Singly bridged calix[8]crowns

Crowned calix[8]arenes are obtained by direct alkylation of p-tert-butylcal ix[8]arene (1) with poly(ethylene glycol) ditosylates in the presence of va rious bases. K2CO3 promotes the preferential formation of 1,3-calix[8]crown s. Cs2CO3 mainly gives the 1,5-isomers, which are selectively obtained in h igh yields when shorter chains are used (1,5-crown-2, 88%; 1,5-crown-3, 78% ). Nail affords the 1,4-isomers in yields up to 46%, often as the sole crow n derivative, besides unreacted 1. 1,2-Calix-[8]crowns are also obtained in appreciable amount in some instances. The observed regioselectivity is rat ionalized in terms of preferential formation of specific anions in dependen ce of the base strength. Dynamic NMR and modeling studies prove that the po lyether chain, depending on its bridging mode, may significantly reduce the available space for the through the annulus passages leading to derivative s conformationally blocked (on the NMR time scale).