7) one of 3,2 Hz. The first coupling constant provides evidence for an approximate trans ‐diaxal arrangement of H a –C(6) and H a –C(7), and the second for a gauche conformation of H a –C(6) and H e –C(7). The tetrahydropyrazine ring in the racemic 5, 6, 7, 8‐tetrahydropteroic acid (III) and in the diastereomeric 5, 6, 7, 8‐tetrahydro‐ L ‐folic acid (XVII) exists therefore in a half‐chair conformation with a pseudoequatorial position of the side chain at C(6) (Fig.5) .