Highly selective synthesis of a 1,3,5-tris-protected calix[6]arene-type molecular platform through coordination and host-guest chemistry.

An elegant methodology based on the synergistic combination of coordination and host-guest chemistry led to the highly efficient synthesis of a unique C(3v)-symmetrical, calix[6]arene-based molecular platform with three protected amino arms in alternating positions. The key step involves the formation of a stable supramolecular host-guest Zn(II) complex from a C(6v)-symmetrical calix[6]hexaamine. Indeed, in the presence of a polar neutral guest and a strong donor that acts as an exogenous ligand, three alternating amino groups of this calix[6]hexaamine are selectively coordinated to the Zn(II) ion while the three others remain free and are thus much more reactive toward chemical reagents. In addition to this protective role, the metal centre preorganises the C(3v)-symmetrical complex in such a way that the uncoordinated NH(2) groups are directed toward the outside of the cavity; they are then accessible for a chemical transformation. Hence, reaction of these alternating free amino groups with a protective reagent (i.e., Boc(2)O) followed by zinc decoordination quantitatively and selectively yielded the 1,3,5-tris-Boc-protected calixarene derivative on a gram scale. It was shown that the presence of all the partners of the key intermediate Zn complex (i.e., the metal centre, the exogenous ligand and the included guest) is crucial for a high selectivity. Finally, a two step sequence that led to a C(3v)-symmetrical 1,3,5-tris-acetylated calix[6]hexaamine through the removal of the Boc groups illustrates that the 1,3,5-tris-protected calix[6]hexaamine is a promising molecular platform. Examples of such readily available C(3v)-symmetrical calixarene-based building blocks are extremely rare in the literature.