On acetylation (OH group was changed to OCOCH3 group), the alpha# b/ D: q. z3 U1 Y& r
carbon will show a downfield shift while the belta carbon will show a9 A! E# g+ |' j
upfield shif. " I' m3 e0 q8 g8 |( z7 `
( S& d4 l! K2 G& T
See the following article how to revise a structure reported before just using the above rule! ) s9 N& s$ C" w& s/ {
% R$ t" [. o" l/ a+ TOne 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. 8 T7 V$ A" a Y ( M' A J0 o# s$ K7 H* g" @5 ]0 jAnother compound with one OH group acetylated: it could be C-6 OAc, C-7, W/ S# u D" O+ }9 F. ?
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
0 y6 g; T& k. W! O; x, P7 Rin the article). The chemical shifts of the two carbons are 78.1 and% `. K8 x9 {* g) Y6 n( O
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
" J; A$ z- c( h- j 5 o3 m$ N: b( b: q( X$ X& ?The auhors resovled the problem: s5 l4 f6 U6 q F& w# r4 [
+ z7 V; x* I! @2 RIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
+ C: A% L. @/ S# M$ E- X- Cshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
0 l! z7 E( l3 s) |$ A6 BThat assupmtion violates the above rule. 9 ` Z' t6 p" b, y ! q& }9 o% V5 J6 I: n0 ISo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 2 d( O9 n: L, D% `' { : y, V" ~/ b, o$ q+ t% y
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from2 z$ @. ]$ {+ a4 @5 g
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,; @+ R# h j& |7 ~* j4 T
it was the C-7 with the OAC group (compound II), not the C-6 with the
5 l: E4 v6 m" G7 A9 @' xOAC group (compound I). - b, G/ w+ ~( x; \' p" a5 |
3 @7 L, P1 N. N$ E+ t4 k Z
Simple, 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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