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 / c8 Q P9 M: y; N' ~# E7 C2 icarbon will show a downfield shift while the belta carbon will show a ; O6 p4 Q' } z0 P- N8 F2 Jupfield shif.
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See the following article how to revise a structure reported before just using the above rule!

; `* k! R% `1 z4 ]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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! z4 e5 i* b* X; D JAnother compound with one OH group acetylated: it could be C-6 OAc, C-7 5 `& h# `0 k; [4 o6 U3 [$ }OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II ~$ R5 L/ Z1 p: sin the article). The chemical shifts of the two carbons are 78.1 and " `8 J/ K6 T4 r2 d w3 n& n80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

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The auhors resovled the problem:

# ?% S6 d9 n+ E& b- KIf 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. That assupmtion violates the above rule.
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So 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, it was the C-7 with the OAC group (compound II), not the C-6 with the $ [- T2 w8 |$ E0 VOAC group (compound I).
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% X: i( [% b/ U7 d5 t- {& `3 @+ aSimple, 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]

[此贴子已经被作者于2009-5-28 23:48:20编辑过]

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