On acetylation (OH group was changed to OCOCH3 group), the alpha% t& g0 N7 d7 Z4 Z" z# f. ~1 ~( C
carbon will show a downfield shift while the belta carbon will show a+ u& [& b0 [- ]* Z" r
upfield shif. : ?! [/ J: {# I/ g% U
0 S, O8 @1 [' gSee the following article how to revise a structure reported before just using the above rule! ' x: Z1 y7 a6 q0 h) B$ k' v" L
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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. ' I0 A! P" ?( q; f* }$ F. a
8 M$ l' [$ l! v, p/ lAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
N6 U/ ]/ o3 M& L0 k4 ^% XOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II' a9 b' k2 e/ K4 I& e6 t
in the article). The chemical shifts of the two carbons are 78.1 and5 _ f; P! m7 `; ? ^6 ~
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
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The auhors resovled the problem: - G5 ^8 y P) B6 K% M. Q7 f7 ^ . K8 h; c7 D- H/ P
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical, w* P4 q. h0 A6 D! J
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
' S5 g% e3 `* o' Q- v! HThat assupmtion violates the above rule. 0 J; H e+ K u% H' J1 N* \
# [% A$ I+ Q, N* }) K' \ SSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. " A' Y/ x' X$ [, y# K- C: i 7 X1 }0 S$ x* W8 e& x+ [C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from4 g6 j6 O" [" X
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
6 Z9 ~ @& C( j! A( W$ P( Q- x' ^it was the C-7 with the OAC group (compound II), not the C-6 with the
8 T; L4 ^. w5 P( ]4 DOAC group (compound I). * A9 c6 l8 w1 ^
8 H) o7 r7 K* U: [7 Y! \/ uSimple, logic structural revision published in 1981!
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楼主: 上善若水
发表于 2009-5-28 16:27:12
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