Glucose chair conformation

The following are representations of two forms of glucose The six membered ring is known to exist in a chair conformation in each form Draw clear representations of the most stable con formation of each Are they two different conformations of the same molecule or are they stereoisomers Which substituents (if any) occupy axial sites ... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

The chair conformation of cyclohexane has many consequences. We ll see in Section 1.1.9, for instance, that the chemical behavior of many substituted cyclohexanes is influenced by their conformation. In addition, we ll see in Section 2S.5 that simple carbohydrates such as glucose adopt a conformation based on the cyclohexane chair and that their chemistry is directly affected as a result. 

Galactose, a sugar related to glucose, contains a six-membered ring in which all the substituents except the -OH group indicated below in red are equatorial. Draw galactose In its more stable chair conformation. 

The alternative chair conformation, should we draw it instead, would be less favoured than that shown because of the increased number of axial substituents. The conformation of o-glucose is the easily remembered one, in that all the substituents are equatorial. 

In the case of glucose, the mutarotation gives 36 percent a, 64 percent p, and negligible strciight chain. The unequal distribution of the two anomers is due to the fact that the -OH on the anomeric carbon of the p form is equatorial, which for a chair conformer is more stable. The -OH on the anomeric carbon in the a anomer is axial, which means this anomer is slightly less stable. 

The target molecule is 2-hydroxymethyl-4-methoxy-3,7-dioxabicyclo[4.3.0]nonan-8-one (15), a key intermediate in several synthetic routes to thromboxane B2 8. The sequence9 begins with levoglucosan (9) in which the pyranose chair of D-glucose is conformationally locked by 1,6-etherification. Tosylation occurs chemoselectively at the unhindered positions (C-2 and -4). 

D-Glucose (p-bromophenyl)hydrazone113 is intramolecularly cyclized by addition of the 5-OH group across the C-l-N double bond, and crystallizes in that chair conformation in which all of the substituents are equatorially attached. It contains five unique hydro-... 

Catalysis may also be observed via transition states or intermediates which are more than six-membered. An example is the hydrolysis of glucose-6-phosphate dianion which surprisingly is more rapid (5 times) than the monoanion. Presumably the relatively acidic 1-hydroxyl group of glucose (p/Ca = 10.8, 100°C) can act as a general-acid catalyst of phosphate group expulsion (58),3 > even though the required chair conformation has all substituents axial, viz. 

Exercise 20-4 Draw the chair conformation of /3-D-glucose with all of the substituent groups axial. Explain how hydrogen bonding may complicate the usual considerations of steric hindrance in assessing the stability of this conformation relative to the form with all substituent groups equatorial. 

X-ray crystallographic studies have shown that crystalline D-glucose as commonly isolated exists in the a-D-glucopyranose form. Furthermore the stable chair conformation (1), in which the hydroxyl groups on C-2, C-3 and C-4, and the hydroxymethyl group on C-5 are equatorial, is preferred to the alternative chair conformation (6) in which these groups occupy axial positions. 

Conformational representations of the two different forms of glucose are drawn in the usual way. An oxygen atom is present in the six-membered ring, and we are told in the problem that the ring exists in a chair conformation. 

In the cyclodextrins readily obtainable from starch, the six (a-CD), seven (P-CD) and eight (y-CD) a(l- 4)-linked glucose units are "locked up" in a strait-jacket type belt due to adoption of 4Cj chair conformations of the pyranoid rings and a net of 2-OH — OH-3 hydrogen bonds (73, 74). As this structural rigidity even persists on inclusion complex formation, as exemplified by the three represenatives in Fig. 1 (75 - 78) ... 

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