On acetylation (OH group was changed to OCOCH3 group), the alpha$ s8 N0 e. T( T# s
carbon will show a downfield shift while the belta carbon will show a
9 T4 h5 W& C5 B- V8 L3 ]upfield shif. 0 ~+ Y; _5 t" I! c* L ' b3 d- X8 Y* v+ `
See the following article how to revise a structure reported before just using the above rule! 7 |' \. O0 n" N( T" D" u1 k
1 w+ t: P% Z& H$ E0 H
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. * z0 j$ }8 ~* E+ A
; B0 R) t) j9 M9 GAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
. n' f7 B W1 I" i: V3 |% D2 VOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
0 X H: F/ U/ t$ {- P& x) Ain the article). The chemical shifts of the two carbons are 78.1 and
?- ~% V9 v4 `' G. ]& _80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
& D$ M1 ^( K7 @+ V2 n+ L$ {/ v
, h4 j$ \& e, l1 H
The auhors resovled the problem: E" |8 D. i1 ?$ n# s4 i9 v$ ^
2 F/ q* R: V1 C" u! EIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical9 G0 U( ]; y& Q1 {! J
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.8 t2 e" t1 x1 {# f" U( C2 H
That assupmtion violates the above rule. 2 |% B! ^4 S2 G; H5 R& `% l. f ) D- f9 |7 o" R
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. 1 v: B" n8 N" {( _$ \
$ Z+ P1 _1 O8 ` {' U3 b1 w
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
# W. L: { |# ]4 m6 z79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
2 k" U6 \, z: u+ x* X9 Tit was the C-7 with the OAC group (compound II), not the C-6 with the
! U4 V x/ n# YOAC group (compound I). ( ?& l! p* Y2 x/ [6 I8 W8 b& A; ]
" ?1 C N: n) ]+ [* |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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