On acetylation (OH group was changed to OCOCH3 group), the alpha
/ V. I$ D+ ~* C Ucarbon will show a downfield shift while the belta carbon will show a
" Z0 k+ V, l3 zupfield shif. 8 G8 ]/ |" m& Q( \9 Q( [ ' C3 R: n/ q# T( z& a
See the following article how to revise a structure reported before just using the above rule! ' S- N- o! U/ T8 d" K \/ `0 W& T& I: G
. `: Z# s C/ C# C/ `& {6 S
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. 1 P# D, \' s+ d7 b+ f
2 Z: T8 N' A* d: Y3 [+ r4 FAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
6 X$ T' G% D& `; POH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
+ j. m& ?0 P Gin the article). The chemical shifts of the two carbons are 78.1 and
( s- x) S+ j5 t( c* h80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
* y. H8 i6 I# e% Z/ d8 R3 t # z) O( T* |7 ]0 t' K: a
The auhors resovled the problem: & `' F! d; L# ?# w) q( f/ D
- g; ~3 I+ a l* q: w% m
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
3 g t/ Z; u7 m' J/ Ushifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
/ S% y4 Z$ d& ]# U u5 q0 Q; m5 @That assupmtion violates the above rule. + b, D& J6 H- c6 M+ Y3 A 1 `3 C4 E7 C# c2 LSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. ! Q+ e9 A3 b3 S3 _! @9 k4 g- Z [
( a) I3 A' B/ B3 F4 g6 pC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from& S4 c0 S4 Z& c( j: ~
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,0 i* d$ V0 d8 l% Z# I q6 t
it was the C-7 with the OAC group (compound II), not the C-6 with the
# B$ Q" Y( ~$ W- H. @OAC group (compound I). ' Y8 V" D5 q/ t
5 ]5 e/ {1 [9 `1 j5 ^
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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