Synthesis and Stereochemistry of Stereoisomeric 1,3‐Oxazino‐ and 1,3‐Thiazino[4,3‐ a ]isoquinolines

Starting from the 6,7‐dialkoxy‐1‐[bis(hydroxymethyl)methyl]‐1,2,3,4‐tetrahydroisoquinolines 2 and 3 , the 4 ‐imino‐substituted 1‐(hydroxymethyl)‐9,10‐dialkoxy‐ 2H, 4H ‐1,6,11b‐tetrahydro‐1.3‐oxazino‐and ‐thiazino [4,3‐a]isoquinoline diastereomers 6a‐c, 7a‐c, 8a‐c, 9a‐c, 14 , and 15 and the 4‐substituted 1,6,7,11b‐tetrahydro 1,3‐oxazino[4,3‐ a ]isoquinoline diastereomers 16–24 were prepared. the relative configurations and the predominant conformation of these products were determined by NMR spectroscopy and for 18 by X‐ray diffraction methods. the prepared 1,3‐oxazino[4,3‐ a ]isoquinoline diastereomers have predominantly trans conformations ( 16, 18, 22, 23 ), whereas cis conformations ( cis ‐A) prevail for 20, 21 and f 24 . Thus, the first evidence for either trans ‐ or cis ‐A conformations in 1,3‐oxazino[4,3‐ a ] isoquinolines is presented.