On acetylation (OH group was changed to OCOCH3 group), the alpha
2 g0 V( V) t' k& f) E' {( _* ~carbon will show a downfield shift while the belta carbon will show a
! `* A0 w) z' \; z! Q6 mupfield shif. - v9 k/ f* d6 D9 {7 \) u: }' G
* N9 E6 I! m" b* }9 vSee the following article how to revise a structure reported before just using the above rule! 6 y2 w$ Y7 |& P- [, J 7 H1 s9 t1 o R+ y1 g8 i
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. & I0 g: C- F# J: l) B6 |
% q3 T9 ?$ y& A' r( H9 JAnother compound with one OH group acetylated: it could be C-6 OAc, C-75 K$ z( S( J& h7 D1 n& h' T
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
7 w7 d7 j. N9 B$ @in the article). The chemical shifts of the two carbons are 78.1 and
& f% K" h. B$ w( K80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
) `& H0 _/ z! L% y5 J
/ f9 H: ^9 M; V: U% h- C0 f. tThe auhors resovled the problem: ; u4 d9 w: a" Y! j# ] $ ~4 p- n( R0 t" H EIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical1 ~# Q( Q. x( N+ L
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased./ ^* D) e: j+ z4 W4 o1 ]4 A
That assupmtion violates the above rule. 9 w1 w/ d4 M3 g3 c( }# P: c
. O2 m1 x4 @ f- H6 t- L# L& _( U7 OSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 5 n. C. T5 L2 l. M. J1 A- q7 a
" g" h I! s4 U/ m( H5 K, t7 x z
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from; q4 B0 x+ d; O( L
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,) Q/ t. L5 V( L3 Y# x" O/ {# _
it was the C-7 with the OAC group (compound II), not the C-6 with the
& @$ v9 @! F; }: w% FOAC group (compound I). 2 h' q7 s3 T# c3 ?: e" u7 ]/ v, m
6 }. ?8 V, G, p2 K# `8 y$ `Simple, logic structural revision published in 1981!
: U8 I0 j& L* n4 G- t
7 G4 i: l' ?/ |7 c; E" m + B; ~1 J! q: c4 }, [0 T! B
楼主: 上善若水
发表于 2009-5-28 16:27:12
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