On acetylation (OH group was changed to OCOCH3 group), the alpha
% h* S; j3 S7 Hcarbon will show a downfield shift while the belta carbon will show a3 u0 `( C }+ c- `
upfield shif. 5 R& Y/ i j/ K : `6 d% e4 S+ a' N2 S7 `See the following article how to revise a structure reported before just using the above rule! , T9 m) R+ w) ]0 }- z% K: b4 J
/ M9 p1 i2 u; B+ J q) O; I! ROne 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. ) u+ J3 L) g; w* | 8 t* g% I! }9 I n: [% g4 N; eAnother compound with one OH group acetylated: it could be C-6 OAc, C-7$ j/ G' n, y: F" p0 S8 M
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
9 ]3 R" [1 b7 F3 h5 I% t0 Oin the article). The chemical shifts of the two carbons are 78.1 and
$ D$ ?- _! g, V# c1 B80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
1 R% n2 T* n$ x* O' L: ~' n! \" N/ q* _ 3 R6 r7 E! Y/ }* f0 e* n
The auhors resovled the problem: 3 ~, t% i, k) E4 W" { , Y4 B; L6 g7 V# O! c# ~If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
, \0 A+ z F+ D; ^shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.4 D# k6 v! k. q) _" I# D- i- z. a* e
That assupmtion violates the above rule. " w$ t5 }: ]) p ' ?/ q, g) f! S+ W/ U* {+ s9 XSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 2 L9 n1 O/ h8 ]" Z/ F
% m+ k% D2 f: o0 j0 v ?3 s" w3 H. e
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from1 R* D; p( Y9 l# @4 V t0 Y6 F
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
% z% e- I2 J6 ?9 H5 Y/ i% Tit was the C-7 with the OAC group (compound II), not the C-6 with the
% T' @4 z* a$ a) ~) Z* `OAC group (compound I). 0 X6 g4 H! v" Y( F4 V" v
c! c+ D* y1 s9 }, r# x
Simple, 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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