On acetylation (OH group was changed to OCOCH3 group), the alpha! k% [1 i, i! u, R( ^4 H, `
carbon will show a downfield shift while the belta carbon will show a
1 R4 e; g: ~7 y; x2 R( ]upfield shif. 1 V; z% i- w# {9 j) Z/ B 7 s* t ^0 y% n8 nSee the following article how to revise a structure reported before just using the above rule! * h3 y5 P4 v' {% i8 x2 T 2 m/ L9 R# m* p5 ?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. 9 J! m0 [, o i2 L6 O. ]" j
: v1 X* y G% j( h4 S6 dAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
) R) n; H b" E5 |4 A" x7 DOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II+ E% J( `' |- `% `8 J+ ?
in the article). The chemical shifts of the two carbons are 78.1 and
2 T0 I& p+ u* d+ u, J80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
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The auhors resovled the problem: ' G4 J$ K1 u; p$ _
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If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
7 y" H% F8 w1 T& Wshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.4 k+ g+ p! r+ k# K; y0 H% K
That assupmtion violates the above rule. $ D2 M3 v9 X! b7 e* C/ w4 N* w' J
& a* ^% E. m) ]8 n7 L1 t1 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. 1 [% z) Y, F9 v& s4 s/ W6 f + s+ \0 |5 L" F) g/ D- uC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
1 A6 Y5 `2 m I& C# {( Z8 Y9 z2 v79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,# r- I% D2 r; Y {" P
it was the C-7 with the OAC group (compound II), not the C-6 with the
( x* `) A2 G$ WOAC group (compound I). # q& L7 r; @/ ^& S! j
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Simple, logic structural revision published in 1981!
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楼主: 上善若水
发表于 2009-5-28 16:27:12
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