On acetylation (OH group was changed to OCOCH3 group), the alpha
# K& E' `, U: X ocarbon will show a downfield shift while the belta carbon will show a
- D, w* ]9 c0 H3 bupfield shif. * }+ I( l( j# T, a, x3 | 1 R) O: p( W* a0 }9 v
See the following article how to revise a structure reported before just using the above rule! " j* Z+ r$ V7 R" q6 D: t q
/ }/ b) X+ `. Z4 A- P+ F6 y
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. # H# m6 E) I4 K& H" g
' X/ p# T) C; c8 r$ R# z: [Another compound with one OH group acetylated: it could be C-6 OAc, C-7. g5 n- k& Z& s
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
9 H' T0 l, m+ j( W0 Din the article). The chemical shifts of the two carbons are 78.1 and
$ Y8 P2 D8 m7 b1 s9 y' {: E80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
e6 w4 H( Q8 _5 L; j, `1 Q4 D 6 z: a `$ l% e* I/ _
The auhors resovled the problem: 6 o# u s) e1 M% G- J* a( ^
' Y! a) V z" hIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical% d* h8 M3 q1 @
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
5 i) `: \0 z4 ?3 Z% hThat assupmtion violates the above rule. $ i2 h7 g: E: [, `
' ]7 ~/ j2 K E$ U2 G g8 {' O
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. ) ^' \2 L* c& F' m: A( i. k ( G# s! D0 b6 M7 l$ @- r* VC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
( R! Q2 y- Z- ?; l79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
- i! p7 j, B9 I' J( nit was the C-7 with the OAC group (compound II), not the C-6 with the
" ^& B. Y' C# V2 @1 I' {OAC group (compound I). 2 W4 c N; r' ?* a% P 2 D- l+ e( }: g/ K5 @6 }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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