hemiacetal


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Related to hemiacetal: Ketal

hem·i·ac·e·tal

(hem'ē-as'e-tăl),
RCH(OH)OR', a product of the addition of an alcohol to an aldehyde (an acetal is formed by the addition of an alcohol to a hemiacetal). In the aldose sugars, the hemiacetal formation is internal and labile, brought about by the 4-OH or 5-OH attack on the carbonyl oxygen, thus yielding the furanose or pyranose structures; the hemiacetal forms of the sugars are involved in all polysaccharides, as glycosyl residues or glycosides.
See also: hemiketal, acetal.

hem·i·ac·e·tal

(hem'ē-as'ĕ-tăl)
A product of the addition of an alcohol to an aldehyde (an acetal is formed by the addition of an alcohol to a hemiacetal). In the aldose sugars, the hemiacetal formation is internal and labile, brought about by the 4-OH or 5-OH attack on the carbonyl O, yielding the furanose or pyranose structures; the hemiacetal forms of the sugars are involved in all polysaccharides, as glycosyls or glycosides.
See also: hemiketal, acetal
References in periodicals archive ?
Bacterial EPS are heteropolysaccharides containing different functional groups in their backbone including amino, sulfate, phosphate, and hydroxyl substituents as well as N-acetylamino sugars and hemiacetal ends; all of these functional groups are great candidates in the reduction, complexation, and stabilization of metal nanoparticles during their synthesis.
On the other hand, almost all of the carbohydrates that exist in nature are glycosidic compounds as a result of dehydrative condensation of hemiacetal and aglycones.
The slight increase in gelatinization temperatures also confirm our observation of higher peak viscosity of [oxAYBS.sup.*] than nAYBS in the viscoAmylograph, reasons of which was attributed to probable formation of crosslinks by hemiacetals that may be formed by low levels of oxidants.
The P450-dependent hydroxylation of the methoxy group yields an intermediate corresponding to the hemiacetal of formaldehyde that can cyclize to form a methylenedioxy bridge via an ionic mechanism involving the methylene oxonium ion intermediate.
The primary products from the methanolysis of CS and DS were presumed to be uronic or iduronic acid-N-acetylhexosamine dimers, methylated at both the carboxylic acid and terminal hemiacetal function (24).
In aqueous solution, they occur as an equilibrium of four related molecular structures; namely the fiavylium cation, quinoidal base, hemiacetal and chalcone forms.
However, by the treatment of 15 with Yb[(OTf).sub.3] in methanol and reflux of the reaction mixture for 6h, 17 (17% yield) and 18 (32% yield) were obtained, probably via a hemiacetal intermediate (3-hydroxy-3,5,7,3', 4'-pentamethoxyflavane) 16 (Scheme 4).
In the particular case of alcohols and acids, it seems to be likely that formation of esters by esterification of alcohols and acids (or by transesterification of two esters), addition reactions of alcohols to aldehydes or ketones to form hemiacetals or acetals, oxidation of alcohols, etc.
El alginato oxidado presento un grado de oxidacion del 90%, ademas en el espectro de [sup.13]C-CP-MAS NMR se observa un pico a 92 ppm lo que indica la presencia de grupos hemiacetales, producto de la interaccion de los grupos aldehidos con los grupos alcoholicos presentes en el alginato (Painter, T.
And the study we cited from the New England Journal of Medicine raised a red flag about the formaldehyde-containing hemiacetals formed when some e-cig flavoring substances are vaped.
The two sets of [sup.1]H NMR chemical shift values for the methylene group C-26 (3.95, 3.62 and 4.06, 4.53) suggested that the saponin was actually a mixture of the 25R and 25S epimeric hemiacetals. Both approaches, comparison of the [sup.1]H NMR chemical shift values with data from the same kind of saponins recorded under the same conditions (Farid et al.