On acetylation (OH group was changed to OCOCH3 group), the alpha% u. I' w8 F/ }# N# h
carbon will show a downfield shift while the belta carbon will show a0 `( T, A# R0 {8 T
upfield shif. ! P1 y: N% v# T # f5 a8 f8 f: y2 kSee the following article how to revise a structure reported before just using the above rule! 0 Q( \8 ~( b5 ` P1 V! ? : r4 _ F% Y- f* K+ 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. 0 [$ q7 J1 o8 p* J' G3 h: N0 q& t . s+ i# H- I# a' o5 I" r/ n% LAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
1 a- z) ^1 \ ?: J3 _& sOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II" T9 z5 N' U# z8 ?8 ~0 I
in the article). The chemical shifts of the two carbons are 78.1 and
7 E6 U! w6 z5 E0 h80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
4 w- L+ L* X) p6 L) l 6 x1 b$ F+ L6 }
The auhors resovled the problem: 3 x3 x5 p- k/ m0 u1 e 3 _; G0 H3 C5 ?! N
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical. i+ L+ M2 ]* W" l* a
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.# ?5 G) A3 m/ m: g
That assupmtion violates the above rule. " Q( o- ]2 K. z- G0 v, D S' Q1 t3 A8 G& D7 aSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. $ _( e" M/ [9 y4 b
8 B3 C2 W) k( Z5 c$ }# J
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from# Y. G) h. _% C' F8 a9 K
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
( ~' n/ N) ~& Y, ? C" g- w: _it was the C-7 with the OAC group (compound II), not the C-6 with the
4 H8 k$ Q( x& Z: |2 e1 @" |OAC group (compound I). / x- w1 Z- g$ p5 X5 G5 u* o3 m; U
& I; L3 A8 u3 M$ T, X2 A
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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