C-Branched, Ether Connectivity

A series of related cascadols , prepared from pentaerythritol, has been reported, 190 but little supportive data are available. 

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Percec et al.130bl have employed 1 — 2 C-branching and ether-type connectivity for the preparation of nonspherical, thermotropic liquid crystalline dendrimers. Observed thermotropic behaviors are predicated on mesogenic monomers that are capable of conformational isomerism. 

C-branched dendritic structures, based on either ether and amide linkages or simply amide connectivity (iii) Tomalia-type PAMAM dendritic structure, differing from the POPAM by the presence of amide bonds, but keeping the amine AB2 as the branching center and (iv) Fr6chet-type dendritic strnctures, based on ether and phenyl groups as AB2 branching center. Description of the different types of covalent dendrimers reported will follow a classification based on the element content, such as only C, C-N, N-O, C-N-0, or with heteroatoms such as P, Si, and so on. Examples of porphyrin-, thiophene-, carbohydrate-, and fullerene-based dendrimers are described separately, since they can be bound to different types of frameworks. 

Poly(ether.siloxanes) contain a poly(methyl.siloxane) polymer, which may be branched, and poly(eiher)-blocks. Its structure may be linear or comblike. The blocks are connected by Si-O-C- or Si-C- bridges., Si-0-C- linked products can, for example, be produced by reacting branched dimethyl(chloro)siloxy-lerminated poly(methyl-siloxanes) (see Section 4.3.3) with monohydroxy-functional polyCethers. 

Detailed n.m.r. studies have been reported on some bicyclic uridine analogues reported last year by Chattopadhyaya s group, formed by intramolecular addition of free radicals to allyl ethers (see Vol. 25, p. 258).i66 The same group have described the preparation of the thymine nucleoside 134 by means of intramolecular free radical trapping by an allyl group tethered by a temporary silicon connection (Scheme 15). Similar sequences were used to make analogous compounds with the three-carbon branch at C-2 in a down position (D-ribo-), and at C-3 in the up orientation, but an attempt to make the compound with the C-2 up stereochemistry led to a tricyclic product after oxidation in which the intermediate bicyclic radical had attacked C-6 of the thymine ring. 167... 

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