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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

& @0 M: o( C3 r

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 alpha7 V' w* p) a3 K; h. C carbon will show a downfield shift while the belta carbon will show a 4 W# w9 ]4 F" d% nupfield shif.
5 R$ P5 K" P9 P+ _3 M' d
: [- u, m0 R: P, O7 ? See the following article how to revise a structure reported before just using the above rule!
6 [0 t% M1 R/ K+ c1 R6 @/ I
/ l9 c% f' {! r9 G* e$ x) \2 m/ r/ G 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.
2 ^' C4 Y o4 s; u& {4 A
- C7 s& ?0 P; c; [ I9 A6 V, p Another compound with one OH group acetylated: it could be C-6 OAc, C-7 ! g: A0 v' Q; C% d4 B" sOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II$ p u- p' T1 c! B# Q8 V' o( l in the article). The chemical shifts of the two carbons are 78.1 and # r* a3 [3 O' K: }80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

: p. Z [3 k. P1 f/ T( p% P6 H! ~9 X
9 f' v, j% |2 I; t, e0 Z5 w The auhors resovled the problem:
; _1 O: S+ l, [3 s: v/ N1 t. {
+ Z- V' F. m0 Y# a7 m/ U3 BIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical 8 _/ f5 x# d* p- O8 q. ~; ]shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. * [9 f, G3 B& Z& m5 Q4 k+ M/ oThat assupmtion violates the above rule.
Z) w; W& ~3 @" B
" P6 k: B$ q. t- j# F% E 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.
, ^6 K2 D9 z" G& g
. V$ P( B2 L( ` C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from/ E9 [, o2 T, O) T _: h [3 ]8 E 79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically, * \% }+ H# r5 V2 A+ pit was the C-7 with the OAC group (compound II), not the C-6 with the ; C% G; W$ R6 I% K0 F0 H7 | _OAC group (compound I).
1 G% M) L: `5 O7 t
9 E. l) S$ U0 GSimple, logic structural revision published in 1981!

4 X, i' O* p6 k0 c3 f3 M

 # G) X0 l/ k w4 ?0 k
5 d/ X" s) J& I9 b8 m

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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]
% ?4 D% f; {! b' p/ a( R p, P
[此贴子已经被作者于2009-5-28 23:48:20编辑过]
n4 w2 F5 `/ h# O0 }
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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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