Functionalization of calix[4]arenes by alkylation with 2-(chloromethyl)pyridine hydrochloride
作者:Sebastiano Pappalardo、Luigi Giunta、Mario Foti、George Ferguson、John F. Gallagher、Branko Kaitner
DOI:10.1021/jo00035a016
日期:1992.4
The syntheses, structures, and conformations of nine of the 13 possible [(2-pyridylmethyl)oxy]calix[4]arene conformers obtainable by direct substitution on calix[4]arenes 1a,b are described. The conformer distribution in the exhaustive O-alkylation of 1a,b with 2-(chloromethyl)pyridine hydrochloride (PicCl.HCl) in N, N,N-dimethylformamide (DMF) is strongly affected by the base applied: NaH induces only cone conformers, while K2CO3 or Cs2CO3 lead preferentially to partial cone and/or 1,3-alternate conformers, depending on the para substituent of the starting calix[4]arene. Single-crystal X-ray analyses on tetra-O-alkylated cone 1k,1 and partial cone 2c have been conducted. Molecule 1k has a distorted cone conformation with pendant OCH2Py groups; a methanol of solvation is hydrogen bonded to one pyridine N atom and is exo to the calix cavity. The crystal structure of 11 contains two independent distorted cone shaped molecules per asymmetric unit which differ principally in the relative orientations of the OCH2Py groups. In the partial cone conformer 2c the conformation adopted is such that the pendant OCH2Py group of the rotated aryl ring lies in, and effectively fills, the calix cavity produced by the remaining three aryl rings; as in 1k a methanol of solvation is hydrogen bonded to a pyridine N atom exo to the calix cavity. Regioselective syn-proximal (1,2-) or syn-distal (1,3-) difunctionalization at the lower rim of calix[4]arenes has been also achieved. Syn-1,2-disubstituted derivatives have proved to be useful intermediates for the stereoselective synthesis of tri-O-alkylated cone conformers, calix[4]arenes with mixed ligating groups in the sequence AABB at the lower rim, for inherently chiral calix[4]arenes, and for the transfer of proximal regioselectivity from the lower to the upper rim. On the basis of stepwise O-alkylation of calix[4]arenes, and with the aid of MM2 calculations on the involved intermediates and their anions, a possible genesis of the various conformers is proposed.