On acetylation (OH group was changed to OCOCH3 group), the alpha
* Y: V6 N5 g* Y9 s; Ucarbon will show a downfield shift while the belta carbon will show a6 q- {% ]& ]1 d9 z* V6 V
upfield shif. ; @' g) g0 ?6 o- c 9 r4 j) s D* n4 p L
See the following article how to revise a structure reported before just using the above rule! 3 S, |' z9 j# V9 B
; ?* l* R# H. x, cOne 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. , @% I6 A. @7 @9 G- ]" { - ^$ E7 i' }/ K" hAnother compound with one OH group acetylated: it could be C-6 OAc, C-7: I/ n5 L* `; k9 Q; R1 {
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II* o' u# W5 ~( m' R! H
in the article). The chemical shifts of the two carbons are 78.1 and1 F; U+ y; n: c( C9 f
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
0 s4 @# j9 B$ y! u" A6 t; g 7 ` Y: s, w6 \: i1 v
The auhors resovled the problem: 7 {9 X4 S7 T4 D0 B5 z 1 ^6 t/ s1 a0 w0 OIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical/ t! B: ^8 r) H0 }) Q1 R; i- u$ X
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased., D0 }% ?" B/ r3 D+ S: Q
That assupmtion violates the above rule. % E" c. ^7 b+ d2 {
$ z" n1 U' G5 A. G6 w" L2 h$ T/ J
So chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. * E5 {7 E0 S5 y' O
% I! E0 W8 A9 c1 D# ?, GC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from0 u6 }( n( f4 r3 |
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
7 u4 ?; W. N3 s; g% ~! |it was the C-7 with the OAC group (compound II), not the C-6 with the
1 k" w" k2 ~" Q! v. HOAC group (compound I). 2 u# X1 ]# T& `. u/ J8 t 1 P# p' C1 _8 ]! f' fSimple, 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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