Stereochemistry of six-membered spiranes arising from the first use of a diaza-trispiro-heneicosane motif in the synthesis of a G-1 dendritic melamine

The first use of a spiranic diamino-nucleophile, 7,11,18,21-tetraoxa-3,15-diazatrispiro[5.2.2.5.2.2]heneicosane, as linker in the synthesis of dendritic melamines, is reported. The chirality of a C(a)4 and/or a C(a)2(b)2 spiranic carbon is discussed and, for the first time, assigned. The nature of exocyclic ligands in aza-positions N-3, -15 leading to versatile types of pseudo-allenic arrangements about the trispiranic skeleton is discussed. When aza-positions are involved in >N–C(double bond; length as m-dashX)–↔–N+double bond; length as m-dash(C–X−)– (Xdouble bond; length as m-dashO, N) bonds, the first NMR evidence for a distinction between rotational (pro)diastereomerism and axial chirality in spiranes is shown. Unexpected supramolecular facets of this stereochemistry are also observed.