On acetylation (OH group was changed to OCOCH3 group), the alpha8 ~$ H/ P# Q7 \& B- j' Q! c7 k
carbon will show a downfield shift while the belta carbon will show a
3 g) V& ?& a3 b1 s0 [upfield shif. + n/ H7 c# I8 | ; A) ~. B- L4 O, Z( y
See the following article how to revise a structure reported before just using the above rule! $ z; F7 N+ O* V
% \ s* c4 T# V, V! R" O0 hOne 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. - U# \- t: s* v j
+ u2 r) J8 c' H4 Y! R8 pAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
* P n& j. R( a. K- c$ A7 TOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
( D: T* y5 v0 @- ]8 pin the article). The chemical shifts of the two carbons are 78.1 and2 R6 v. u' e, x7 C! R
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
_% c' r/ U( h, e/ P
- D6 S. }$ e# v% h) mThe auhors resovled the problem: / n; C- M' t& Z g " Z0 h+ k2 B3 }9 B7 d
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical4 u6 {; W8 e$ @4 e6 O
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
7 d: c+ B( p9 F6 u2 M VThat assupmtion violates the above rule. ' M& J6 Y# l1 V. c L8 ` Z
t) @0 U& o, G7 w3 g. e' L7 vSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. + b# I. F3 o6 }* l2 U + N9 J) x0 ~- z$ s
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from* k( U, p1 Q, A3 q
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
, a3 P# A' j1 K* M% cit was the C-7 with the OAC group (compound II), not the C-6 with the8 s% n& v6 L ?. }
OAC group (compound I). % r2 _. v- x" b
* f3 v5 N) A) dSimple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
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