Synthesis and stereochemistry of stereoisomeric 1,2,3-oxathiazino[4,3-a]isoquinolines

The ring-closures of homocalycotomine and its 1'- and 2'-methyl-substituted diastereomers 3, 4 and 7, 8 with thionyl chloride or with sulfuryl chlorid e led to 1,2,3-oxathiazino[4,3-a]isoquinoline derivatives 9-11 and 18, 19 o r to 15, 17 and 22, 23, respectively. The relative configurations and the p redominant conformations of the cis(1)-trans-cis(2) conformational equilibr ium were studied by means of H-1- and C-13-NMR spectroscopy, with the appli cation of DNOE, 2D HSC and 2D-COSY measurements. In good agreement with the liquid-phase results, the X-ray investigations revealed that 9 and 10 have the cis(1), while 18 has the trans-anellated stereostructure.