215F07-notes-11-12-07 - Chem 215 F07 Notes — Dr Masato...

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Unformatted text preview: Chem 215 F07 Notes — Dr. Masato Koreeda - Page 1 of 2. Date: November 12, 2007 Carbohydrate stereochemistry More about a- or B- at the anomeric carbon — Taken from: upac/ani‘b/061107.html The anomeric center: The new center of chirality generated by hemiacetal or hemiketal ring closure is called the anomeric center. The two stereoisomers are referred to as anomers, designated or or [3 according to the configurational relationship between the anomeric center and a specified anomeric reference atom. The anomeric reference atom and the anomeric configurational symbol (a or B): The anomeric reference atom is the configurational atom and of the parent, unless multiple configurational prefixes are used. If multiple configurational prefixes are used, the anomeric reference atom is the highest—numbered atom of the group of chiral centers next to the anomeric center that is involved in the heterocyclic ring and specified by a single configurational prefix. In the or anomer, the exocyclic oxygen atom at the anomeric center is formally cis, in the Fischer projection (i.e., the same side with respect to the carbon main chain), to the oxygen attached to the anomeric reference atom; in the [3 anomer these oxygen atoms are formally trans. The anomeric symbol on or followed by a hyphen, is placed immediately before the configurational symbol D or L of the trivial name or of the configurational prefix. D—sugar on! at C—1 (anomeric center) CH20H 1 H O H same side of the ring 8H H anomeric reference I HO OH H OH position a-D—glucopyranose B! at C—1 (anomeric center) CHQOH ¢ H H O OH OH H HO and O on the HO H anomeric reference opposrte Sides of the ring H OH position B—D—glucopyranose L-sugar same side of the ring Q H HO H O OH OH H H H anomeric reference I position H OH a-L-arabinopyranose Chem 215 F07 Notes — Dr. Masato Koreeda — Page 2 of 2. Date: November 12, 2007 Carbohydrate stereochemistry practice examples 1. Draw the OL—pyranose forms and B—furanose forms of each of the following. For pyranose forms draw as a Haworth projection and as a conformational representation (i.e., chair form). fl) 3 CHQOH O=CH CH CH O HO H HO H HO H Ho H HQ H H OH H OH H OH H OH HO H H OH H OH HO H CH20H CH20H CHQOH CH20H H. Draw the open—chain form as a Fischer projection for each of the following. H HOCH 2 H 20H OH OH OH OH OH OH H OH OH OH CHZOH ...
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