Reactions of the anomeric center-i

Transcription

1 Reactions of the anomeric center-i 1

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3 The anomeric centre H H H 2 H H!-glucopyranose "-glucopyranose 38% 62% Axial Equatorial 3

4 Acetal forma1on and hydrolysis R Me H H + Me R H H H R H Me R Me H Me + H + Me R Me Me H H + H + Me R H H Me R H Me H R Me H H H Me + R H H + R Me H 4

5 Acetals and hemiacetals are in the same oxidation state as carbonyls and it is therefore not surprising that they are readily interconverted and you should think of the process of aldehyde-tohemiacetal-to-acetal as one reversible rxn. 5

6 Fischer glycosyla1on H H +H + H -H + H H H H H H R +H + -H + H H R H H Glycosyl ca1on Presence of an oxygen subs:tuent α to the anomeric carbon, makes any carboca:on formed there more stable R H 6

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8 Maltose, Malt sugar beautybodyandskin.com buyfoodsale.com 8

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11 thin- layer chromatography (TLC) "N" for normal soybean "S" for sugar standards "H" for high sucrose soybean 11

12 hip:// TLC_Student.html 12

13 Fischer glycosida:on/glycosyla:on Equilibrium reac:on Treated with aqueous acid can reverse the reac:on Source of anhydrous acid: Bronsted acid/lewis acid Under thermodynamic control Stable under almost all other condi:ons except aqueous acid 13

14 Steric effect H H H 2 H H!-glucopyranose "-glucopyranose 38% 62% Thermodynamic stable pdt 14

15 Anomeric effect (A) (B) 15

16 Effec:ve only: X = electronega:ve atom,, F, Cl, Br No stabilising electronic interac:on at equatorial posi:on Several consequences: Shortening of - C ring bond length Lengthening of C- X bond 16

17 Ac Ac Ac Ac Ac Ac 1:5 Ac Ac Ac Ac Cl 1:32 Cl CH 3 K eq = 3.4 in CCl 4 K eq = 1.8 in CH 3 CN CH 3 Modern Physcial rganic Chemistry Page

18 Mutarota1on H H H bond H H H 2 H 6 rotation H H! H H " H H! " 18

19 Position of the equilibrium Glucose: thermodynamic stability of pyranose ring In this case, all substituents can adopt equatorial positions ther sugars may have larger amounts of the furanose form at equilibrium 19

20 Relative amounts of α and β pyranose pdt Anomeric effect of α form Steric effects favor equatorial substituents Steric effect may, at least in part, counteract the anomeric effect Competition between electronic effects (favor α anomer) and steric effects (favor α anomer) 20

21 H H H H Me Ts, Me 25 o C, 24h 25 o C, 4d Ts, Me 45% + 55% Ts, Me reflux 21

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24 Summary To understand the underlying reasons for the anomeric effect and to be aware of some of the other consequences To understand and describe the process of mutarotation To be able to ratinalise the relative proportions of α/β and furanose/pyranose forms present at equilibrium for glucose 24

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31 SWEETING AGENTS 31

32 one of five types of taste sensed by humans. Sweetness, sal:ness, sourness, biierness and savouriness. Sucrose is the standard sweetener used in cuisine. A less expensive alterna:ve known as high fructose corn syrup (HFCS) developed in the late 1950's widely used in baked goods and beverages 32

33 HFCS is made from corn syrup by enzyma:c conversion of glucose to fructose. Fructose is 2.3 :mes as sweet as glucose and 75% sweeter than sucrose, then HFCS provides a prac:cal subs:tute for sucrose in a variety of applica:ons, and is available in composi:ons ranging from 45 to 90% fructose. Since the specific rota:on of these sugar solu:ons changes from +66.5º for pure sucrose to º for the hydrolysis mixture (fructose is strongly levorotatory), the resul:ng glucose fructose mixture is called invert sugar. invertase 33

34 FDA approval in not significantly metabolized, so no calories in 1879 at Johns Hopkins University 34

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36 Sweet triangle For sweetness to be perceived, molecules of a substance must ac:vate receptor sites in taste bud proteins on the tongue. The A(H) and B regions encompass func:ons of higher electronega:vity, the distance between A and B (2.4 A A) C, represents a hydrophobic and lipophilic region of the molecule 36

37 Stevia rebaudiana Bertoni Steviol 37

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