Synthesis and X-ray structure of a tris(trimethylsilyl)oxacalix[3]arene

The silylation of hexahomotrioxacalix[3]arene macrocycles 1a-d has been investigated. Reaction of macrocycles 1a-d with 1-(trimethylsilyl)imidazole or bis(trimethyl silyl)trifluoroacetamide results in a mixture of two tris(trimethylsilyl) isomers 2 and 3, The ''partial cone'' isomer 2 has two TMS groups on one side of the macrocycle and the third TMS group on the opposite side, whereas the ''cone'' isomer 3 has all of its TMS groups on the same side of the macrocycle, The isomers were found to exhibit no equilibration up to 350 K, The dependence of the selectivity of the silylation (2 vs, 3) on the nature of the silylating agent, the reaction conditions, and the para-substituent of the oxacalix[3]arenes 1a-d has been examined, The first crystal structure of a silylated calixarene, partial-cone isomer 2a, has been solved.