On acetylation (OH group was changed to OCOCH3 group), the alpha3 q- i, ^# t( l- S' a
carbon will show a downfield shift while the belta carbon will show a$ I! p2 j9 ]' `9 H
upfield shif. " O/ j4 q2 p6 @. y9 V & I2 V/ F( o% e( ]6 u2 rSee the following article how to revise a structure reported before just using the above rule! 2 ]& g4 ^$ `4 V/ t : h$ t; g5 {: ], @) `( f/ x) u# g( I
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. * V3 [% r A$ V% P2 h 5 {% h3 S8 M eAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
Y% u8 v7 F9 o7 \7 _& t8 m' i6 AOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
* z& p0 X2 d4 {2 F- M: N" ein the article). The chemical shifts of the two carbons are 78.1 and- a; Y4 e# Y- B9 y. E9 m3 W
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
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2 A1 t t( B8 J7 D4 ?- K# ~4 _1 TThe auhors resovled the problem: / G" |- d$ O! r* Y4 `- w/ Y
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If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical* {3 C) q+ I* b- y. r8 I2 X3 A
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
+ p0 \3 Q% X" HThat assupmtion violates the above rule. , A3 C! ^# M( N- z8 h ) i" n& L2 q8 R( ?) i \! b* ~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 o0 R1 R% E! ]( Y Y( Z* e$ y; U2 w8 l: aC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from X' }& P# a$ {
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,: ]) D4 L! d8 j
it was the C-7 with the OAC group (compound II), not the C-6 with the. [ M8 ? U4 N( o3 z
OAC group (compound I). 7 E+ ?, L; D' w6 I% H, H t * i* L1 u. ~1 N" ~$ o8 G
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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