On acetylation (OH group was changed to OCOCH3 group), the alpha7 `2 g% \5 U7 ^ }, }
carbon will show a downfield shift while the belta carbon will show a
, o. V2 a7 i3 h* U* Q$ f2 Iupfield shif. ' \. K: g- K; `, ~% l8 d, [ % c" w6 r6 b) W* p1 H& l& h5 LSee the following article how to revise a structure reported before just using the above rule! 9 b: C7 H5 t3 l( h! u- b; i/ A3 B
8 F! t' B" i i
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. 7 c x9 E: N E0 Y 6 c0 _8 X ?3 Y, ^
Another compound with one OH group acetylated: it could be C-6 OAc, C-77 X( \! D2 a, L$ p9 X8 h5 n+ s
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
/ @& W- z) r" u7 M4 L4 |. Pin the article). The chemical shifts of the two carbons are 78.1 and; q9 R+ G5 M# l' E. A" }
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
' k7 q8 b% z$ q N% k) o6 z J+ l% {7 D
8 b4 c: H/ b- C, m8 B8 X2 |; ?The auhors resovled the problem: . |# h; c: x7 S/ q3 Y% Z) E " T% d5 N$ }, i/ D# R& bIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
- \* s% J* |- M# ^* p+ S2 qshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
2 t+ n9 h& L2 ~$ F2 [% U2 nThat assupmtion violates the above rule. . V) y9 Q- c: b8 j+ h! F % o( p; M; H* ~5 h$ G
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. ) H& m8 J* v% r( t
( B9 I3 }* z/ C- o/ j' Y
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
o/ M; ~6 x, q' ] a& t. Y79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
% `0 r# p4 v; G, o8 L/ u9 iit was the C-7 with the OAC group (compound II), not the C-6 with the3 w6 X, q4 b* g3 E: G5 y
OAC group (compound I). 7 e8 d ~+ G) E( b/ x1 s; k, T * E5 K: }. e; n' g" m
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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