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请教这四种类似结构某个C化学位移的相对大小

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发表于 2009-5-28 16:27:12 | 显示全部楼层

4>3, 4>1

3>2,
1>2

3>1 (-OCO- alpha +56.5; belta +6.5; -OH alpha +49.0, belta +10.1) although 3 is close to 1

4>3>1>2

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according to your conclusion,maybe 3 is the  lowest

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发表于 2009-5-28 22:02:51 | 显示全部楼层
 Why?
The conclusion is 4>3>1>2.
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发表于 2009-5-28 23:13:32 | 显示全部楼层
  On acetylation (OH group was changed to OCOCH3 group), the alpha4 @" v, b2 ~" J( @- | carbon will show a downfield shift while the belta carbon will show a 9 m* l& Y' d- N6 |& qupfield shif.
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, ^. a, A, W6 r7 n* L+ |# X4 F See the following article how to revise a structure reported before just using the above rule!
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' C% s1 }' g4 @& [5 t1 I: P; i One compound with C-6 OH, C-7 OH (compound IV in the article):  chemical shift values of C-6 79.3 ppm, C-7 76.2 ppm were known.
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! L E! S( M$ I+ u8 V8 AAnother compound with one OH group acetylated: it could be C-6 OAc, C-7# H0 R7 {. a5 f$ {- i OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II . ?* z, |: l/ {8 Kin the article). The chemical shifts of the two carbons are 78.1 and - }* M" h: x4 V9 b; X; b6 T80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

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2 k7 k4 \" K0 G$ B6 ?: @The auhors resovled the problem:
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; g2 H3 t, Q2 r, ~' r$ H) m& h" nIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical2 i+ D0 y1 F6 i8 X shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. , ]) Y2 O* ]- L0 `6 SThat assupmtion violates the above rule.
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) B( N9 [! }" w7 A5 q2 z7 P0 l2 ASo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5.
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5 a' ^. D, S, K, O6 H6 ` C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from+ Q2 P9 i3 G5 h5 m P" d 79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,/ }4 C; o a7 _2 R it was the C-7 with the OAC group (compound II), not the C-6 with the3 o) G0 U; R3 u* @6 w% d OAC group (compound I).
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5 g7 @$ t+ n( Z9 O* I5 FSimple, logic structural revision published in 1981!

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发表于 2009-5-28 23:17:25 | 显示全部楼层
 

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发表于 2009-5-28 23:32:40 | 显示全部楼层
More example:

CH3CH2OH  chemical shift values: 18 (CH3), 58 (CH2).
CH3CH2OAc chemical shifts values: 14 (CH3), 60 (CH2).

These data should be remembered to remind the above rule.

alpha carbon 60>58;

belta carbon 14<18.

[em79][em79]
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[此贴子已经被作者于2009-5-28 23:48:20编辑过]
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 楼主| 发表于 2009-5-31 10:07:36 | 显示全部楼层
受益匪浅
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