请教这四种类似结构某个C化学位移的相对大小

zlfbaggio

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

: j- l- G$ X% A W& c

according to your conclusion，maybe 3 is the  lowest

北冥有鱼

 Why?
The conclusion is 4>3>1>2.

北冥有鱼

  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.
5 ~; m" R( M U- B2 }5 j6 W
See the following article how to revise a structure reported before just using the above rule!
; x3 [& W! N! e8 z3 J+ l
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.
3 V) T- ^ a9 U8 D$ F/ Y$ w# M
) ~! N& d0 B" \3 zAnother compound with one OH group acetylated: it could be C-6 OAc, C-7 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?


The auhors resovled the problem:

If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. That assupmtion violates the above rule.

# 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.

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, it was the C-7 with the OAC group (compound II), not the C-6 with the OAC group (compound I).

! @; |, J" W3 |. Z' rSimple, logic structural revision published in 1981!



  ! V6 Q% E* _& [0 h9 Q) V
1 `4 n: T. D, c- m1 e6 m

北冥有鱼

 

北冥有鱼

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]

[此贴子已经被作者于2009-5-28 23:48:20编辑过]

上善若水

受益匪浅