Natural polyether-containing

The reaction of carbonyl compounds and allylmetal reagents is an important transformation in organic synthesis. Advances in stereoselective carbonyl allylation reactions have been spurred by interest in the synthesis of polypropionate-derived natural products, carbohydrates and other polyhydroxylated compounds. These reactions are ideally suited for the construction of stereochemically rich acyclic skeletons. Additionally, cyclic polyether-containing natural products, among others, have inspired chemists to investigate ring-closing allylation reactions. This review will focus on recent developments in the allylation reaction, with special emphasis on its application towards the synthesis of natural products. 

A wide variety of photo-responsive aromatic polyethers containing azo linkages in the main chain were prepared by nucleophilic aromatic displacement reactions. These polymers undergo cis-trans isomerism in solution when exposed to ultraviolet light. This photo induced isomerization was found to have a profound affect on solution viscosity of the polymers. The polymers were amorphous in nature and undergo crosslinking reactions at elevated temperatures. 

Polyester (Section 20 17) A polymer in which individual structural units are joined by ester bonds Polyether (Section 16 4) A molecule that contains many ether linkages Polyethers occur naturally in a number of antibi otic substances... 

Multiblock systems. A somewhat similar approach is involved in the production of thermoplastic polyurethane elastomers. In this case the chain contains soft segments that are largely aliphatic polyether in nature and also hard segments that are primarily polyurea (see Chapter 27). 

Polyurethane foams may be rigid, semi-rigid or flexible. They may be made from polyesters, polyethers or natural polyols such as castor oil (which contains approximately three hydroxyl groups in each molecule). Three general processes are available known as one-shot, prepolymer or quasi-prepolymer processes. These variations lead to 27 basic types of product or process, all of which have been used commercially. This section deals only with flexible foams (which are made only from polyesters and polyethers). Since prepolymers and... 

The synthesis of PU can be carried out by the reaction described in Eq. (2.24). If the functionality of the hydroxy-containing compounds or the isocyanate is increased beyond 2, branched and possibly crosslinked polymers are produced. Because the nature of the polyol (polyether, polyester, polybutadiene, etc.) and isocyanate components can vary widely, PU are among the most versatile polymers, producing a wide variety of materials such as elastomers, foams, coatings, adhesives, or fibers. 

The work reported here is concerned with the syntheses and properties of polyether-ester block copolymers containing poly (tetramethylene ether) units of molecular weight of approximately 1000 as the amorphous polyether blocks and a variety of esters as the crystallizable hard segments. The purpose of this study is to correlate changes in synthesis and properties of these thermoplastic and elastomeric copolymers with changes in the concentration and nature of the ester segments, particularly the types of diol and diacid. 

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