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

) y2 K" C( E8 s5 g

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 % Q! y* n! {! p* A% c9 ]upfield shif.

See the following article how to revise a structure reported before just using the above rule!
' w P% O8 K7 G% @# ^' K; ?
, @5 ?9 h- V; s, {1 e' t7 AOne 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.
1 o0 Q1 F. [' k/ B9 h; N
* B: j* c7 y$ ?7 V" ], J' JAnother compound with one OH group acetylated: it could be C-6 OAc, C-7 ) _* D$ m; e2 p0 Q4 D( J2 \* TOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II 6 @( P( m3 ^( P# W3 din 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?

1 H. p/ Y+ w6 d/ o' A" ?, F
. f& Y: } A6 f5 c9 \The auhors resovled the problem:

If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical 1 ?2 I7 @- q5 @1 ]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.

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.
4 ~+ ~5 F( D6 Q1 B7 g
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 T/ S* c- `& s( V2 e- j0 ?it was the C-7 with the OAC group (compound II), not the C-6 with the 7 F. v6 W* W' zOAC group (compound I).

. R% q" c( B7 d6 Z5 P2 B4 S0 P. bSimple, logic structural revision published in 1981!



 
3 p, p3 Y9 c, ~0 ^

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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