Structural analysis of some bis-(8-isopropyl-isoquinolinium) derivatives reveals a preferential folded conformation leading to a stereoselective attack by sodium borohydride

•Bis-isoquinoliniums are reduced with NaBH4 to get 1,2,3,4-tetrahydroisoquinolines.•A stereoselectivity is obtained for the analogues with propyl and meta-xylyl linker.•Folded conformation is the key of this stereoselectivity.