On acetylation (OH group was changed to OCOCH3 group), the alpha- f' O! K' ~+ W, o M% [! _- Y
carbon will show a downfield shift while the belta carbon will show a; }$ [. H: h, {- w& l
upfield shif. 0 [. L- D' ^/ D8 J: Q* [7 d; T2 [ 9 Z! _3 C/ w6 u1 L; Q( uSee the following article how to revise a structure reported before just using the above rule! * _5 {( ]/ R) u$ X, k9 ^0 U# U: J
& u2 E4 e8 \# N
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. % D: o' F; o1 |/ g & ^4 |) k! _; o. C. LAnother compound with one OH group acetylated: it could be C-6 OAc, C-7/ d, f+ o9 S8 f3 {7 k0 H6 b( z" w0 M5 u
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II: ^8 c, ~0 y3 N, {6 v; L0 }& C3 K
in the article). The chemical shifts of the two carbons are 78.1 and( E0 E. l& r# R( t# o9 l: _
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
6 c9 f* R' x! p5 ~
+ P1 \7 a' z4 c* Q/ S: ~# t" IThe auhors resovled the problem: & x0 y# e9 |& B, }2 Q9 W 9 T! A$ M: r* S3 \: ~If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical+ [& I3 [9 o2 t( Z
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
- |+ h M3 K; E/ ?# ]8 W4 IThat assupmtion violates the above rule. ' Q2 k( @6 o8 ^9 l* e. `0 j
! K+ ]3 l1 X" ?2 X! w9 ^: W; P
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. : l6 c2 _) t0 v, H* p+ p4 a . @5 |- D8 {! h. MC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
J3 f% g9 u4 T% k% }: Z$ Q& o, C79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
: d( |5 J' G- w' L8 F. Z! Wit was the C-7 with the OAC group (compound II), not the C-6 with the# ]1 ^5 u- M1 K0 H# t
OAC group (compound I). 7 a0 b' o* S2 z$ y, g - p4 G& s, R" |3 E2 ISimple, 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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