On acetylation (OH group was changed to OCOCH3 group), the alpha
4 F7 G. S7 O0 s/ w! Bcarbon will show a downfield shift while the belta carbon will show a: I- r+ p+ i) s' g% ?$ T* ]2 ^, q
upfield shif. ( i7 X8 u9 X! v * }) k8 y5 a& w+ V: L6 f' OSee the following article how to revise a structure reported before just using the above rule! " O5 ]( d9 F6 Y0 Q2 t ; @& V# i# e6 x9 aOne 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. 9 }5 h* U( q( K9 e* h) X/ u* i' q
0 e0 @4 P; W7 w2 F- U* fAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
/ A3 N+ ^. b/ [% z2 bOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II! q& s# }( V0 }$ K+ y: O* H: n
in the article). The chemical shifts of the two carbons are 78.1 and8 ~- @5 L( a/ B6 k3 ]( S
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
2 Q* R" A7 s$ G& F, ?4 ` 9 ?+ x! B: U) K1 b3 L, d& T: AThe auhors resovled the problem: - U- }" y1 l$ D2 U, f* M" N( r
* @" n. W/ o& A/ d8 F+ y9 `, J4 ]If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical3 Q% C; }+ x; o& C i) n
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.4 T- F/ U9 i5 M) P4 ?: t) B, d
That assupmtion violates the above rule. 6 Z8 [2 g# B5 P5 O( W3 j# l ~& l
$ N& m5 w- i/ Q5 ?
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. G" k! U2 c: E1 Q
; a* x0 K8 o) t* Q
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
3 V3 U7 |; ]4 U! k# {6 r% A* u79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
. |- p4 ^( a+ Sit was the C-7 with the OAC group (compound II), not the C-6 with the
# ^4 x z6 ~6 i. ?6 {: L: W9 U8 nOAC group (compound I). # G5 L. Z/ g# y @6 i/ k
* x7 t8 b" C2 k6 l4 z
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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