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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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

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 . }1 W- x; q- lcarbon will show a downfield shift while the belta carbon will show a upfield shif.
* F- ]7 N; n$ R
See the following article how to revise a structure reported before just using the above rule!

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.

: U# v: G6 [3 A+ b, sAnother 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 80.5. How to determine which carbon (C-6 or C-7) has the OAC group?


2 W+ Z3 ]! ~ n" r$ dThe auhors resovled the problem:
; I+ n- K. j' j
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. m4 ~- X, q2 w0 M6 w4 OThat assupmtion violates the above rule.
9 y R" S4 o, _) `
; x$ B2 \1 {* k, ~$ JSo 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 7 `8 P+ B; G& W4 U$ L) z79.3 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 OAC group (compound I).

Simple, logic structural revision published in 1981!

$ Q$ s2 H# x; M2 T

 
' O; `% S4 s- z! b. J9 I& z; S9 Q* `

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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