On acetylation (OH group was changed to OCOCH3 group), the alpha
' X3 Q: Y8 j& l6 h2 v7 ?$ Bcarbon will show a downfield shift while the belta carbon will show a7 W9 |8 Q6 E" }7 i/ ^& \
upfield shif. : }9 {7 Y7 k; e7 ]
; k- e- i# Q9 a7 @" x
See the following article how to revise a structure reported before just using the above rule! " E% q$ D; l' x9 o# h
$ y6 y) o i$ uOne 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. * P6 g/ c* N( n7 |# _
, y p" D; h9 J- {3 R j7 v
Another compound with one OH group acetylated: it could be C-6 OAc, C-7! ~/ Q8 p6 k5 H" b8 G' ]8 b2 M8 z
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
& h& t" P5 F5 `, j0 _% @in the article). The chemical shifts of the two carbons are 78.1 and" ]7 Y+ f% l5 a
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
. W# I8 I [! H/ W( g+ F1 t
7 t Z# n# j5 t c7 oThe auhors resovled the problem: , f; D' k V- P1 ~ 5 e0 r) d! o' t7 A
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical/ c% g `2 M. {
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
6 D5 ]3 F7 e! j% GThat assupmtion violates the above rule. 9 X1 K5 i! p2 B+ K
* m' j' k& w* i) Y) ^: kSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. $ A1 o4 r% S6 W: z4 T
3 g' C# {. \0 M, l5 `* m% B
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
9 v: [0 Z0 N9 \; I! V79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
a+ E9 W0 u, l6 {8 h1 R+ Uit was the C-7 with the OAC group (compound II), not the C-6 with the
9 ^3 B$ }6 M$ r4 E# v' Z) t8 uOAC group (compound I). 3 _- v! {- R% c, q6 ^( S6 A. \* ]% R " Q3 p# r# _# W3 X: m: V( U! fSimple, 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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