Metal-capped α-cyclodextrins:: Squaring the circle

This report describes the four-step synthesis of 6A,6D-diamino-6A,6D-dideoxy-2(A),2(B),2(C),2(D),2(E),2(F),3(A),3(B),3(C),3(D),3(E),3(F), 6(B),6(C),6(E),6(F)-hexadeca-O-methylcyclomaltohexaose (5), a methylated alpha -CD that bears two -CH2NH2 ligands located on diametrically opposed glucopyranose rings. Reaction of 5 with K2PtCl4 afforded the water-soluble chelate complex [PtCl2(5)] where the metal center is bonded to cia-arranged nitrogen atoms. A single-crystal X-ray structure of the latter reveals the high distortion imposed on the CD structure by the short metallo-organic cap. The cyclodextrin core adopts an unprecedented elongated, almost rectangular shape, the shortest and longest O(4)(n) ... O(4)(n+3) distances being respectively 5.44 and 9.98 Angstrom. Two opposing glucose rings are no longer in the usual C-4(1) chair conformation, but adopt an elongated S-O(2) skew-boat structure. The observed CD-flattening produces a highly preorganized hydrophilic pocket that complexes through multiple hydrogen bonding a single water molecule. Complex [PtCl2(5)] and the corresponding synthetic intermediates were characterized by elemental analysis, MS, and IR and NMR spectroscopy.