Acetalsketals

This kind of cyclization is often used to provide protection for c/s-diols and thiols. It is based on the reaction of aldehydes or ketones with 1,2-diols in the presence of an acid catalyst and proceeds via the formation of a hemiketal that is further rearranged to the dioxolane product (called acetonide or iso-propylidene when acetone is used as the ketone; see reaction [V]).

since they exhibit a broad ability to cyclize and are able to bridge diols up to a 1,4 position. The cyclic products (reaction [VI]) are stable enough to be analysed by GC and may even be made in the GC inlet.

However, hydrolytic stability is not good and the products of reaction with a single OH group cannot be analysed. This can cause problems with, for example, steroids containing an additional alcohol group or groups next to the diol. The required additional treatment (acetylation, silylation) often results in unwanted by-products. Various alkyl groups, such as t-butyl, have been investigated with a view to improving the stability of the product, but the results have not always been as expected. The n-bu-taneboronates offer a convenient compromise between volatility and stability. For ECD, 2,4-dichloro-and 3,5-bis(trifluoromethyl)benzeneboronic acid have proved to be the best, since PFB-boronates are unstable to hydrolysis. Some useful applications include treatment of monosaccharides with ben-zeneboronic acid in acetone to give mixed aceton-ide-boronate products, as shown in reaction [VII]).

Acetonide derivatives are prepared, e.g. from a-monoacylglycerols (^-isomers do not react), and separated by GC. Corticosteroids with c/s-C-20,21- or C-17,20-dihydroxy groups form acetonides under mild conditions and the products are suitable for GC analysis. The acetonide group is also stable under conditions of further treatment such as silylation or acetylation. The reaction is specific to c/s-diols since the trans-C-20,21 diol does not form an acetonide. Siliconides are also formed by a similar mechanism when dimethylchlorosilane is used in pyridine, but multiproduct formation and instability of the derivatives has prevented wider use.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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