Stereoselective dialkylation of the proximal hydroxy groups of calix- and thiacalix[4]arenes.

Treatment of p-tert-butylcalix[4]arene (C1) and its sulfur-bridged analog T1 with 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane in the presence of imidazole gives proximally O,O'-disiloxane-1,3-diyl-bridged calixarenes C2 and T2 in excellent yields, respectively. Subsequent base-catalyzed etherification of the remaining hydroxy groups with alkyl halides gives syn- and anti-O'',O'''-dialkylated products, the stereoselectivity of which varies depending on the nature of the macrocycle, as well as the metal cation of the base employed. Thus, conventional calixarene C2 preferentially affords syn compounds of 1,2-alternate conformation (C3) with the aid of tert-BuOK and K(2)CO(3) and anti counterparts of partial-cone conformation (C4) with Cs(2)CO(3).