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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

# C- z+ u7 j9 _6 ?# `& `

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* l! w% O1 S, W2 b- Y2 C carbon will show a downfield shift while the belta carbon will show a - o: ^& t [8 h. Q/ r6 M Tupfield shif.
' l1 J/ ?5 _ ?9 \/ l/ U9 I, Y) i
( l+ l4 c9 o9 B- ~% L2 A See the following article how to revise a structure reported before just using the above rule!
% [; L! m& D' ?; W. ?& Z m
# b. G# T2 }. ] T/ E5 i 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.
# [- M, ^' O5 e
' _# V2 ^, Q+ Z) S- L Another compound with one OH group acetylated: it could be C-6 OAc, C-7$ |, r8 Z. o4 w* E/ U- E* G7 n3 P OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II & o2 U6 u. Y1 s; U( W2 E- k4 Qin the article). The chemical shifts of the two carbons are 78.1 and 7 J9 a' ~: T$ U+ a80.5. How to determine which carbon (C-6 or C-7) has the OAC group?

1 Z& M7 V% x" ?+ `: b" _9 w
( G* ^+ ]; v4 I0 J4 l6 V The auhors resovled the problem:
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! Y" `8 | x( A If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical . ~0 @$ A4 a+ w% M0 [9 o- p- [shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.. b, Z! T3 J2 m/ s$ v That assupmtion violates the above rule.
6 J# e' d& w3 i ^* |
4 B7 W8 u- O' Z* h5 O( E! QSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5.
, a' m$ \/ W. T7 h+ S! k
3 N- o9 K/ P" i. a* f3 ]C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from 6 E6 {: ]& ^# n F @1 v) D79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,% g! n" i) p% B7 @$ K; N it was the C-7 with the OAC group (compound II), not the C-6 with the; Y0 N5 U- ^, d4 T. a$ F h OAC group (compound I).
4 ^7 q0 n5 M+ E m* a8 B3 P
1 s6 _* s9 A- ISimple, logic structural revision published in 1981!

* K/ t$ ]( F; ^6 t0 @8 x1 c

 ' ^, f" q9 I% T3 B3 j2 {; H
/ b# e& K1 _# U+ U+ [1 c) b

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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]
" n+ B6 h- H/ I! R1 B; n
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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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