グルコース環の開環
Kidsらには中々わかりにくい反応らしい。直鎖と環状のグルコースの存在条件とアルデヒド基へのNucleophilic攻撃。
Chemistry 420 - Principles of Biochemistry
Ring-Closure of Glucose and Fructose
Ring-Closure of Glucose and Fructose
www3.nd.edu/~aseriann/glufru
Section 11.3: Hemiacetals, hemiketals, and hydrates
chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_11%3A_Nucleophilic_carbonyl_addition_reactions/Section_11.3%3A_Hemiacetals,_hemiketals,_and_hydrates
One of the most important nucleophilic addition reactions in biological chemistry is the addition of an alcohol nucleophile to a ketone or aldehyde.
When an alcohol adds to an aldehyde, the result is called a hemiacetal; when an alcohol adds to a ketone the resulting product is a hemiketal.
図１
(The prefix ‘hemi’ (half) is used in each term because, as we shall soon see, a second addition of an alcohol nucleophile can occur, resulting in species called acetals and ketals.)
The reactions in the figure above are highly reversible: hemiacetals and hemiketals easily convert back to aldehydes and ketones plus alcohol. The mechanism for the conversion of a hemiacetal back to an aldehyde is shown below:
図２
When in aqueous solution, aldehyde and ketones groups can form hydrates: this is simply the result of addition of water to the carbonyl.
図３
In many cases, the hydrate is actually the predominant form in water, although it is customary to show the aldehyde or ketone structure in structural drawings.
図４
The cyclic form of glucose is called glucopyranose. As you know, nucleophilic attack on a planar carbonyl group can occur from either face, leading to two different stereochemical outcomes - in this case, to two different diastereomers. In carbohydrate nomenclature, these two diastereomers are referred to as the alpha and beta anomers of glucopyranose.
Because the formation of glucopyranose occurs spontaneously without enzyme catalysis, shouldn’t equal amounts of these two anomers form?
In fact, this does not happen: there is almost twice as much of one diastereomer than the other at equilibrium. Why is this?
Remember (section 3.2C) that six-membered rings exist predominantly in the chair conformation, and that the molecule favors the chair conformation in which unfavorable interactions between substituents are minimized – in most cases, this is the conformation in which larger substituents are in the equatorial position. In the lower-energy chair conformation of the major beta β-anomer of glucopyranose, all of the hydroxyl groups are in the equatorial position, but in the minor alpha anomer one hydroxyl group is forced into the axial position. As a result, the alpha anomer is higher in energy, and less is present at equilibrium.
Fructose in solution forms a six-membered cyclic hemiketal called fructopyranose when the hydroxyl on C6 attacks the C2 ketone carbon.
Notice in the above figure that the percentages of alpha and beta anomers present at equilibrium do not add up to 100%. This is because fructose can also exist in solution as a five-membered cyclic hemiketal, referred to in carbohydrate nomenclature as fructofuranose.
In the formation of fructofuranose from open-chain fructose, the hydroxyl group on the fifth carbon attacks the ketone.
図６
In aqueous solution, then, fructose exists as an equilibrium mixture of 70% beta-fructopyranose, 23% beta-fructofuranose, and smaller amounts of the open chain and cyclic alpha anomers.
Although we have been looking at specific examples for glucose and fructose, other five- and six-carbon monosaccharides also exist in solution as equilibrium mixtures of open chains and cyclic intramolecular hemiacetals and hemiketals.
Shorter monosaccharides are much less likely to undergo analogous ring-forming reactions, however, due to the inherent instability of three and four-membered rings.