Conformational Immobilization via Bridging

Replacement of all four of the hydrogens of the OH groups of a calix[4]arene with alkyl, acyl, or aroyl groups generally results in conformational immobilization, with the result that non-interconverting cone, partial cone, 1,2-alternate, and 1,3-alternate conformers can be formed. Although approximate guidelines 

CH3OCH2CH2OTS and NaH produces the cone conformer of the tetraether in 81% yield, while with t-BuOK as the base the partial cone conformer is formed in 64% yield. Treatment of4 with p-bromobenzenesulfonyl chloride, on the other hand, yields the cone conformer with NaH but the 1,3-alternate conformer with 1-methylimidazole.  

Alkylating agents of the structure RCOCH2X generally provide high yields of cone conformer with NaH as base, e.g. 4 reacts with Et2NCOCH2Cl to give a 

Acylation of 4 gives (a) with acetyl chloride/NaH an 89% yield of the cone conformer (b) with acetic anhydride/H2S04 a 42% yield of the partial cone conformer along with 19% 1,3-alternate and 6% 1,2-alternate con-former (c) with acetic anhydride/p-toluenesulfonic acid a mixture containing 

Iwamoto, K. Fujimoto, K. Matsuda, T. Shinkai, S. Tetrahedron Lett. 1990, 31, 7169. 

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