Diastereoselective Functionalization of a Spherand‐Type Calixarene

Condensation of 5,5′‐di‐ tert ‐butyl‐2,2′‐dihydroxybiphenyl ( 1 ) with formaldehyde yields a cyclic trimer 2 (and tetramer 3 ) having three (four) methylene groups less than a calix[6]‐arene (calix[8]arene). Alkylation of the (flexible) trimer with ethyl bromoacetate gives exclusively the stereoisomer 4 with C 2 symmetry, while the isomer with D 3 symmetry is not observed. Two isomers 6a and 6b ( C 2 and C 1 symmetry) are obtained by treatment with tert‐butyl bromoacetate which both are converted by transesterification with methanol into the same hexamethyl ester 5 having C 2 symmetry. These results are rationalized by restricted rotation around ArAr bonds for larger O‐alkyl groups also around ArCH 2 Ar bonds. The structure of the hexaethyl ester 4 is also confirmed by single crystal X‐ray analysis.