请教这四种类似结构某个C化学位移的相对大小

zlfbaggio

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

北冥有鱼

 Why?
The conclusion is 4>3>1>2.

北冥有鱼

  On acetylation (OH group was changed to OCOCH3 group), the alpha ( P- ]- F( ]2 Z* `1 X5 s# Xcarbon will show a downfield shift while the belta carbon will show a upfield shif.
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5 i I/ K. n* b# d6 h. B7 BSee the following article how to revise a structure reported before just using the above rule!
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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.

: t6 {- l, d3 Y1 y- \Another compound with one OH group acetylated: it could be C-6 OAc, C-7 OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II in the article). The chemical shifts of the two carbons are 78.1 and 5 T) v4 r' o) M# h6 P1 W: z+ c80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

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P' M% ?7 Q( T: |$ YThe auhors resovled the problem:

If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. - O$ w: S" w* u3 q ~That assupmtion violates the above rule.

- t' M$ s2 A' w5 F$ dSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5.

C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from 79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically, 5 O3 d8 w9 a" h1 G; Hit was the C-7 with the OAC group (compound II), not the C-6 with the u; Y0 t* }, bOAC group (compound I).
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Simple, logic structural revision published in 1981!

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北冥有鱼

 

北冥有鱼

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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