On acetylation (OH group was changed to OCOCH3 group), the alpha7 V' w* p) a3 K; h. C
carbon will show a downfield shift while the belta carbon will show a
4 W# w9 ]4 F" d% nupfield shif. 5 R$ P5 K" P9 P+ _3 M' d : [- u, m0 R: P, O7 ?
See the following article how to revise a structure reported before just using the above rule! 6 [0 t% M1 R/ K+ c1 R6 @/ I / l9 c% f' {! r9 G* e$ x) \2 m/ r/ G
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. 2 ^' C4 Y o4 s; u& {4 A
- C7 s& ?0 P; c; [ I9 A6 V, p
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
! g: A0 v' Q; C% d4 B" sOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II$ p u- p' T1 c! B# Q8 V' o( l
in the article). The chemical shifts of the two carbons are 78.1 and
# r* a3 [3 O' K: }80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
: p. Z [3 k. P1 f/ T( p% P6 H! ~9 X 9 f' v, j% |2 I; t, e0 Z5 w
The auhors resovled the problem: ; _1 O: S+ l, [3 s: v/ N1 t. { + Z- V' F. m0 Y# a7 m/ U3 BIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
8 _/ f5 x# d* p- O8 q. ~; ]shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
* [9 f, G3 B& Z& m5 Q4 k+ M/ oThat assupmtion violates the above rule. Z) w; W& ~3 @" B
" P6 k: B$ q. t- j# F% E
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. , ^6 K2 D9 z" G& g
. V$ P( B2 L( `
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from/ E9 [, o2 T, O) T _: h [3 ]8 E
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
* \% }+ H# r5 V2 A+ pit was the C-7 with the OAC group (compound II), not the C-6 with the
; C% G; W$ R6 I% K0 F0 H7 | _OAC group (compound I). 1 G% M) L: `5 O7 t
9 E. l) S$ U0 GSimple, 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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