Conformational analysis of S,IO-methenyl-(6RS)-5,6,7, S-tetrahydro-~-folic acid2)Summary Conformational analysis of the diastereomeric 9,9-dideu~terio-5,10-methenyl-(6RS)-5,6,7,8-tetrahydro-~-folic acids (IV) in the lOO-MHz-'H-NMR. spectrum shows a trans-diaxial arrangement of Ha-C (6) (A -part) and Ha-C (7) (X-part) (.IAX= 10.6 Hz) and a gauche arrangement of Ha-C(6) and He-C(7) (M-part) (JAM= 4.2 Hz). Accordingly the tetrahydropyrazine ring in IV exists in the same manner as in tetrahydrofolic acid but with a more marked half chair conformation and with a pseudo equatorial position of the C (6), C (9) bond.A similar analysis of the diastereomeric 7,7-dideuterio-5,10-methenyl-(6RS)-5,6,7,8-tetrahydro-~-folic acids (VIII) in the 360-MHz-lH-NMR. spectrum shows an almost identical situation with higher values of S between Ha-C(6) (A-part) and H-C(9,6) (B-part) on one side ( J = 11.2) and between Ha-C(6) and H-C(9a) (X-part) on the other side ( J A X = 9.2). This can be reached only when the imidazolinium ring of VIII is nearly planar (Fig. 3). 1. Einleitung. -In Fortsetzung der Arbeiten iiber die Konfiguration von 7,8-Dihydro-[2] und 5,6,7,8-Tetrahydropterin-Derivaten [ 11 dehnen wir mit der vorliegenden Arbeit die Untersuchungen auf das Gemisch der diastereomeren 5,lO-Methenyl-(6RS)-5,6,7,8-tetrahydro-~-folsauren (IX) aus. Bekanntlich ist eines der beiden Diastereomeren IX eine Zwischenstufe sowohl fur die chemische wie fur die enzymatische fiberfiuhrung von 5bzw. lO-Forrnyl-5,6,7,8-tetrahydro-~-