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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

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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 ' r+ L( Y8 M: g, q, qcarbon will show a downfield shift while the belta carbon will show a, f/ F* s2 V0 Z6 | upfield shif.
2 J/ b u# W; A2 Q% J0 u8 t1 b; G# h
4 s) x C7 P4 i/ R$ g6 ]See the following article how to revise a structure reported before just using the above rule!
2 t$ C/ ]0 Y1 R" b5 u$ R( I0 J% d- e
! G; |& L% d" s" s" H 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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# d+ l/ D3 `) Y0 c8 K/ |Another compound with one OH group acetylated: it could be C-6 OAc, C-7: [1 Y {; { @5 R- _# B OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II 3 E5 Z. ?/ x9 O& sin the article). The chemical shifts of the two carbons are 78.1 and 3 }( a8 V2 D# S$ e! c3 w80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

3 @6 H6 Z* _0 s4 O6 I
; d& S( U c9 Q) z- e# _The auhors resovled the problem:
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6 b6 `* i/ ^1 V! x5 w3 X2 \" Y- O If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical . N" e6 x3 L( I. C9 O: Jshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.' x- R* r: o- C: l7 D: ?: M That assupmtion violates the above rule.
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. T. ~# t3 w" b& f: R8 PSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5.
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8 C2 b; l: y4 P6 r. [& ~; a C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from. d8 \: t& }; i. C8 O, j 79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,. A) t+ e$ R6 \ it was the C-7 with the OAC group (compound II), not the C-6 with the * x4 D0 N' C& x4 Y% HOAC group (compound I).
) k) K% _, V& S" i! ?
5 {1 f/ ?2 J" B/ n Simple, logic structural revision published in 1981!

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 , h' B' e% e: g3 {0 H
# y, h' R# J4 X1 n1 G

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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]
m" t( K0 o7 B/ C
[此贴子已经被作者于2009-5-28 23:48:20编辑过]
5 E# G# r( p$ ]8 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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