On acetylation (OH group was changed to OCOCH3 group), the alpha7 m3 b1 u/ y4 v8 ~+ W- b% k
carbon will show a downfield shift while the belta carbon will show a
. p) p1 s0 O3 G4 \2 U- T% u6 M; |upfield shif. ' N" o: p/ F# t. V
& f, m3 _4 w3 B, t" g. VSee the following article how to revise a structure reported before just using the above rule! % I% L6 s- K) K2 _+ \( u
/ U, P, R$ M1 Q( `! wOne 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 b/ I" o; ^ E
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Another compound with one OH group acetylated: it could be C-6 OAc, C-7
: {4 g, l2 v4 }6 ?7 M2 ?, NOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
# q6 j D4 i& ?4 ~: Y4 ain the article). The chemical shifts of the two carbons are 78.1 and$ x2 M6 B+ U9 q' d. P$ k/ w
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: 6 G0 ^+ X- g- q1 q% a& t
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If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
* U7 s1 t+ {5 A9 n2 oshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.8 v- M5 S0 `3 H4 _# P
That assupmtion violates the above rule. 0 N" q, g. h$ ?' R. ?$ o
& S- d: x1 H- L! Q2 q7 }7 S7 ?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. 8 ]! x' B& M3 k7 C6 v) t
9 h' G% O* U% h. P* w3 NC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
1 f0 u9 N3 p: ]0 r2 ]79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
* |( P5 A; {( @it was the C-7 with the OAC group (compound II), not the C-6 with the
2 X' E! F6 \# O, V) VOAC group (compound I). 4 c1 h& {% M- l- m% o9 Y * D& N+ t7 Q. M2 I+ i, f# u! J
Simple, 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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