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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! }' C$ D# z) m carbon will show a downfield shift while the belta carbon will show a' r. m# }$ N5 d1 J: Z( Q! E. z4 r upfield shif.
' k1 N2 ]0 r! _. J% J
/ ^) _7 I# i0 E: rSee the following article how to revise a structure reported before just using the above rule!
- }! t! r% C: Q4 L2 @8 l
3 z$ R5 d2 { \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.
& j4 u( X' C. j( }# s |
; t9 S$ z% H" i" PAnother compound with one OH group acetylated: it could be C-6 OAc, C-7 - O8 D! l9 B/ iOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II ) a& X' z- f$ uin the article). The chemical shifts of the two carbons are 78.1 and 6 L" W5 w H/ G" s+ E" b80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

* _2 Y+ z2 ?, \2 S; n
- S3 N7 |, c. p1 TThe auhors resovled the problem:
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" h. s0 p6 } q' B* M; A( u7 J9 K- IIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical: Z, u, ]" h# [) z- S9 {+ u shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. & l/ b+ ?* a3 f" h' q4 ZThat assupmtion violates the above rule.
( R5 l$ {# z# o% q5 r
2 q" V# s' j! p6 J4 t 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.
+ e3 t8 g5 p7 {3 N2 A1 w- l
- |7 P) A/ D6 C/ ?. W, ]C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from. x% L& I/ w6 J% v x. t 79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically, J; l9 `, N. _1 v( V% t2 b it was the C-7 with the OAC group (compound II), not the C-6 with the % G& q% W, x+ iOAC group (compound I).
2 E% g3 w9 i* o, w
( K. S/ ?$ [' } Simple, logic structural revision published in 1981!

( K1 M( S8 S8 w0 s

  # `2 S( U: K' l5 y8 S p' P% m$ G
$ B+ _! m+ T3 `' ^

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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]
1 r `) E; j, K+ G2 I, w1 m4 z
[此贴子已经被作者于2009-5-28 23:48:20编辑过]
$ ?( `; U& J5 ~
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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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