On acetylation (OH group was changed to OCOCH3 group), the alpha0 J' j% c$ A$ V$ V
carbon will show a downfield shift while the belta carbon will show a. C0 ` y [5 k
upfield shif. - q) ?7 A( v9 {3 g+ n% l
9 k: s) U9 c8 M0 c7 n; \2 }% ~- I
See the following article how to revise a structure reported before just using the above rule! 3 n& q% U: p$ }) ]8 F2 `5 R- x
: O* @( I3 |9 s8 |( Z* j# J2 COne 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. $ U0 g" [; I. E0 B/ H ; J, l# ]3 y1 [8 P* P4 Y0 V% j: cAnother compound with one OH group acetylated: it could be C-6 OAc, C-7/ u, N( p- |# {8 S4 d( {
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
! D @# P; R# F, M/ Y3 [. e$ D# [in the article). The chemical shifts of the two carbons are 78.1 and9 A% ?1 K- ?9 E6 F* j5 |: v- K" ]
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
" [. W- G* u: ~6 t9 v( l; V
+ @( C T, V5 h: SThe auhors resovled the problem: / e" |4 o# M- P4 c+ ~
2 x- h, D' b% O+ ^* m
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
3 l8 e6 @- v& z- pshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
% L7 S7 E& z. T1 vThat assupmtion violates the above rule. S$ r: s `* m8 M; i/ L/ S- x3 ?" k ! b. E5 z2 k: hSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 7 Z8 o* y1 ? a; a& q) I
$ i' n2 u, w( f3 b3 OC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
' |8 O5 I8 {% @ Y* w5 x79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
8 U7 N! K" z0 G& `8 _/ S; e Git was the C-7 with the OAC group (compound II), not the C-6 with the
" J( |# Q& k X% a# u4 {OAC group (compound I). / R8 [0 W0 d5 p9 Z, b# `4 x, W
1 e4 i9 c8 Z2 B) e, v" r, ?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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