On acetylation (OH group was changed to OCOCH3 group), the alpha
- C/ j' d) q& Z, Ncarbon will show a downfield shift while the belta carbon will show a& k+ V% M3 G B6 n
upfield shif. 4 r' c2 x' ~+ w0 i/ Q6 \. P$ n
( i+ I" j: H7 u1 G0 Z* I& R& j: eSee the following article how to revise a structure reported before just using the above rule! 1 g% k& T' W' u& M ; I3 d. t2 J/ e, J4 t; }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. 8 K* }2 t+ ]: t
8 F$ J4 R0 a2 a6 X5 E. W+ X! r
Another compound with one OH group acetylated: it could be C-6 OAc, C-7 C. a- Z/ a) }2 r
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II: w" W/ S% k' g) i' I$ [: {
in the article). The chemical shifts of the two carbons are 78.1 and
/ f( |# R. e9 Y7 H80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
1 R$ u" O+ p6 V- O. D. m + p7 g; F7 e1 ]8 C @The auhors resovled the problem: % B0 X7 ~/ u6 D3 ]# C, N
2 ^; R5 G4 d6 N) Q2 ~; L) F( A2 D/ QIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
! }8 U. e' Q q$ }* X, Z) Lshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.% _8 S7 ^. C6 y3 U( t
That assupmtion violates the above rule. $ k C* r: t, H" H _
! j( r8 [3 f% a! a: _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 q5 s7 ~! V5 V) k* T; k
6 H: W' e- V7 H+ L4 Z/ N
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from$ y5 B, ^3 n' o1 T0 F- a
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,& }5 H4 m8 |& Z) [
it was the C-7 with the OAC group (compound II), not the C-6 with the K- b, E9 Y% S8 t& ^" e# y
OAC group (compound I). 5 ?* T. g3 U, X7 Y* \: R! i , B" a5 b. s& m. N. V; cSimple, 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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