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楼主: 上善若水

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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发表于 2009-5-28 16:27:12 | 显示全部楼层

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

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发表于 2009-5-28 22:02:51 | 显示全部楼层
 Why?
The conclusion is 4>3>1>2.
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发表于 2009-5-28 23:13:32 | 显示全部楼层
  On acetylation (OH group was changed to OCOCH3 group), the alpha- f' O! K' ~+ W, o M% [! _- Y carbon will show a downfield shift while the belta carbon will show a; }$ [. H: h, {- w& l upfield shif.
0 [. L- D' ^/ D8 J: Q* [7 d; T2 [
9 Z! _3 C/ w6 u1 L; Q( uSee the following article how to revise a structure reported before just using the above rule!
* _5 {( ]/ R) u$ X, k9 ^0 U# U: J
& u2 E4 e8 \# N 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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& ^4 |) k! _; o. C. LAnother compound with one OH group acetylated: it could be C-6 OAc, C-7/ d, f+ o9 S8 f3 {7 k0 H6 b( z" w0 M5 u OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II: ^8 c, ~0 y3 N, {6 v; L0 }& C3 K in the article). The chemical shifts of the two carbons are 78.1 and( E0 E. l& r# R( t# o9 l: _ 80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

6 c9 f* R' x! p5 ~
+ P1 \7 a' z4 c* Q/ S: ~# t" IThe auhors resovled the problem:
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9 T! A$ M: r* S3 \: ~If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical+ [& I3 [9 o2 t( Z shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. - |+ h M3 K; E/ ?# ]8 W4 IThat assupmtion violates the above rule.
' Q2 k( @6 o8 ^9 l* e. `0 j
! K+ ]3 l1 X" ?2 X! w9 ^: W; P 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.
: l6 c2 _) t0 v, H* p+ p4 a
. @5 |- D8 {! h. MC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from J3 f% g9 u4 T% k% }: Z$ Q& o, C79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically, : d( |5 J' G- w' L8 F. Z! Wit was the C-7 with the OAC group (compound II), not the C-6 with the# ]1 ^5 u- M1 K0 H# t OAC group (compound I).
7 a0 b' o* S2 z$ y, g
- p4 G& s, R" |3 E2 ISimple, logic structural revision published in 1981!

& u( m) _$ }, s# Q; o

 5 Z6 g/ J ~7 ?2 t" ~
) s/ p2 M6 Z$ g$ q% S

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发表于 2009-5-28 23:17:25 | 显示全部楼层
 

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发表于 2009-5-28 23:32:40 | 显示全部楼层
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]
S( n5 c- _: e; x
[此贴子已经被作者于2009-5-28 23:48:20编辑过]
( U B; u" p' N5 r; s+ ~) u
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 楼主| 发表于 2009-5-31 10:07:36 | 显示全部楼层
受益匪浅
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