On acetylation (OH group was changed to OCOCH3 group), the alpha
, r* \+ X, }3 r, ccarbon will show a downfield shift while the belta carbon will show a
5 J6 B, B$ {% t9 E7 n9 zupfield shif.
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1 |+ T* k3 Y! A& @2 N. h: u7 |See the following article how to revise a structure reported before just using the above rule!
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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.
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& ?) W4 s' J$ G2 C3 C/ oAnother compound with one OH group acetylated: it could be C-6 OAc, C-7' i* ?" m; A% d/ ~9 s6 D+ F
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
Q1 j7 }- X. \# `in the article). The chemical shifts of the two carbons are 78.1 and
5 T* T3 e* R% ~. |2 [* f" i) ^# b80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
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The auhors resovled the problem:
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) v0 S) r9 D; W/ l* V, r' e& \ G8 bIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical" `- s; n8 I G. W3 `
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
8 E3 ^$ N, y) w" u/ IThat assupmtion violates the above rule.
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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.
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C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
% c8 } m3 A' {. J$ l; ~9 A79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
! \! ]6 T5 h5 s Z0 f7 Yit was the C-7 with the OAC group (compound II), not the C-6 with the) X. V5 x2 |- Q, b( l; |0 q
OAC group (compound I).
, p) W8 x- i1 E3 c7 Y
# P* M0 y7 z+ f a- I ]1 ESimple, 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
[em79][em79]
