On acetylation (OH group was changed to OCOCH3 group), the alpha, `" K4 ~3 S2 f
carbon will show a downfield shift while the belta carbon will show a
% |9 Y% }9 P3 H1 b! P; lupfield shif. : N+ S$ C! B5 ~ f5 L $ }0 i! o" w# l9 |2 U; X2 f5 C5 p( C
See the following article how to revise a structure reported before just using the above rule! 9 U8 u$ i; X8 v5 `( m 2 V. {- M; X8 b0 ^+ C$ z
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. % p* B/ |8 @( d0 v + q2 U: ?! b: z9 jAnother compound with one OH group acetylated: it could be C-6 OAc, C-79 Y) G9 L' K4 f& y; C' W% X, n3 c
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II' H( x% m! i8 g% @) ]5 h1 d; B" R
in the article). The chemical shifts of the two carbons are 78.1 and6 `3 c" A5 w7 k R* U- U
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
8 {$ b3 n* S+ }& \+ c
5 s: q2 Q$ Q$ r: q2 |9 f* D' W
The auhors resovled the problem: + i, G0 V3 [1 Z" ~5 W; o ! K# A6 [2 e1 T0 bIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical# H+ e- b$ g: [5 y \
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
- [, |: f$ l- ?! s+ i F- I) T0 VThat assupmtion violates the above rule. , \7 D. J; _/ w/ E0 R2 P * F; _4 v; |) O- H2 fSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 9 w, W* x: p3 N: O( z$ g- K
. v8 }5 ~, o# F+ s6 [- V' V$ x
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
3 Z2 t/ g. F' o7 G# Y2 i& t79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,# O; P5 O% o4 F- E7 {
it was the C-7 with the OAC group (compound II), not the C-6 with the
) ?- X( j* Y! t7 t* n! h* FOAC group (compound I). , Z4 v% S) X8 m% A6 f
: v' ` ~; {# w1 }0 w
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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