Isomérie Z-E des spirochromènes oxa- et thiazolidiniques-1,3 à deux atomes de carbone asymétriques. Comparaison avec les spirochromènes oxa- et thiaziniques-1,3 homologues
The synthesis of oxa- and thiazolidine-1,3 spirochromenes with two asymmetric carbon atoms in an α position to oxygen or sulphur leads generally to a mixture of two diastereoisomers for which the Z configuration is predominant. Examination of Dreiding models and application of the Karplus equation allow a good solution of this problem. On the other hand, only one isomer exists in the 2,4,5,6-tetrahydro oxa- or thiazine-1,3 homolog compounds. The study of the geometry of the aza ring proves then that the configuration may be Z or E according to whether one of the asymmetric carbon atoms is in an α or β position to oxygen or sulphur.