On acetylation (OH group was changed to OCOCH3 group), the alpha+ V, D9 ]2 |$ R4 a* C
carbon will show a downfield shift while the belta carbon will show a- p( {/ d6 G+ g6 Q: C" j9 F
upfield shif. ) L6 _ m a1 B G# R9 V) a# V* s0 k 8 c* k; J* R: Y
See the following article how to revise a structure reported before just using the above rule! ! G3 O. G/ }& h: K, B p 6 |3 q1 s, a$ F! `. `. m% Z# dOne 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. . Q. J. T1 S- o0 x
0 ~$ f* k+ i; C( yAnother compound with one OH group acetylated: it could be C-6 OAc, C-71 l0 u3 W- z6 H9 W3 f1 y& J
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
- m4 {, {9 E' Q7 f/ z; ?2 c! {4 rin the article). The chemical shifts of the two carbons are 78.1 and
' S+ }6 r7 B( `3 F: l9 A( n4 ~80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
( m) ^; L9 L& S' X9 }/ b+ |
}. \+ v4 ^; [7 N! K1 S' GThe auhors resovled the problem: 2 ]! w* y% M) k! w% ~ $ F) \! P/ A, oIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
+ l: y+ k* @& w. M, V9 q7 K( T, w# Oshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.4 V0 m3 \! i0 S" s* y+ o1 o E
That assupmtion violates the above rule. 3 d6 g" z, x/ A1 I8 O
8 z, b e$ N! J) j; A8 ]' zSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 4 h- W$ I; {6 O( r* E D
2 T! m+ V! f3 B0 `" S
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
! E% M( K6 A" J79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
8 f+ q+ D% C# Y$ [( eit was the C-7 with the OAC group (compound II), not the C-6 with the: R8 V* M! K5 L' Q( W. S
OAC group (compound I). 6 ]% A6 T+ e3 _; h/ Z, J* @! K : c8 r: l' r( B# zSimple, 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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