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The first synthesis of all four stereoisomers of 1 ,3,5,7-tetrahydroxy-1,3,5,7-tetraisopropylcyclotetrasiloxane, [i-PrSiO(OH)]4 (all-trans-, cis-cis-trans-, cis-trans-cis-, and all-cis-1), is presented. The starting compounds, all-trans-, cis-cis-trans, cis-trans-cis-, and all-cis-1,3,5,7-tetraaryl-1,3,5,7-tetraisopropylcyclotetrasiloxanes, were prepared by the hydrolysis of the corresponding arylisopropyldichlorosilanes, i-PrArSiCl2 (Ar = Ph, p-tolyl), and subsequent separation of isomers. A combination of dephenylchlorination of tetraarylcyclotetrasiloxanes and the following hydrolysis proved to be an efficient method for the stereospecific transformation of aryl-substituted cyclotetrasiloxanes into (i-PrSiO(OH))4. For example, treatment of cis-trans-cis-1,3,5,7-tetraphenyl-1,3,5,7-tetraisopropylcyclotetrasilane with HCl and AlCl3, followed by hydrolysis in the presence of pyridine, resulted in the exclusive formation of cis-trans-cis-1 in 92% yield. The structures of cis-cis-trans-1, cis-trans-cis-1, and all-cis-1 were determined by X-ray crystallography. All isomers were found to construct unique packing structures by intermolecular hydrogen bonding; cis-trans-cis-1 composed an infinite antiladder structure, and cis-cis-trans-1 formed a sheetlike structure. [ABSTRACT FROM AUTHOR]

An X-ray crystallographic analysis of the bis-N[SUPδ]-Boc-tetra-N[SUPα]-methyl derivative of gramicidin S, cyclo(-VaI-MeOrn(Boc)-Leu-D-MePhe-Pro-)[SUB2], was undertaken successfully (R-factor = 0.088). As expected, the main chain adopts an antiparallel pleated β-sheet conformation, but the pleated sheet is slightly twisted, and the sense of twisting is opposite to that found in the reported crystal structures of the gramicidin S-urea complex and the bis-N[SUPδ]-(trichloroacetyl) and bis-Nδ-(m-bromobenzoyl) derivatives of gramicidin S. In agreement with the observed resistance toward N-methylation, the urethane NH groups of the protected Orn side chains are hydrogen bonded to the carbonyl groups of the D-Phe residues. However, the side-chain-main-chain hydrogen bonding is in the i → i - 3 mode, although hydrogen bonding in the i → i + 2 mode was deduced from a [SUP1]H NMR study of protected gramicidin S derivatives and was actually found in the crystal structures of the diacylated gramicidin S. [ABSTRACT FROM AUTHOR]

The article describes the synthesis of octacarboxyphenyl functionalized cage, octakis[(p-carboxyphenyl)dimethylsilyl]silicate (OCPDS). According to the authors, the spherosilicate cage structure remains intact during the hydrolysis of octakis[dimethyl(4-tribromomethylphenyl)-silyl]silicate. They add that OCPDS shows lower thermal decomposition temperature due to easier decomposition of the carboxyphenyl groups than the tolyl groups.