Direct reduction to give the corresponding ether is difficult as the intermediate hemiacetal tends to decompose to give an alcohol and an aldehyde ( which is rapidly reduced to give a second alcohol ).
32.
A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide ( or a molecule derived from a saccharide ) and the hydroxyl group of some compound such as an alcohol.
33.
For example, the aldohexose glucose may form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose.
34.
Ring closure creates another chiral center at the anomeric carbon ( the one with the hemiacetal or acetal functionality ), and therefore each open-chain stereoisomer gives rise to two distinct stereoisomers ( anomers ).
35.
The enol is stabilized by conjugation with the imine, and after the attack on a second formaldehyde, a hemiacetal is created-- the alkoxide is readily stabilised by the COOH group next to it.
36.
As a reaction to create an acetal proceeds, water must be removed from the reaction mixture, for example, with a Dean-Stark apparatus, lest it will hydrolyse the product back to the hemiacetal.
37.
The alcohol was oxidized to the aldehyde using a Parikh-Doering oxidation and TIPS group removal gave hemiacetal "'19 "'called ( + )-diaboline which is acylated Wieland-Gumlich aldehyde.
38.
The oxidation of the aldehydes is proposed to proceed via the formation of hemiacetal-like intermediates, which arise from the addition of the O 3 CrO-H " bond across the C = O bond.
39.
At pH 7, glucose exists in solution in cyclic hemiacetal form as 63.6 % ?-D-glucopyranose and 36.4 % ?-D-glucopyranose, the proportion of linear and furanose form being negligible.
40.
The inventor of a heat-stable ( and therefore useful ) POM homopolymer was Dal Nagore, who discovered that reacting the hemiacetal ends with acetic anhydride converts the readily depolymerizable hemiacetal into a thermally stable, melt-processable plastic.
