On acetylation (OH group was changed to OCOCH3 group), the alpha) I$ ~6 B( X8 J$ W! U
carbon will show a downfield shift while the belta carbon will show a% }" b `3 `, C6 N
upfield shif. ' R) D( l, C% R# n6 f% P ) {. S& f) |! T( ?, A% E
See the following article how to revise a structure reported before just using the above rule! - C- b! z" @) N% I 2 o5 P- i( u# Y% Z" UOne 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. % p- s" @; Q+ o M' g9 ~6 C2 U8 P7 _
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Another compound with one OH group acetylated: it could be C-6 OAc, C-7- m' [" E: j2 Q" p L2 S' n
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II; Z2 k* \8 W2 b r5 v
in the article). The chemical shifts of the two carbons are 78.1 and
8 R; x0 Z* E; |" L5 K80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
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The auhors resovled the problem: / l0 a/ q/ @7 e( S ' Q6 g `0 Z% `# ^
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
: e4 g R( ]3 N9 a7 `shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
& c- F1 f% b/ \That assupmtion violates the above rule. 4 o& E$ Y+ T z4 ?
, b$ ~" r* b6 {3 i% l# V
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. / {0 j: G' ~ `3 q( p* A 3 _, \1 ]: V; I* UC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from9 D4 h1 n" e p! r
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,8 v- Q; o; u& S. a X7 c$ X% P* f
it was the C-7 with the OAC group (compound II), not the C-6 with the; K+ }" J# k4 f) A- H
OAC group (compound I). 1 p7 b( \; Y# K; o 3 V% l1 ?' \5 q. d: |
Simple, logic structural revision published in 1981!
4 }$ l+ N3 K4 O" E! J
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楼主: 上善若水
发表于 2009-5-28 16:27:12
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