Acetals hemiacetals

One of the most important methods of pyrazole synthesis involves the reaction of 1,3-diketones with hydrazine derivatives. The method has wide scope, especially considering that the diketones may be replaced by acetals, hemiacetals, chlorovinyl ketones, tetrahalides, etc. Most accessible by this path is 3,5-dimethylpyrazole (10), obtained in 85% yield from acetylacetone and hydrazine hydrate.134-138... 

We need now to discuss hemiacetals though you may well wonder why - they retain the carbonyl oxygen and they are unstable. We need to discuss them as a preliminary to the much more important acetals. Hemiacetals are halfway to acetals. 

The NMR spectrum of 26 showed well-dispersed signals of 76 protons, which included seven methyl singlets and two methyl doublets. The 13C NMR spectrum revealed a ketonic carbonyl carbon (5 221.2), two acetal/ hemiacetal carbons (8 108.9,119.8), four oxygenated quaternary carbons, four oxygenated methines, and an oxygenated methylene carbon. In addition to the pyrazine carbons, two sp2 carbon signals resonating at 8 121.2... 

In 1999, new cycloparvifloralone-type sesquiterpenes were reported to consist of a unique acetal-hemiacetal and/or ortholactone structure as shown in Fig. (11). Cycloparvifloralone (65) occurred in the leaves oil. parviflorum, and (ll)7,14-ortholactone-14-hydroxy-3-... 

Problem 14.23. For each of the following, indicate whether the compound is an acetal, hemiacetal, ketal, hemiketal, or something else ... 

Ans. Acetals, hemiacetals, ketals, and hemiketals have two different oxygens bonded to the same carbon. 

In contrast to acetals, hemiacetals can revert to aldehydes and alcohols through both acid and base catalysis. Some catalysts incorporate both acid and base functions and can serve as bifunctional catalysts. This type of... 

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