On acetylation (OH group was changed to OCOCH3 group), the alpha
) X6 D8 w1 V. @carbon will show a downfield shift while the belta carbon will show a& r6 E$ j: \7 v9 m! T
upfield shif. 8 \. m# u0 r8 H' R
- b/ u& }/ c; @See the following article how to revise a structure reported before just using the above rule! & N; u- O$ |$ K# }4 f) p! k 6 N1 z* @+ i' Y T. iOne 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. . o7 n0 W N/ y
) [8 }. [# p8 P \' @Another compound with one OH group acetylated: it could be C-6 OAc, C-7
2 i9 ~- i" }# K) P& X$ F$ ?; K# g+ pOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
7 G- t$ s; E3 g; [, g. b Tin the article). The chemical shifts of the two carbons are 78.1 and3 \: f- ?/ L3 ~# L* J& v6 c
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
; k" g( ]' b0 H- m! W 1 J h3 x4 y$ N9 vThe auhors resovled the problem: 5 i9 y5 Q8 R7 u" O& q - l) i0 e& z/ p5 F5 W* r/ [& W( o9 \If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical. m3 i4 s4 K. K4 H9 ]
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
( c. S! [! I/ K* {# ?That assupmtion violates the above rule. 7 \/ I/ T0 I, L3 R4 W ) a8 }% w; l* u7 s0 i
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. & K7 [: a/ N, w8 ~/ a8 K : o( W2 `" l" A! L# |, jC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
3 N! d- j. _9 R/ s7 ?5 B" x79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,, @5 ]/ l& I0 U4 u; y
it was the C-7 with the OAC group (compound II), not the C-6 with the( P6 d/ B- z u! F$ L) d2 b
OAC group (compound I). - E* F7 w' l5 ~ ) _ ]* w8 ]9 B( x9 {* k. pSimple, 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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