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

$ @+ s' z5 }/ h; l# E( `% c0 N

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 carbon will show a downfield shift while the belta carbon will show a upfield shif.

! h& G* o$ h# V) n9 S3 b) {See the following article how to revise a structure reported before just using the above rule!

' C0 `- d0 `5 G: k* z# GOne 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.

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 , B$ B3 K5 }/ Q2 l4 Qin 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?


The auhors resovled the problem:
9 U2 y' p* |; }) A C8 \0 i* c
/ W9 g* Y; J8 E: ^, g! v2 cIf 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.
3 c2 r1 }8 i$ i1 p2 e
5 y' j$ h7 U6 c& c" M- T$ Y+ CSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5.

/ v3 r8 O$ I4 M' g: O- HC-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 OAC group (compound I).
, C$ H* w& n% |: Q: U
; V* [7 c/ N" {1 r8 ZSimple, logic structural revision published in 1981!



  + O' ~- v+ H( C7 m
: ]! i* @0 y3 v4 @7 W1 [

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]
8 [, H, z6 @7 j) K$ S4 ^# S Z, n" c

[此贴子已经被作者于2009-5-28 23:48:20编辑过]

3 `0 [4 ~ ^7 j; t& t$ I

上善若水

受益匪浅