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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

" H+ f4 i5 \) |- a7 N. P! j

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 j6 W. { q4 p& p carbon will show a downfield shift while the belta carbon will show a" X/ f7 ?7 ~ ~" ? L# }* h upfield shif.
8 A' h& t7 q) }
) G% c' H' x3 G9 x8 h) T, ` See the following article how to revise a structure reported before just using the above rule!
2 m1 R9 O0 Y1 S( m5 ~7 C2 a. E
1 Q" t) F$ M" b' E+ ~. QOne 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.
* x# f/ N2 @/ d4 x; P
* }& Y+ P+ E" n9 U3 p9 ~. G# WAnother compound with one OH group acetylated: it could be C-6 OAc, C-78 b0 Y5 l) q" u- ?5 s; R9 P OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II3 t7 G! k+ n9 P8 m- W" d7 p. e in the article). The chemical shifts of the two carbons are 78.1 and! K* C8 ~$ k- Z* v 80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

( q; d9 _4 c7 h: h0 B; y7 m: Y
, M9 I. p& a2 L4 R# u! i/ E The auhors resovled the problem:
) ^- k k: u8 r0 P4 I7 w1 O
8 n0 R, D# f& SIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical % Z3 L3 D7 \% @ W ^shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. J y: P. M' |' T1 b: ~5 y/ q+ E That assupmtion violates the above rule.
0 [# ^/ A, \( H9 V2 T7 B; w
) C- X P/ X$ a. ?7 t, u. @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.
! S/ u* N5 x' S$ B
. ^6 K1 j) G/ f) Q0 e7 l9 i C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from / i" g/ o% q8 s5 a79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,$ T* ~2 M8 C8 ], s7 L it was the C-7 with the OAC group (compound II), not the C-6 with the / i4 S/ f5 d5 o3 _3 N% y1 a4 g, uOAC group (compound I).
6 m) n: y0 _2 f% {; M7 w6 z
. l& H8 [% y6 N- B8 W" b Simple, logic structural revision published in 1981!

% P' k" E- |# r1 {' z5 W

  / `# X2 Z6 `' o* z' ]+ p
% u- K. ?* p* o' t f/ H

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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]
B9 C1 v- P" Z( Q0 s
[此贴子已经被作者于2009-5-28 23:48:20编辑过]
. Y" K- Z0 f T6 s( t
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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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