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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 4 j+ T/ i: m. i1 o% @carbon will show a downfield shift while the belta carbon will show a ' F' q: _1 A7 O# \ lupfield shif.
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* s, h* b1 _( n1 i& T See the following article how to revise a structure reported before just using the above rule!
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. e! H4 M4 ]. n: L& s; }5 g8 X 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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) ~! N& d0 B" \3 zAnother compound with one OH group acetylated: it could be C-6 OAc, C-7& a) u; l' a# a OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II + Y' j, L# G: C) k) z6 f9 K% p) d5 Xin the article). The chemical shifts of the two carbons are 78.1 and * |; F. Y+ j' Q80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

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* l- o2 R7 V0 @3 o The auhors resovled the problem:
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0 j8 F' v( j2 k If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical- E; a3 Y* y8 J shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased." H7 k+ L7 z; M; g- ?" B That assupmtion violates the above rule.
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# i& j' F0 ]9 p3 _3 a/ X# w; DSo 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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6 U7 @0 y/ m' U9 m& s; P C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from ( Z! t( Z7 I1 d5 j5 g3 o79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,4 R; ^) S: [" F% G6 G it was the C-7 with the OAC group (compound II), not the C-6 with the) u `1 W6 w3 E7 s8 q& l: W" K OAC group (compound I).
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! @; |, J" W3 |. Z' rSimple, logic structural revision published in 1981!

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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]
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[此贴子已经被作者于2009-5-28 23:48:20编辑过]
R3 R( C* G% n. [6 S9 Q. _+ ]
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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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