CHIRAL RECOGNITION BY CYCLODEXTRIN CAVITY

S. Makedonopoulou¹, I. M. Mavridis¹, K. Yannakopoulou¹, D. Mentzafos², V. Lamzin³

¹Institute of Physical Chemistry, NCSR Demokritos, Athens, Greece
²Laboratory of Physics, Agricultural University of Athens, Greece
³European Molecular Biology Laboratory, c/o DESY, Hamburg, Germany

mavridi@cyclades.nrcps.ariadne-t.gr

Keywords: cyclodextrins, spiroacetal, chiral recognition, enantiomeric resolution, inclusion complex, supramolecular chemistry

It has been found that hexakis(2,3,6-O-methyl)-á-cyclodextrin (TMáCD) binds selectively[1] (R)-(-)-1,7-dioxaspiro[5.5]undecane (R)-(-)-spiroacetal). The enantioselective complexation of the (S)-enantiomer of spiroacetal by heptakis(2,3,6-O-methyl)-â-cyclodextrin (TMâCD) has also been studied in the solid state. The TMâCD/(S)-(+)-(spiroacetal) complex crystallizes in P2₁2₁2₁, a=11.08830, b=25.97405, c=29.99488 A at room temperature (RT). Since the crystal display diffraction quality, unusual for cyclodextrin crystals, we have collected high resolution data at low temperature using synchrotron radiation.

At the temperature of 100K the TMáCD/(R)-(-)-spiroacetal complex crystallizes in P2₁2₁2₁, a=14.636, b=21.637, c=23.448 A, different from the C222₁ of the structure at RT. Data were collected up to a resolution of 0.55A. The guest is found along the TMáCD 6-fold axis in two positions related by a local 2-fold non-crystallographic axis, whereas at the RT structure those were related by an exact 2-fold axis. It is interacting with the enclosing and neighboring host molecules by numerous van der Waals contacts, the shortest being that of spiroacetal oxygen atoms and TMáCD methoxy hydrogen atoms (O…C distance close to 3 A).

At the temperature of 100K the TMâCD/(S)-(+)-spiroacetal complex crystallizes in P2₁2₁2₁, a=10.936, b=25.529, c=29.640 A . Data were collected up to a resolution of 0.65 A. The guest is also found disordered over two positions and all its atoms, but the central spiro carbon atom, are involved in medium strength, multiple van der Waals interactions of the type C-H…O (C…O distances greater than 3.5 A) with the host atoms.

Detailed comparison of the structures at high resolution will be attempted in order to understand better the enantiospecificity of the macrocycles for the enantiomers, since resolution techniques using cyclodextrins have many applications.

[1] "Chiral Recognition of (R)-(-)-1,7-Dioxaspiro[5.5]undecane by Hexakis (2,3,6-tri-O-methyl)-á-cyclodextrin", K. Yannakopoulou, D. Mentzafos, I.M. Mavridis & K. Dandika Angew. Chem. Int. Ed. Engl. 1996, 35, 2480.
[2] "Structure of the 1:1 Complex of Hexakis(2,3,6-tri-O-methyl)-á-Cyclodextrin with (R)-(-)-1,7- Dioxaspiro[5.5]undecane", D. Mentzafos, I. M. Mavridis, K. Yannakopoulou, J. Incl Phenom., in print.

Microsymposium C7