Key Words: 1,3-0xazino[4,3-a]isoquinoline / 1,3-Thiazino[4,3-a]isoquinoline / Conformational analysisStarting from the 6,7-dialkoxy-l-[bis(hydroxymethyl)methyl]-these products were determined by NMR spectroscopy and for 18 1,2,3,4-tetrahydroisoquinolines 2 and 3, the 4-imino-substituted by X-ray diffraction methods. The prepared 1,3-oxazino[4,3-a]-isol-(hydroxymethyl)-9,IO-dialkoxy-2H,4H-l,6,7,11 b-tetrahydro-1,3-quinoline diasteromers have predominantly trans conformations oxazino-and -thiazino[4,3-a]isoquinoline diastereomers 6a-c, 7a -c, (16, 18, 22, 23), whereas cis conformations (&A) prevail for 20, 21 8a-c, 9a-c,14, and 15 and the4-substituted 1,6,7,llb-tetrahydro-and 24. Thus, the first evidence for either trans-or cis-A confor-1,3-oxazino[4,3-a]isoquinoline diastereomers 16-24 were prepared. mations in 1,3-oxazino[4,3-a]isoquinolines is presented. The relative configurations and the predominant conformation of