On acetylation (OH group was changed to OCOCH3 group), the alpha
|# L o( T0 hcarbon will show a downfield shift while the belta carbon will show a
+ C2 h: I0 j2 x2 yupfield shif. - o" Q5 R T/ ^ $ D+ s5 i$ b& a7 O
See the following article how to revise a structure reported before just using the above rule! ' d8 V. M6 q. t( s
+ a, z/ A- |5 ~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. # D) I; W: R Q# |: S" Z7 j9 o # K D: q; B+ k
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
2 h+ n0 \3 j* M3 L# j+ oOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
+ q/ K" V- Q5 E% s2 Q qin the article). The chemical shifts of the two carbons are 78.1 and
3 k" d! l% j i; @80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
( b R z! U3 s P o: F& D9 M
+ c7 O1 z, [ F: P+ x6 K7 P5 u
The auhors resovled the problem: 2 ^0 s e1 l* e& Z/ {3 L1 |5 A2 U: N # h" [9 N, U: ^6 G2 A7 l
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical B5 A& d5 }" R, E9 O) w& X
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
% ]1 K* g9 m" D# K" l& SThat assupmtion violates the above rule. 0 F. J( K# X8 N' c
5 x- k. N( X9 Z& z4 G. i% k( cSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. * j$ s9 b# n$ z: J* J4 i
- v' A! M1 x( B* f) Q
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from/ L# F$ g+ [$ J6 u. P3 N
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,; C- U; e2 j/ _& c
it was the C-7 with the OAC group (compound II), not the C-6 with the
1 I- v# Z9 a1 `# BOAC group (compound I). : n' H2 C4 m' y1 i3 K7 }! o/ x( A 0 n) f H+ ]3 X5 }# D5 Z; N( ySimple, 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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