Stereochemistry of a Spherand-Type Calixarene

STEREOCHEMISTRY OF A SPHERAND-TYPE CALIXARENE

Kasim Agbaria,^a Silvio E. Biali,^a Volker Böhmer,^b Jörg Brenn,^c Shmuel Cohen,^a Michael Frings,^b Flavio Grynszpan,^a,§ Jack McB. Harrowfield,^d Alexander N. Sobolev,^d Iris Thondorf^c

aDepartment of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel, ^bJohannes Gutenberg-Universität, Fachbereich Chemie und Pharmazie, Abteilung Lehramt Chemie, Duesbergweg 10-14, 55099 Mainz, Germany, ^cMartin-Luther-Universität Halle-Wittenberg, Fachbereich Biochemie/Biotechnologie, 06099 Halle, Germany, ^dDepartment of Chemistry, University of Western Australia, Nedlands, Perth, Australia.

The stereochemistry of the spherand-type calixarene 1 was analyzed in terms of the configuration of the three 2,2'-dihydroxybiphenyl subunits (R or S) and the disposition of the methylene groups (crown or twist). X-ray crystallography indicated that the neutral 1a and its mono- or dianion (in form of the salts 1a^- ^·C₅H₅NH⁺ and 1a^2- ^·2 Et₃NH⁺) exist essentially in the same conformation (RRS/SSR-twist). This asymmetric RRS/SSR-twist form is the lowest energy conformer according to molecular mechanics calculations. Low temperature ¹H NMR data indicated the presence of a major conformer of C₁ symmetry, in agreement with a RRS/SSR-twist form. The lowest energy topomerization pathway mutually exchanges two pairs of methylene protons and is ascribed to an enantiomerization process involving rotation around an Ar-Ar bond.

^{§Present address: The Scripps Research Institute, Department of Molecular Biology, 10550 N. Torrey Pines Rd., La Jolla, CA 92037, USA.

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